How Flood Risk and Drainage Affect Development Potential?
Flood risk can determine whether development is acceptable, while drainage can determine whether the proposal can operate safely without increasing flooding elsewhere.
The planning system seeks to direct development away from areas at the greatest risk of flooding. Councils must consider whether proposed homes, employment buildings, infrastructure and access routes will remain safe throughout the lifetime of the development and whether the proposal could increase flood risk to neighbouring land or communities.
The Environment Agency Flood Map for Planning provides an important starting point by identifying land within Flood Zones 1, 2 and 3. However, the mapped Flood Zone does not provide a complete assessment of every source of flooding and does not replace a site-specific Flood Risk Assessment where one is required.
Land within Flood Zone 1 is generally at the lowest probability of river or sea flooding, but it may still be affected by surface water, groundwater, sewer, reservoir or ordinary watercourse flooding. Conversely, land within Flood Zone 2 or Flood Zone 3 may still have some development potential where the proposed use is appropriate, the Sequential Test is satisfied and safe development can be demonstrated.
Drainage conditions can be important even where the site has no recorded history of flooding. Development increases the amount of impermeable surface and can cause rainfall to run off more quickly. Without suitable storage and controlled discharge, this can overload watercourses, sewers or drainage systems.
Ground conditions also affect drainage feasibility. Some soils allow water to infiltrate readily, while clay, made ground, contamination or a high groundwater table may make infiltration drainage unsuitable. The capacity and location of nearby watercourses, public sewers and drainage networks must therefore be investigated.
Flood risk and drainage can influence the developable area, density, road levels, finished floor levels, open space, access arrangements and location of attenuation ponds or basins. Land may need to be reserved for flood storage or sustainable drainage features, reducing the area available for buildings.
Early assessment is particularly important when land is being submitted through a Call for Sites, HELAA, SHLAA or Local Plan process. Councils need confidence that a site is genuinely suitable and deliverable. A proposal that ignores mapped flood risk or assumes drainage can be resolved later may be assessed as uncertain or unsuitable.
Flood risk and drainage should be considered before the proposed development capacity is fixed. A technically realistic masterplan should place vulnerable development in the safest parts of the site and reserve sufficient land for drainage, flood storage and safe access.
Check the Environment Agency mapping
Use the Flood Map for Planning
The Environment Agency Flood Map for Planning can be used to identify the mapped probability of flooding from rivers and the sea and to request flood risk data for a particular site. The map is an important early screening tool but it should not be treated as a complete flood risk assessment. It does not show every local watercourse, drainage problem, groundwater issue or surface water flow route that could affect development.
Site-specific levels, topography, historic flood information, local drainage records and hydraulic modelling may reveal a different or more detailed picture than the national mapping alone.
Flood Risk and Drainage Factors That Can Influence Development Potential
A flood risk review should consider all relevant sources of flooding, the vulnerability of the proposed development, the safety of future occupants and whether surface water can be managed without increasing risk elsewhere.
River Flooding
Rivers, streams and ordinary watercourses can overtop their banks during periods of heavy rainfall. The probability, depth, velocity and duration of flooding can influence whether development is acceptable, where buildings should be located and whether floodplain compensation or hydraulic modelling is required.
Coastal and Tidal Flooding
Land near the coast, estuaries and tidal rivers may be affected by tidal flooding, storm surges and rising sea levels. Development assessments may need to account for climate change, defence standards, residual risk and what could happen if a flood defence were overtopped or breached.
Surface Water Flooding
Intense rainfall can flow across the ground or collect in low-lying areas before reaching a watercourse or drainage system. Surface water flow routes can cross land in Flood Zone 1 and may affect building positions, road levels, access routes and the location of drainage features.
Groundwater Flooding
Groundwater flooding occurs where the water table rises above ground level or enters basements, foundations and drainage systems. It can be difficult to predict and may affect infiltration drainage, earthworks, construction methods and the suitability of below-ground structures.
Sewer and Drainage Flooding
Public sewers and private drainage systems can become overloaded, blocked or damaged. Historic sewer flooding records, network capacity and the proposed discharge arrangement may need to be reviewed with the relevant water and sewerage company.
Reservoir and Canal Flooding
Some sites may lie within an area that could be affected by reservoir failure, canal breach or overtopping. Although such events may have a low probability, emergency planning, residual risk and safe access can still require consideration.
Ordinary Watercourses
Small streams, ditches and channels may not always appear clearly on national flood maps but can still create significant local flood risk. Their condition, ownership, capacity and maintenance responsibilities should be investigated before the development layout is fixed.
Culverts and Restricted Channels
Culverted watercourses can become blocked or lack capacity during major storms. Development may need to retain access for maintenance, provide stand-off distances or investigate whether opening a culvert could improve flood risk, drainage, ecology and placemaking.
Safe Access and Escape
A development may need to provide safe access and escape during a flood event. Even where buildings are positioned outside the highest-risk area, the scheme may remain problematic if residents, emergency services or vehicles would need to cross deep or fast-flowing floodwater.
Understanding Flood Zones 1, 2 and 3
Flood Zones indicate the probability of flooding from rivers and the sea. They are an important starting point when determining whether a site and proposed land use are appropriate.
Flood Zone 1
Flood Zone 1 has a low probability of flooding from rivers or the sea. Most forms of development are generally directed towards this zone where reasonably available sites exist.
Flood Zone 1 does not mean that land is free from all flood risk. Surface water, groundwater, sewer, reservoir and ordinary watercourse flooding must still be considered, particularly for larger sites.
Flood Zone 2
Flood Zone 2 has a medium probability of river or sea flooding. Some development types may be appropriate, but the Sequential Test and a site-specific Flood Risk Assessment may be required.
The assessment should demonstrate that the development will remain safe, will not increase flooding elsewhere and can accommodate climate change over its intended lifetime.
Flood Zone 3
Flood Zone 3 has a high probability of flooding from rivers or the sea. The acceptability of development will depend on the proposed use, the Sequential Test, the Exception Test where applicable and the findings of a detailed Flood Risk Assessment.
Part of Flood Zone 3 may also form functional floodplain, where land stores or conveys floodwater. Vulnerable development is normally heavily restricted within functional floodplain.
Flood Zone boundaries do not always follow field, ownership or development boundaries. A site can contain more than one Flood Zone, allowing the proposed development to be directed towards the areas at lowest risk while higher-risk land is used for open space, drainage or flood storage.
The Sequential Test and Exception Test
The planning system uses the Sequential Test to direct development towards land at the lowest probability of flooding. In some circumstances, the Exception Test must also be passed before development can be considered acceptable.
Sequential Test
The Sequential Test considers whether there are reasonably available alternative sites appropriate for the proposed development in areas with a lower probability of flooding. The relevant search area and the range of alternative sites will depend on the type, scale and purpose of the proposal.
- The proposed use and its flood risk vulnerability are identified.
- The appropriate geographical search area is established.
- Reasonably available alternative sites are reviewed.
- The proposal is directed towards the parts of the site at lowest risk.
Exception Test
Where the Sequential Test is passed but the proposed development remains within an area of higher flood risk, the Exception Test may be required. Both parts of the test must normally be satisfied.
- The development must provide wider sustainability benefits that outweigh the flood risk.
- A site-specific Flood Risk Assessment must demonstrate that the development will be safe for its lifetime.
- The proposal must not increase flood risk elsewhere.
- Flood risk should be reduced overall where possible.
The Sequential Test should not be treated as a simple comparison of Flood Zone numbers. The assessment must be proportionate, evidence-based and aligned with the relevant planning policy, development type, site allocation status and availability of suitable alternatives.
What Flood Risk and Drainage Assessments May Be Needed?
The evidence required will depend on the Flood Zone, site area, development type, drainage conditions, local watercourses and the scale and sensitivity of the proposal.
Site-Specific Flood Risk Assessment
A Flood Risk Assessment, or FRA, examines the sources and probability of flooding affecting a site and considers whether the proposed development will remain safe throughout its lifetime. It should also demonstrate that the development will not increase flood risk elsewhere and should reduce risk where reasonably possible.
Surface Water Drainage Strategy
A drainage strategy explains how rainfall runoff will be collected, conveyed, stored, treated and discharged. It should identify the proposed drainage hierarchy, calculate runoff rates and volumes, include climate change allowances and demonstrate how exceedance flows will be managed safely.
Infiltration Testing
Infiltration testing may be required where soakaways, infiltration basins or permeable drainage are proposed. The testing helps establish whether the ground can absorb water at a suitable rate and whether groundwater, contamination or unstable ground could make infiltration inappropriate.
Hydraulic Modelling
Hydraulic modelling may be required where national mapping does not provide enough detail, where a watercourse crosses or adjoins the site or where flood levels, depths and velocities need to be established. Modelling can also test the effect of culverts, bridges, channel works or development platforms.
Sequential Test Assessment
A standalone Sequential Test report may be needed where development is proposed in an area at risk of flooding and the site is not already appropriately tested through the development plan. The report should assess reasonably available alternative sites in lower risk areas.
Exception Test Evidence
Where the Exception Test applies, planning and technical evidence will be required to demonstrate wider sustainability benefits and show through the FRA that the development will remain safe, will not increase flooding elsewhere and will reduce risk overall where possible.
Topographical Survey
A detailed topographical survey records ground levels, banks, ditches, channels, culverts, roads and surrounding features. Accurate level information is essential for understanding flood routes, calculating storage and designing safe development platforms and drainage systems.
Flood Warning and Evacuation Plan
Some proposals may require a flood warning, emergency access or evacuation plan. This should explain how occupants will receive warnings, where safe refuge is available and how people can leave the site without entering hazardous floodwater.
What Should a Site-Specific Flood Risk Assessment Consider?
A proportionate FRA should establish the flood risk affecting the existing site and explain how the proposed development will respond to that risk over its full lifetime.
Existing Flood Risk
The assessment should identify all relevant sources of flooding, available mapping, historic events, local drainage records, site levels and nearby watercourses. It should not rely solely on the mapped river or sea Flood Zone.
Development Vulnerability
Different land uses have different levels of vulnerability. Housing, care facilities, emergency infrastructure, commercial uses and open space may be treated differently when determining whether development is appropriate within a particular Flood Zone.
Finished Floor Levels
Finished floor levels may need to be raised above the predicted flood level with an appropriate allowance for climate change and freeboard. Raising levels must not displace floodwater or create unacceptable effects on surrounding land.
Safe Access
The FRA should consider whether residents, visitors and emergency services can enter and leave safely during a design flood event. Flood depth, velocity, duration and warning time can all affect whether an access route is acceptable.
Climate Change
Flood levels and drainage calculations must normally account for the effects of climate change. The appropriate allowance will depend on the source of flooding, development lifetime, location and applicable national guidance.
Floodplain Compensation
Where development or earthworks would remove floodplain storage, compensatory storage may be required at a similar level and in a hydraulically connected location. The replacement storage must be effective and should not transfer risk elsewhere.
An FRA should inform the development layout rather than simply justify a design that has already been fixed. The safest areas should accommodate the most vulnerable uses, while higher-risk land may be better used for open space, habitat, drainage or flood storage.
Sustainable Drainage Systems
Sustainable Drainage Systems, commonly known as SuDS, manage rainfall in a way that more closely reflects natural drainage while controlling flood risk, improving water quality and creating environmental benefits.
Attenuation Basins
Basins temporarily store surface water during storms and release it at a controlled rate. They can form part of the site's green infrastructure and may remain dry between rainfall events.
Ponds and Wetlands
SuDS ponds and wetlands can provide permanent water storage, water treatment, biodiversity and attractive open space. Their design must consider safety, maintenance, ecology and seasonal water levels.
Permeable Paving
Permeable paving allows rainfall to pass through the surface into a stone storage layer below. It can reduce runoff from parking areas, driveways and lower-speed roads while providing treatment.
Swales
Swales are shallow vegetated channels that convey, store and treat surface water. They can run alongside roads and open spaces and may provide an attractive alternative to buried pipework.
Green Roofs
Green roofs absorb and delay rainfall while providing habitat and helping regulate building temperatures. They may be particularly useful on apartments, commercial buildings and dense developments.
Infiltration Features
Soakaways, infiltration trenches and basins allow water to soak into the ground. Their use depends on suitable soil, groundwater conditions, separation from buildings and the absence of unacceptable contamination risks.
SuDS should be designed as part of the masterplan from the beginning. Retrofitting attenuation storage after the layout has been fixed can reduce housing numbers, create poor-quality open space or result in drainage features being placed in unsuitable locations.
The Surface Water Drainage Hierarchy
A drainage strategy should consider the most sustainable discharge options first and explain why any preferred method is technically suitable or why a higher option cannot reasonably be used.
Reuse Rainwater
Rainwater may be collected and reused for irrigation, toilet flushing or other appropriate purposes, reducing the volume discharged from the site.
Infiltrate to Ground
Where soil and groundwater conditions are suitable, rainfall may be discharged through soakaways, permeable paving, infiltration basins or trenches.
Discharge to Watercourse
A nearby river, stream or ordinary watercourse may provide a suitable discharge point, subject to capacity, ownership, levels, ecology and the necessary consents.
Discharge to Sewer
Connection to a surface water or combined sewer is generally considered after more sustainable options have been examined and will depend on available network capacity and approval from the sewerage undertaker.
Why Must Drainage and Flood Risk Assessments Allow for Climate Change?
Development must be designed for future conditions rather than relying only on present-day rainfall and flood levels. Climate change can increase river flows, rainfall intensity, sea levels and the frequency of severe weather events.
Future Flood Levels
River and coastal flood risk assessments may need to apply allowances for increased peak river flows, sea level rise and other future changes. The relevant allowance depends on the location, development type, flood source and expected lifetime of the proposal.
These allowances can affect predicted flood extents, finished floor levels, floodplain compensation, access arrangements and the amount of land that must remain undeveloped.
Increased Rainfall
Surface water drainage calculations must normally include an allowance for increased rainfall intensity. Attenuation ponds, basins, tanks, swales and pipe networks must be capable of managing the future design storm rather than only current rainfall conditions.
A drainage strategy that fails to provide sufficient future storage may cause flooding within the development or increase runoff towards neighbouring land and drainage systems.
Climate change allowances can materially affect development capacity. Larger drainage features, higher finished floor levels and wider flood corridors may be required, particularly where a site is already affected by river, surface water or groundwater constraints.
What Should a Surface Water Drainage Strategy Demonstrate?
A drainage strategy should show that runoff can be managed safely throughout the lifetime of the development and that the proposed system is technically feasible, maintainable and integrated into the site layout.
Runoff Rates and Volumes
The strategy should calculate the existing runoff from the site and the volume generated after development. Discharge rates may need to be limited to an appropriate greenfield, brownfield or agreed controlled rate.
Attenuation Storage
Sufficient storage must be provided to hold water during severe storms before it is released at a controlled rate. Storage may be provided through basins, ponds, tanks, oversized pipes, permeable paving or a combination of features.
Discharge Point
The proposed discharge point must be identifiable and deliverable. Rights, levels, downstream capacity, land ownership and necessary consents should be investigated before relying on a watercourse, sewer or adjoining drainage system.
Exceedance Routes
Severe storms can exceed the design capacity of a drainage system. The masterplan should therefore provide safe surface water routes that direct exceedance flows away from homes, entrances, critical infrastructure and neighbouring properties.
Water Quality
Surface water can collect sediment, oil and other pollutants from roads, parking areas and roofs. SuDS features should provide an appropriate treatment train before water is discharged to the ground, a watercourse or drainage network.
Maintenance and Adoption
The strategy should explain who will own, inspect and maintain the drainage system. SuDS features require long-term management to prevent blockages, erosion, vegetation failure and loss of storage capacity.
Common Drainage Constraints Affecting Development Land
A drainage solution may appear straightforward at an early stage but become more complex where the land has difficult ground conditions, limited outfalls or existing water infrastructure.
Clay or Poorly Draining Soil
Clay soils and other low-permeability ground can make soakaways ineffective. Alternative storage and controlled discharge arrangements may be required, increasing the amount of land and infrastructure needed.
High Groundwater
A high groundwater table may prevent infiltration, reduce available storage and create a risk of groundwater entering basements, foundations or drainage systems.
Contaminated Ground
Infiltration may be inappropriate where water could mobilise contamination or create a pathway towards groundwater. The drainage and land remediation strategies must therefore be coordinated.
Limited Sewer Capacity
Existing sewers may lack capacity to accept additional surface or foul water. Reinforcement, off-site works, pumping or an alternative discharge point may be required.
Third-Party Land
A proposed outfall may need to cross land outside the applicant's ownership. Easements, rights and access for future maintenance must be secured before the drainage strategy can be regarded as deliverable.
Existing Drainage Infrastructure
Pipes, culverts, drains, sewers and easements can constrain building positions and roads. Accurate surveys may be needed to confirm their location, depth, condition and ownership.
Which Authorities Consider Flood Risk and Drainage?
Different organisations have different responsibilities. The relevant consultee will depend on the source of flooding, type of watercourse, drainage proposal and scale of development.
Environment Agency
The Environment Agency has a key role in relation to main rivers, river and sea flood risk, flood mapping and certain higher-risk developments. It may provide flood data, review hydraulic modelling and comment on whether proposals are safe and policy compliant.
Lead Local Flood Authority
The Lead Local Flood Authority, or LLFA, is normally responsible for local flood risk from surface water, groundwater and ordinary watercourses. It commonly advises the planning authority on drainage strategies and SuDS for major development.
Local Planning Authority
The local planning authority determines whether the proposed development complies with national and local flood risk policy. It considers the Sequential Test, Exception Test, technical consultee responses and whether conditions or planning obligations are required.
Water and Sewerage Company
The relevant water company may need to confirm whether foul and surface water connections are available, whether the network has capacity and whether reinforcement or other off-site works are required.
Flood Risk, Local Plans and Site Allocations
Flood risk is a major consideration when councils decide which sites should be allocated for future housing, employment or mixed-use development. Local planning authorities should apply a sequential approach and seek to direct growth towards land with the lowest probability of flooding wherever reasonably possible.
During a HELAA, SHLAA, Call for Sites or Local Plan assessment, councils may consider Flood Zones, surface water mapping, historic flooding, ordinary watercourses, reservoir risk, drainage feasibility and whether safe access can be achieved. A site may be rejected or given a lower capacity where the flood constraints have not been properly understood.
A landholding partly affected by flooding may still represent a realistic development opportunity. The developable area can sometimes be placed within Flood Zone 1 or other lower-risk parts of the site, while land at greater risk provides open space, flood storage, ecological corridors or SuDS.
Landowners promoting a site should avoid presenting a gross capacity based on the entire ownership where substantial areas are affected by flood risk or required for drainage. A realistic concept plan should identify the net developable area and show sufficient space for attenuation, watercourse buffers and exceedance routes.
Where flood mapping is uncertain or appears overly broad, technical evidence may help refine the position. A topographical survey, flood data request or hydraulic model can sometimes demonstrate that part of the land is at lower risk than suggested by generalised mapping.
Map Every Flood Source
Review Flood Zones, surface water routes, watercourses, groundwater, sewers and historic flood information before preparing a capacity plan.
Define Safe Development Land
Direct vulnerable development towards lower-risk areas and reserve sufficient land for flood storage, drainage and safe access.
Demonstrate Deliverability
Explain how runoff will be discharged, what assessments are needed and whether any off-site works, consents or third-party rights are required.
Can Flood Risk Prevent Land From Being Developed?
Flood risk can prevent or substantially restrict development where a safe and policy-compliant scheme cannot be achieved. However, mapped flooding does not automatically mean that the whole site has no development potential.
Where Development May Be Possible
- Vulnerable development can be located within the lowest-risk part of the site.
- The Sequential Test and Exception Test can be satisfied where required.
- Safe access and escape can be provided throughout the design flood event.
- Flood storage can be retained or appropriately compensated.
- A deliverable SuDS and drainage strategy can be secured.
Where Development May Be Difficult
- The proposed buildings would occupy functional floodplain.
- No safe access or emergency escape route can be provided.
- Raising the site would displace floodwater towards neighbouring property.
- No technically feasible surface water discharge point is available.
- The required flood mitigation or drainage infrastructure makes the scheme undeliverable.
How Can Flood Risk and Drainage Affect Development Capacity and Land Value?
Flood risk and drainage can affect the amount of land that is realistically capable of being developed. Areas required for flood storage, watercourse buffers, attenuation ponds, basins, swales and exceedance routes may reduce the net area available for housing, commercial floorspace, roads and parking.
The development capacity may also be influenced by raised floor levels, access requirements, floodplain compensation and the need to avoid functional floodplain. A site that appears large on an ownership plan may therefore accommodate substantially fewer units after flood and drainage constraints are taken into account.
Technical investigations and infrastructure can increase costs. These may include topographical surveys, Flood Risk Assessments, infiltration testing, hydraulic modelling, drainage design, pumping stations, sewer reinforcement, culvert works and off-site drainage connections.
Flood risk can also affect planning certainty, marketability and lender confidence. A site requiring a complex Sequential Test or relying on access through floodwater may carry greater planning risk than a comparable site within Flood Zone 1 with a clear drainage outfall.
Sustainable drainage can nevertheless add value where it is designed positively. Ponds, wetlands, swales and landscaped basins can create attractive green infrastructure, support biodiversity and improve the quality and marketability of a completed development.
Our Flood Risk Review
How We Assess Flood Risk, Drainage and Development Potential
We undertake a high-level review of flood risk and drainage alongside the planning, access, landscape, ecology, heritage, infrastructure and deliverability considerations affecting the land.
Flood Mapping
We review Flood Zones, surface water mapping, reservoirs, nearby watercourses and other available flood risk information affecting the land.
Site and Drainage Context
We consider site levels, low points, ditches, culverts, possible outfalls, nearby sewers and whether infiltration appears likely to be feasible.
Constraints and Capacity
We consider whether flood storage, SuDS, buffers, safe access or drainage infrastructure may reduce the realistic developable area.
Recommended Next Steps
We identify whether a Flood Risk Assessment, drainage strategy, topographical survey, infiltration testing or hydraulic modelling may be required.
The purpose of the initial review is to establish whether flood risk and drainage appear to be manageable design considerations, material constraints affecting capacity or issues requiring specialist investigation before the land is promoted further.
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If you would like a free, no-obligation assessment of your land, simply send us the location of your site and we will carry out a high-level review of its development potential. You do not need to provide a Flood Risk Assessment, drainage strategy, topographical survey or infiltration testing at this stage. Simply let us know where the land is located using one of the following:
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Search Our Live Planning Guides
Search our live planning pages to identify current opportunities for promoting land through the planning system. You can find open Call for Sites exercises, monitor the progress of emerging and reviewed Local Plans and identify current HELAA and SHLAA assessments being undertaken by local councils. These resources can help landowners understand when councils are seeking new development sites, where future growth may be planned and when land can be submitted for consideration for housing, employment or mixed-use development.
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View Land AssessmentsRelated Guides
Flood risk and drainage are important parts of a wider development assessment. Planning policy, access, ecology, landscape, infrastructure, location and deliverability must also be considered before the realistic development potential and value of land can be established. These related guides explain how councils assess potential development sites and how landowners can promote land through the planning system.
Is My Land Suitable for Development?
Understand how flood risk, drainage, planning policy, access, ecology, landscape, infrastructure and deliverability influence whether land may be suitable for development.
Click hereCan My Land Be Developed?
Review the principal planning and technical considerations that determine whether land has realistic development potential and what evidence may be needed.
Click hereCan I Get Planning Permission on My Land?
Learn how planning policy, flood risk, drainage feasibility, technical evidence and site design can influence the prospects of obtaining planning permission.
Click hereLocal Plan Allocation Guide
Find out how councils compare potential development sites and how flood risk, drainage and deliverability can influence site selection and allocation.
Click hereWhat is a HELAA or SHLAA?
Understand how councils assess whether potential development sites are suitable, available and achievable through housing and economic land availability assessments.
Click hereSuitable, Available and Achievable Sites
Learn how flood risk, drainage infrastructure, technical constraints and viability are considered when councils assess whether a site is realistic and deliverable.
Click hereCall for Sites Guide
Learn how to submit land to a council and why flood mapping, drainage feasibility and a realistic net developable area should be addressed within the submission.
Click hereWhy Location Matters for Development Land
Understand why settlement relationships, services, infrastructure, policy designations and environmental constraints can cause similar sites to have very different development prospects.
Click hereLand Value With Planning Permission
Discover how net development capacity, flood mitigation, drainage infrastructure, planning obligations and abnormal costs can influence the value of development land.
Click hereFrequently Asked Questions About Flood Risk, Drainage and Development Land
Can land in a Flood Zone be developed?
Land within a mapped Flood Zone is not automatically incapable of development. The acceptability of a proposal depends on the Flood Zone, the vulnerability of the proposed use, whether the Sequential Test and Exception Test apply and whether a site-specific Flood Risk Assessment can demonstrate that the development will remain safe without increasing flood risk elsewhere.
Does Flood Zone 1 mean that land cannot flood?
No. Flood Zone 1 identifies land with a low probability of flooding from rivers or the sea, but the land may still be affected by surface water, groundwater, sewers, reservoirs, canals or ordinary watercourses. Local drainage conditions and historic flooding should therefore be considered even where the site lies entirely within Flood Zone 1.
When is a Flood Risk Assessment required?
A site-specific Flood Risk Assessment may be required where development is proposed within Flood Zones 2 or 3, where a site exceeds the relevant size threshold within Flood Zone 1 or where other evidence identifies a material source of flooding. The exact requirement should be checked against national guidance and the local planning authority's validation requirements.
What is the Sequential Test?
The Sequential Test seeks to direct development towards reasonably available land with the lowest probability of flooding. It normally considers the proposed use, appropriate search area and whether alternative sites in lower-risk locations could accommodate the development. The test may also be applied within a site by locating vulnerable uses in the safest areas.
What is the Exception Test?
The Exception Test may apply where development remains proposed in an area of higher flood risk after the Sequential Test has been satisfied. It requires evidence that the development will deliver wider sustainability benefits and that it will remain safe throughout its lifetime without increasing flood risk elsewhere.
What is a sustainable drainage system?
A sustainable drainage system, or SuDS, manages rainfall using features such as ponds, basins, swales, permeable paving, infiltration systems and wetlands. SuDS are designed to slow runoff, provide storage, improve water quality and create environmental benefits. They should be incorporated into the masterplan from the outset rather than added after the development layout has been fixed.
Can part of a site affected by flooding still be developed?
Yes. A site may contain areas with different levels or sources of flood risk. Development can sometimes be directed towards the safest land, while higher-risk areas are retained as open space, flood storage, ecological habitat or sustainable drainage. The proposed capacity should be based on the realistic net developable area rather than the entire ownership.
Why is infiltration testing required?
Infiltration testing helps establish whether water can soak into the ground at a suitable rate. The results inform the design of soakaways, infiltration basins, permeable paving and other drainage features. Testing can also identify whether clay soils, high groundwater, contamination or unstable ground make infiltration unsuitable.
Can drainage requirements reduce the number of homes on a site?
Yes. Land may need to be reserved for attenuation ponds, basins, swales, watercourse buffers, flood storage and safe exceedance routes. Drainage infrastructure can therefore reduce the net developable area and the number of homes or amount of commercial floorspace that can be delivered. These requirements should be reflected in the masterplan and land valuation.
How do flood risk and drainage affect land value?
Flood risk and drainage can affect land value by reducing development capacity, increasing technical costs and creating greater planning uncertainty. Flood mitigation, hydraulic modelling, sewer reinforcement, pumping and off-site drainage works may all affect viability. However, well-designed SuDS, ponds, wetlands and green corridors can improve the quality and marketability of a completed development. Learn how planning permission can affect land value.
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Send us the location of your land and we will undertake a free initial review of the mapped Flood Zone, surface water risk, nearby watercourses, drainage context and wider development potential.
You do not need a Flood Risk Assessment, drainage strategy, topographical survey or infiltration testing at this stage. A postcode, map pin or what3words reference is usually enough for us to begin.