Flood risk assessments for Wandsworth planning applications, with the council's exit test
Wandsworth's Local Validation Checklist (July 2026) asks every application in flood zones 2, 3a and 3b for a flood risk assessment, and any proposal where safe access or egress cannot be achieved for a flood emergency plan. We write the assessment for a fixed £395, and say at the start if your address needs none.
Fixed fee from £395 · all prices
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What Wandsworth's July 2026 checklist says
Item 20 of the council's Local Validation Checklist (July 2026) says all applications that fall within a Flood Risk Area should submit a flood risk assessment, and spells that out as "all applications for developments located within flood zones 2, 3a and 3b". It frames the assessment against the national planning policy tests of safe, resilient and drained development.
One of those tests is that the development "incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate". So a short drainage note belongs in the assessment, even for a modest extension. Our SuDS drainage statement is the longer version for schemes that need one.
Flood Risk Assessments in Wandsworth
from £395
Every price is the full fixed fee you pay. No VAT is charged. Council application fees are paid to the council separately.
What you get
- Your address checked on the Environment Agency flood map and the council's strategic flood risk assessment
- A check against each Flood Zone 1 trigger the council lists
- The sequential test question answered for sites in flood zone 2 or 3
- Safe access and escape routes considered, with a view on whether an emergency plan is asked for
- A basement check against the strategic flood risk assessment's Thames rules
- The assessment as a PDF for the planning application
- Most vulnerable use placed safely
Within the site, the more vulnerable parts sit where flood risk is least.
- Flood resistant and resilient
The building could be back in use quickly without significant refurbishment after a flood.
- Sustainable drainage
Included, unless there is clear evidence that it would be inappropriate.
- Residual risk managed
What is left after the defences and the design, handled safely.
- Safe access and escape
Routes in and out, as part of an agreed emergency plan where one is needed.
Flood Zone 1 and the triggers that still apply
In flood zone 1 the checklist lists three cases that need an assessment: applications on 1ha or more, land the Environment Agency has identified as having critical drainage problems, and land that a strategic flood risk assessment shows at increased flood risk in future. It adds land subject to other sources of flooding where the development would bring in a more vulnerable use.
So a house on a dry street can still trigger one if the street sits in a critical drainage area, or if you are converting, say, a commercial ground floor to a flat. We check each trigger against the address before you pay for anything.
For areas at residual tidal risk, the same assessment says all basements, basement extensions and basement conversions must have safe access threshold levels and internal staircases to floors above the extreme water level.
The sequential test and the emergency plan
Two further documents hang off the assessment in flood zones 2 and 3. A sequential test is needed when your development is in flood zone 2 or 3 and a sequential test has not already been done for a development of that type on that site. The checklist says it can sit inside the planning statement or stand alone.
The emergency plan has a threshold that is easy to miss. In flood zones 2 and 3 a flood emergency plan is required for 10 or more dwellings or 1000sqm of non-residential space, and also "on any other proposal where safe access/egress cannot be achieved". That last clause can catch a single house with a basement flat or an upper floor you cannot reach. We test the route out as part of the assessment, and say so if a plan is likely.
Behind the Thames defences
The council's Level 1 strategic flood risk assessment (November 2020, prepared with Merton) says large areas defined as flood zone 3a in Wandsworth are protected from tidal flooding by the tidal defences, and are therefore at residual risk of flooding if the defences are breached.
That changes what the assessment must show. A home can sit in zone 3a and be protected most of the time, and the council still wants to know how the building and the people in it would cope if the wall failed. We take the extreme water level the Environment Agency gives for your stretch of river and set it against your floor levels and your exit.
Basements along the river
For areas at residual tidal risk, the same assessment says all basements, basement extensions and basement conversions must have safe access threshold levels and internal staircases to floors above the extreme water level. The only exception it gives is where the applicant shows a permanent fixed barrier that keeps floodwater out of any sleeping accommodation below that level. It also says self-contained residential basements and bedrooms at basement level will not be permitted.
That is the council's own evidence, not a rule we invented, and it is why we ask about the basement before the drawings. If you are planning one, start with the basement conversion drawings page, and send us the address first so the levels agree.
Fee, and what we check first
The fixed fee is £395, the total you pay, and no VAT is charged. It covers the flood map and trigger check, the written assessment and a drainage note for a house, extension or small conversion. The assessment and a full SuDS statement together are £625.
For major development or a site needing river modelling, a specialist is brought in and quoted separately. Send the address and a line on what you want to build. If the Environment Agency map and the council's triggers say you need nothing, we tell you so.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| flood zones 2, 3a and 3b | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: (source wording: "all applications for developments located within flood zones 2, 3a and 3b") | www.wandsworth.gov.uk |
| 1ha | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: in flood zone 1 (source wording: "Applications 1ha in size or greater") | www.wandsworth.gov.uk |
| critical drainage problems | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: in flood zone 1 (source wording: "Land which has been identified by the Environment Agency as having critical drainage problems") | www.wandsworth.gov.uk |
| sustainable drainage systems | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: (source wording: "it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate") | www.wandsworth.gov.uk |
| sequential test | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: (source wording: "Applicants will need to undertake a sequential test if both of the following apply: Your development is in flood zone 2 or 3; A sequential test hasn't already been done for a development of the type you plan to carry out on your proposed site.") | www.wandsworth.gov.uk |
| safe access/egress | Wandsworth Local Validation Checklist (July 2026), item 20 Flood Risk Assessment: (source wording: "In flood zones 2 and 3, all planning applications (or other relevant applications) on sites of 10 or more dwellings or 1000sqm of non-residential development or more, or on any other proposal where safe access/egress cannot be achieved, a Flood Emergency Plan must be submitted.") | www.wandsworth.gov.uk |
| residual risk | Wandsworth and Merton Level 1 Strategic Flood Risk Assessment (AECOM, November 2020), section 6.3: (source wording: "Large areas defined as Flood Zones 3a in London Borough of Wandsworth are protected from tidal flooding by tidal flood defences. They are therefore at residual risk of flooding in the event of a breach in tidal flood defences.") | www.wandsworth.gov.uk |
| internal staircases | Wandsworth and Merton Level 1 Strategic Flood Risk Assessment (AECOM, November 2020), section 6.3: (source wording: "In areas at residual risk of tidal flooding, i.e. along the Thames in London Borough of Wandsworth, all basements, basement extensions and basement conversions must have safe access threshold levels and internal staircases provided to access floors above the appropriate extreme water level") | www.wandsworth.gov.uk |
| permanent fixed barrier | Wandsworth and Merton Level 1 Strategic Flood Risk Assessment (AECOM, November 2020), section 6.3: (source wording: "The only exception to this is where the applicant has demonstrated that a permanent fixed barrier is in place to prevent floodwater from entering any sleeping accommodation that is located below the extreme water level.") | www.wandsworth.gov.uk |
The full guide lives on our sister site
We still prepare this for a fixed £395. The detailed guide to it is on fixedfeeplans.co.uk, part of the same practice.
Questions people ask
Does a house extension in Wandsworth need a flood risk assessment?
What is the Wandsworth emergency plan threshold?
Do I need a sequential test?
Can I add a basement bedroom near the Thames?
Does the assessment secure permission?
Is the checklist current?
Photographs: PAUL FARMER (CC BY-SA 2.0).
