Flood risk assessments for Hounslow planning applications, with the drainage form
Hounslow's planning validation checklist says a flood risk assessment must accompany proposals of one hectare or more in Flood Zone 1 and all proposals for new developments located in Flood Zones 2 and 3, and it asks for a Drainage Assessment Form beside it. We write the assessment for a fixed £395 and set the drainage information out so the form and the report agree.
Fixed fee from £395 · all prices
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What Hounslow's checklist says
The council's planning validation checklist, which went through public consultation between 14 October and 29 November 2019, says a flood risk assessment identifies and assesses the risks of all forms of flooding to and from the development and shows how they will be managed. It says one needs to accompany proposals of one hectare or greater in Flood Zone 1 and all proposals for new developments located in Flood Zones 2 and 3.
That is the version we could read on 29 October 2026, and it is several years old. Before we issue anything we check whether the council has published a newer list, and we tell you if the trigger has changed.
Flood Risk Assessments in Hounslow
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
- A written assessment for Flood Zone 2 or 3 sites
- Help completing Hounslow's Drainage Assessment Form
- Floor levels and a safe route out tied to your drawings
- A basement check against the joint West London SFRA table
- The assessment as a PDF for the planning application
- Flood zone
Zone 1, 2 or 3 for your address, including the Thames and the Brent.
- Is it new development?
The checklist wording for Zones 2 and 3 is about new developments.
- Drainage Assessment Form
Completed and sent with the application form, as the checklist asks.
- Basement test
Zone rules and access above the design flood level, where a basement is proposed.
- Write and submit
Assessment and drainage information in one PDF, referenced to your drawings.
The Drainage Assessment Form
The checklist says a Drainage Assessment Form will need to be completed and submitted with the application form, and that the Lead Local Flood Authority will check that sustainable drainage is used wherever possible and that a detailed drainage plan and its maintenance are submitted.
For major and a number of minor applications, the flood risk and drainage document is to address a list that includes drainage layout drawings, an infiltration feasibility assessment, greenfield runoff rate calculations and consultation with Thames Water if connecting to the sewer network. For a house or small extension most of that list falls away, and we tell you which parts apply. Where a fuller strategy is needed, our SuDS drainage statement is a separate service, and the assessment with a statement together is £625.
It also says new basement dwellings may be permitted in Flood Zone 2 only if both criteria of the Exception Test are met.
The Thames and the Brent
The council's flood risk sequential and exception test document for its Local Plan 2020 to 2041 says Hounslow sits on the River Thames and contains its confluence with the River Brent at Brentford, that both rivers create a small risk of fluvial and, for the Thames, tidal flooding, and that the riverside areas are well supported by flood defences.
A well defended site can still be in a flood zone, and the zone is what the checklist reads. If your address is in Brentford, Isleworth, Syon or beside the Brent, we check the zone and what the defences do for your floor level before suggesting a report.
Basements and the joint West London SFRA
Hounslow shares a strategic flood risk assessment with five other west London boroughs. The West London SFRA planning requirements table says basements should not be permitted in Flood Zone 3b, that basement dwellings are categorised as highly vulnerable and should not be permitted in Flood Zone 3a, and that all basement rooms must have internal access and egress to a higher floor above the design flood level.
It also says new basement dwellings may be permitted in Flood Zone 2 only if both criteria of the Exception Test are met. That SFRA dates from 2018 and is due a revision, so we confirm the current position before you commission basement drawings. Our basement conversion drawings are prepared with the flood assessment in mind.
Fee and next step
The fixed fee is £395, the total you pay, and no VAT is charged. It covers the flood map check, the written assessment and a drainage note for a house, extension or small conversion. For a major scheme or one needing bespoke river modelling a specialist is brought in and quoted separately. If the map shows Flood Zone 1 with no surface water risk, we tell you so and you pay nothing. Send the address and what you want to build.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| all proposals for new developments located in Flood Zones 2 and 3 | Hounslow planning validation checklist (public consultation 14 October to 29 November 2019), Flood Risk Assessment (source wording: "An FRA needs to accompany development proposals of one hectare or greater in Flood Zone 1 and all proposals for new developments located in Flood Zones 2 and 3.") | www.hounslow.gov.uk |
| 14 October and 29 November 2019 | Hounslow planning validation checklist (source wording: "This checklist has been through 6 (six) weeks of public consultation between 14 October and 29 November 2019.") | www.hounslow.gov.uk |
| Drainage Assessment Form | Hounslow planning validation checklist (source wording: "A Drainage Assessment Form will need to be completed and submitted with the application form.") | www.hounslow.gov.uk |
| greenfield runoff rate calculations | Hounslow planning validation checklist, flood risk and drainage document items for all major and a number of minor applications (source wording: "Greenfield runoff rate drainage calculations" and "Consultation with Thames Water if connecting to the sewer network") | www.hounslow.gov.uk |
| River Brent | Hounslow, Flood Risk Sequential and Exceptions Tests for sites proposed for allocation in the Hounslow Local Plan 2020-2041 (source wording: "Hounslow is situated on the River Thames, and also contains its confluence with the River Brent, at Brentford.") | www.hounslow.gov.uk |
| Flood Zone 3b | West London SFRA Level 1 (Barnet, Brent, Ealing, Harrow, Hillingdon, Hounslow), Table 4-1, basements (source wording: "Basements should not be permitted in Flood Zone 3b.") | westlondonsfra.london |
| Flood Zone 3a | West London SFRA Level 1, Table 4-1, basements (source wording: "Basement dwellings are categorised as ""Highly vulnerable"" infrastructure by the PPG and therefore should not be permitted in Flood Zone 3a.") | westlondonsfra.london |
| internal access and egress | West London SFRA Level 1, Table 4-1, basements (source wording: "All basement rooms must have internal access and egress to a higher floor above the design flood level") | westlondonsfra.london |
| both criteria of the Exception Test | West London SFRA Level 1, Table 4-1, basements, Flood Zone 2 (source wording: "If both criteria of the Exception Test are satisfied, ""Highly vulnerable"" new basement dwellings may be permitted") | westlondonsfra.london |
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 Hounslow house extension need a flood risk assessment?
What is the Drainage Assessment Form?
Is the checklist you quote current?
Can I convert a basement to a flat in Flood Zone 3a?
Will the assessment secure permission?
Photographs: David Howard (CC BY-SA 2.0).
