Guide
Flood risk assessment and SuDS compliance
How to assess flood risk for development sites and comply with Sustainable Drainage Systems (SuDS) requirements. Covers Flood Risk Assessments, Sequential and Exception Tests, and the major regulatory divergence between England and Wales on mandatory SuDS.
Why flood risk matters for your development
Flood risk is a significant planning constraint in England and Wales. If you're developing land or changing the use of property, you need to understand how flood risk affects your project and what assessments or approvals you'll need.
The regulatory position differs dramatically between England and Wales on sustainable drainage. In Wales, SuDS approval is mandatory and constructing without it is a criminal offence. In England, SuDS remains voluntary at national level, though many local planning authorities require it through planning conditions.
Getting flood risk wrong can be costly. Planning applications may be refused, developments may be uninsurable, and in Wales you face criminal prosecution for non-compliance with SuDS requirements.
Flood Zones explained
The Environment Agency classifies land into Flood Zones based on the probability of flooding. Your development site's flood zone determines what assessments you need and whether your proposed use is appropriate.
When you need a Flood Risk Assessment
A site-specific Flood Risk Assessment (FRA) is a detailed study that identifies flood risks to your development and proposes mitigation measures. It must be prepared by a competent professional with appropriate qualifications.
FRA is required for
- All development in Flood Zones 2 and 3, regardless of size
- Development of 1 hectare or more in Flood Zone 1
- Any site with known critical drainage problems
- Development that may increase flood risk elsewhere
Even if your site is in Flood Zone 1, check with your local planning authority whether an FRA is needed. Some areas have localised flood risks from surface water, groundwater, or ordinary watercourses that aren't reflected in the main flood zone maps.
What an FRA must include
- Assessment of all sources of flood risk (river, sea, surface water, groundwater, sewers)
- Climate change allowances applied to flood levels
- How the development will be safe throughout its lifetime
- How the development will not increase flood risk elsewhere
- Proposed mitigation measures (finished floor levels, flood-resistant construction, safe access and egress)
Sequential and Exception Tests
If you want to develop in Flood Zones 2 or 3, you may need to pass the Sequential Test and possibly the Exception Test before your planning application can be approved.
Passing the Sequential Test
You must demonstrate that there are no reasonably available sites at lower flood risk where your development could be located. This means searching for alternative sites in Flood Zone 1, then Flood Zone 2, before accepting a site in Flood Zone 3.
The search area depends on your development type. For a local convenience store, you might only need to search within your target neighbourhood. For a regional warehouse, you'd need to search a wider area.
When the Exception Test applies
If you pass the Sequential Test but still propose development in Flood Zone 3a, you may also need to pass the Exception Test. This requires demonstrating:
- The development provides wider sustainability benefits to the community that outweigh flood risk
- The development will be safe for its lifetime, will not increase flood risk elsewhere, and will reduce risk overall where possible
Both parts must be satisfied. The Exception Test is rigorous and not all proposals will pass.
Wales: Mandatory SuDS requirements
If you're developing in Wales, you must obtain SuDS Approval Body (SAB) approval before construction begins. This applies to all developments meeting the threshold, regardless of whether planning permission is required.
The SAB approval process
The SuDS Approval Body is your local authority's designated team for assessing drainage applications. The process runs parallel to planning permission but is a separate approval.
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Check if SAB approval is required
Calculate your construction area. If it's 100 square metres or more, or involves more than 1 dwelling house, you need SAB approval. This threshold is lower than many developers expect.
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Design your SuDS to Welsh statutory standards
Your drainage system must meet the Statutory Standards for Sustainable Drainage Systems in Wales. Consider attenuation, treatment, and amenity benefits. Early engagement with SAB is recommended.
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Submit pre-application enquiry (optional but recommended)
Pay for pre-application advice from your SAB. They'll review your proposed design and flag issues before formal submission. This can save time and money on the formal application.
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Submit SAB application with fees
Apply to your local authority SAB with full drainage design, construction area calculations, and application fee. The application must include detailed drainage layout, calculations, and maintenance plan.
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SAB assessment period
Standard assessment takes 7 calendar weeks from validation. If your project requires Environmental Impact Assessment, allow 12 weeks. The SAB may request additional information, which stops the clock.
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Receive decision
SAB will approve (possibly with conditions), request amendments, or refuse. Approval may be conditional on inspections during construction. You can appeal refusal to Welsh Ministers.
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Begin construction only after approval
Do not start any drainage construction work until you have SAB approval in writing. Beginning without approval is a criminal offence.
Wales SAB fees
SAB applications incur fees based on construction area size, plus inspection fees where conditions are attached.
Wales: Criminal penalties for non-compliance
Unlike England, constructing without SuDS approval in Wales is a criminal matter, not just a planning enforcement issue.
Flood insurance considerations
Flood risk affects your ability to insure property. This matters both during development and for future occupiers or purchasers.
Flood Re scheme
The government-backed Flood Re scheme helps households in high flood risk areas obtain affordable insurance. However, it has significant limitations for business development.
Insurance implications for businesses
Flood Re does not cover commercial properties. If you're developing business premises in a flood risk area, you must consider:
- Commercial flood insurance availability - May be expensive, have high excesses, or be unavailable for high-risk locations
- Future saleability - Properties that are uninsurable or have prohibitive insurance costs are harder to sell or let
- Lender requirements - Banks typically require flood insurance as a condition of commercial mortgages
- Business continuity - Even with insurance, flood events cause significant business disruption
Properties built after 1 January 2009
Flood Re excludes properties built after 1 January 2009. This deliberate exclusion encourages flood-resistant construction in new developments. If you're building new residential property in a flood risk area, future occupiers will need to obtain insurance on the open market without Flood Re support.
This makes it even more important to incorporate robust flood mitigation measures in new developments, both to obtain planning permission and to ensure future insurability.
Climate change considerations
Flood Risk Assessments must account for climate change. The Environment Agency publishes climate change allowances that must be applied to flood levels when assessing risk and designing mitigation.
What climate change means for your development
- Higher flood levels - Peak river flows and sea levels will increase over the development's lifetime
- More intense rainfall - Surface water flooding becomes more likely and severe
- Lifetime assessment - Residential development typically assessed over 100 years, commercial over 60 years
- Adaptive design - Consider how the development could be adapted if flood risk increases beyond predictions
The allowances are updated periodically. Use the latest Environment Agency guidance when commissioning your FRA.
Practical tips for development in flood risk areas
Site selection
- Check the Environment Agency's flood maps early in site selection
- Request pre-application advice from the local planning authority
- Consider whether the Sequential Test can realistically be passed
- Factor FRA and SuDS costs into your development appraisal
Design considerations
- Raise finished floor levels above flood levels (with climate change allowance)
- Consider flood-resistant construction (water-resistant materials, raised electrics)
- Ensure safe access and egress routes during flood events
- Design SuDS to provide multiple benefits (attenuation, water quality, amenity, biodiversity)
Cross-border projects
If you work across England and Wales, remember:
- Wales: SuDS approval mandatory, criminal offence without approval, separate SAB application
- England: SuDS through planning conditions, no separate approval body, no criminal penalty
Do not assume that experience in one nation applies to the other.
Environment Agency consultation
The Environment Agency (EA) is a statutory consultee for planning applications in flood risk areas. Their response carries significant weight with planning authorities.
When EA must be consulted
- All applications in Flood Zones 2 and 3
- Development of 1 hectare or more in Flood Zone 1
- Applications within 20 metres of a main river
- Development in an area with critical drainage problems
EA objections
If the EA objects to your application on flood risk grounds, the planning authority should not approve it unless there are exceptional circumstances. If they do approve despite EA objection, the application is referred to the Secretary of State who can call it in for decision.
Addressing EA concerns early in the process is essential. Consider pre-application discussions with EA as well as the planning authority.