Flood risk assessments for Southwark planning applications, every basement extension included
Southwark's full planning checklist asks for a flood risk assessment on all applications in Flood Zones 2 and 3, including minor development and changes of use, and on all basement extensions. Its guidance sets floor levels against a Thames breach. We write the assessment for a fixed £395 and check the basement question before anything else.
Fixed fee from £395 · all prices
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What Southwark's checklist asks for
The council's full planning application checklist, last updated on 16 October 2026, lists five cases for a flood risk assessment: all applications in Flood Zones 2 and 3, including minor development and changes of use; all developments of sites more than 1 hectare in Flood Zone 1; all basement extensions; all major applications in Critical Drainage Areas for surface water flooding; and any application in Flood Zone 1 where the Environment Agency has notified a critical drainage problem.
It cites Southwark Plan 2022 policy P68, says you can check the site on Southwark Maps with the flooding layer, and notes that you will usually need a flood risk specialist. Of the five triggers, the basement one is the easiest to miss: it applies in any zone.
Flood Risk Assessments in Southwark
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 flooding map layer
- A check against each of the council's five flood risk assessment triggers
- A Thames breach level check for floor levels and the way out of the building
- A basement check against the council's Basements and Flooding guide for developers
- A sewer and surface water note, with a one way valve where a basement connects to a sewer
- The assessment as a PDF for the planning application
- Every form of flooding
The risks of all forms of flooding to and from the site.
- Managed over time
How the risks are managed, with climate change allowed for.
- Reduce probability and effect
Ways to cut both the chance and the consequences of flooding.
- Surface water design
The design of surface water management systems, including sustainable drainage.
- Safe access
Safe access to and from the site where flood risk exists.
All basement extensions, in any zone
The council's Basements and Flooding guide for developers says a Basement Impact Assessment is not a replacement for a site-specific flood risk assessment or drainage strategy, and adds a warning: if an assessment is not submitted where one is required, "refusal of the application may be recommended".
So a basement under a terrace in Bermondsey or Dulwich needs both documents, and they have to agree on levels and groundwater. If you are planning one, the basement conversion drawings and the flood assessment are worth starting together.
It says all residential floor levels should be a minimum of 300 mm above the 2100 Maximum Likely Water Level anticipated through a breach of the Thames defences, and that access and escape routes should be considered.
Behind the Thames defences: the breach level
The council's Strategic Flood Risk Assessment (January 2017) says the borough is currently protected from combined tidal and fluvial flooding by the River Thames Tidal Defences up to the 1 in 1000 year event, so the risk is residual: it comes from overtopping or a breach.
That is why its guidance looks at a breach and not at the defended river level. It says all residential floor levels should be a minimum of 300 mm above the 2100 Maximum Likely Water Level anticipated through a breach of the Thames defences, and that access and escape routes should be considered. We work out which breach level applies to your street and set your floor heights against it.
Basements by flood zone
The basements guide goes zone by zone. No basements are permitted in Flood Zone 3b. In Flood Zone 3a basements must be restricted solely to non-residential uses, with thresholds raised above the 2100 year maximum water level estimated through a breach of the River Thames defences, internal access to upper floors and flood resilient construction. In Flood Zone 2 an Exception Test is required for any residential or other highly vulnerable use, and all basements must be flood resistant with internal access above that level.
In plain terms, a basement bedroom or flat is where the rules stop you, and a utility room, store or plant space is where they let you in. We say which side your idea falls on before you pay for drawings.
Critical Drainage Areas, sewers and surface water
A basement is low, so sewer flooding matters as much as the river. The guide says drainage connections from basements to sewers should be fitted with a one way valve to stop drains flooding the basement if they surcharge, and that Thames Water requires a pumped sewage system in basements where sewer flooding is on record locally.
Where a basement extends beyond the footprint of the building the guide expects no net increase in surface water drainage from the site, and asks for runoff to be reduced towards greenfield rates. We put that into the assessment, with a short sustainable drainage note, and for larger schemes our SuDS drainage statement is the fuller version.
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 with a full SuDS statement is £625.
For major development, or a site that needs its own breach modelling, a specialist is brought in and quoted separately. Send the address and a line on what you want to build. If none of the five triggers applies, we tell you so and you pay nothing.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| Flood Zones 2 and 3 | Southwark Council, Planning checklists, full planning application, flood risk assessment entry (page last updated 16 October 2026): (source wording: "all applications in Flood Zones 2 and 3, including minor development and changes of use") | www.southwark.gov.uk |
| 1 hectare | Southwark Council, Planning checklists, full planning application, flood risk assessment entry (page last updated 16 October 2026): (source wording: "all developments of sites more than 1 hectare in Flood Zone 1") | www.southwark.gov.uk |
| all basement extensions | Southwark Council, Planning checklists, full planning application, flood risk assessment entry (page last updated 16 October 2026): (source wording: "all basement extensions") | www.southwark.gov.uk |
| Critical Drainage Areas | Southwark Council, Planning checklists, full planning application, flood risk assessment entry (page last updated 16 October 2026): (source wording: "all major applications in Critical Drainage Areas for surface water flooding") | www.southwark.gov.uk |
| refusal of the application may be recommended | Southwark Council, Basements and Flooding guide for developers: (source wording: "If an FRA is not submitted for a development for which it is required, refusal of the application may be recommended.") | www.southwark.gov.uk |
| 1 in 1000 year | Southwark Strategic Flood Risk Assessment (January 2017): (source wording: "the Borough is currently protected from combined tidal and fluvial flooding by the River Thames Tidal Defences (TTD) up to the 1 in 1000 year event. The risk is therefore of a residual nature") | www.southwark.gov.uk |
| 300 mm | Southwark Strategic Flood Risk Assessment (January 2017): (source wording: "All residential floor levels should be situated a minimum of 300 mm above the 2100 Maximum Likely Water Level, anticipated through a breach of the River Thames defences.") | www.southwark.gov.uk |
| 2100 year maximum water level | Southwark Council, Basements and Flooding guide for developers: for Flood Zone 3a (source wording: "Basements must be restricted solely to non-residential uses within Flood Zone 3a ... Basement thresholds must be raised above the 2100 year maximum water level, estimated through breach of the River Thames defences.") | www.southwark.gov.uk |
| one way valve | Southwark Council, Basements and Flooding guide for developers: (source wording: "Drainage connections from basements to sewers should be fitted with a one way valve to prevent drains flooding the basements if they surcharge.") | www.southwark.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 Southwark need a flood risk assessment?
Do I need one for a basement in Flood Zone 1?
What does 'above the 2100 Maximum Likely Water Level' mean?
Can I have a basement bedroom in Southwark?
Does an assessment secure planning permission?
Photographs: Fernweh (CC BY-SA 2.0).
