Flood Risk Assessment in Brentford

A flood risk assessment in Brentford has to deal with the tide twice: along the Thames frontage, and again up the tidal lower reach of the River Brent, where Hounslow's draft Local Plan expects the walls between the lock and weir and the Thames to be raised. We assess Brentford sites for planning applications, home and commercial purchases, lenders and insurers, and prepare the sequential and exception tests and surface water drainage strategies that often go with them.

The page treats Brentford as Hounslow Council's three Brentford wards draw it. Brentford East runs round Lionel Road and the Gtech Community Stadium; Brentford West climbs from Brentford station and the Butts to the Golden Mile on the Great West Road; Syon & Brentford Lock takes in the High Street, the canal basin, Brentford Dock, Brentford End and Syon Park. Old Brentford and New Brentford, the town's two historic halves, both fall inside, and nearly all of the outline lies in the TW8 postcode district. Isleworth, Osterley and Chiswick lie outside the outline.

The Thames defences protect Brentford to a 1-in-1,000-year standard, yet after the July 2021 storms the TW8 postcode district logged more flood reports than any other part of Hounslow.

To talk a site through with one of our flood risk consultants, call 01293 214444 or email enquiries@unda.co.uk.

How far does the tide reach into Brentford?

Well beyond the riverbank. The tidal Thames forms the town's southern edge from Syon Park past Brentford Dock towards Kew Bridge, and the tide also works its way up the lower Brent through the canal locks. The Canal & River Trust puts the Thames's spring tidal range at up to 7 metres, which is why Thames Lock can be worked only for part of each tide.

• Two locks, two waters. Thames Lock is the final lock on the Grand Union Canal and its gateway to the Thames. It is a tidal lock, worked around high water, and the Trust cannot operate it while the Thames Barrier is shut for planned testing. Upstream, Brentford Gauging Lock marks the canalised Brent, which shares its channel with the canal north towards Hanwell.
• Walls hold the tide out. The Environment Agency's page for the Twickenham policy unit of the Thames Estuary 2100 plan lists flood walls and embankments along the Thames, the lower Brent and the Crane, with the Thames Barrier downstream. Hounslow's 2022 flood strategy says these defences protect to a 1-in-1,000-year event.
• The walls will go higher. Hounslow's draft plan records that TE2100 raises the Thames defences, and the lower Brent defences between the lock and weir and the Thames, by up to 0.3 m by 2050 and a further 0.5 m by 2090. The Twickenham unit runs from Teddington Lock along the north bank to Brentford and is under policy P5 for the tide and P3 for river flooding.
• Protected, yet still Flood Zone 3. The Flood Map for Planning draws riverside Brentford as though the walls were absent, so an assessment has to explain what happens if they are overtopped or breached. Our guide to residual flood risk covers how that is done.
• The Barrier and high river flows. The same TE2100 page asks the Environment Agency to work with Hounslow and Richmond councils to find areas that benefit from shutting the Thames Barrier during river floods, with other measures in place before the Barrier's operating rules change in 2035. Our article on the Thames Barrier explains when and why it closes.

Check a postcode in Brentford, Brentford End or Syon

Enter a postcode below to screen it against Environment Agency data for rivers and the sea, surface water and reservoirs, or open the full flood risk map for any address in England. The coral line follows the three Brentford wards; the Thames marks most of its southern side, and Isleworth to the south-west and Chiswick to the east are left out.

Rain, groundwater and sewers: Brentford's other flood sources

The walls deal with the tide; most of the flooding the council has logged in Brentford in recent years came from rain. Hounslow's 2021 Surface Water Management Plan splits the town into two drainage catchments either side of the Brent, East Brentford and West Brentford, and finds surface water trouble in both.

The SWMP traces most of the incidents it holds for East Brentford to TW8 9, then W5 4 and TW8 0, all lying below a major surface water flow path that runs from Ealing down to the Thames. Our guide to surface water flooding and planning explains how an assessment deals with flow paths like this, and our summary of the 2025–30 London Surface Water Strategy sets out the city-wide response.

What the records show about flooding in Brentford

Brentford's best-documented flood came down the Brent rather than up the Thames, and the more recent records are dominated by rainfall.

1. January 1841. After a rapid thaw the Brent overflowed into the Grand Junction Canal, as the Grand Union was then called. The Spectator reported that the water rose to within a foot of the top of the arch of Brentford Bridge, that about thirty boats and barges were swamped and that houses in that part of the town were flooded. Its report records at least two people drowned.
2. 2011. Hounslow's first Surface Water Management Plan made Boston Manor Road a critical drainage area (CDA33). By 2021 a Brentford North flood alleviation scheme was under way there.
3. April 2021. The council's updated Surface Water Management Plan put the Brentford End hotspot among its five highest priorities for further investigation across the borough.
4. July 2021. Flooding hit many parts of Hounslow. The TW8 district recorded nine incident reports, more than any other postcode district in the borough; no Section 19 investigation followed because the areas were already being studied through the Thames Water Strategic Partnership.

Section 19 investigations are triggered in Hounslow when four or more properties on a street flood internally within six months, or four or more in one of the 35 SWMP hotspots, which is why the hotspots matter to anyone building near one.

Which policies shape a flood risk assessment in Brentford?

Hounslow Council is both the planning authority and the Lead Local Flood Authority, so one council decides the application and reviews its drainage, while the Environment Agency advises on the Thames and the Brent. The policy in force is EQ3 of the Hounslow Local Plan 2015–2030. It expects an assessment that follows the Environment Agency and the SFRA, defences kept in place for the life of the development, SuDS aiming for greenfield runoff and, beside the Thames, nothing that stops the wall being raised or set back later.

A replacement, the Hounslow Local Plan 2020–2041, was submitted for examination on 23 June 2025. Hearings ended in July 2026 and the council published updated modifications that September, so the plan is not yet adopted. Its version of EQ3 goes further:

• River and tidal buffers. Strips of 8 m beside rivers and 16 m beside tidal water, left undeveloped or natural where feasible.
• Room for higher defences. Proposals are expected to show they allow for the raising or set-back of defences that TE2100 needs over the development's lifetime.
• Lowest risk first, inside the site. The most vulnerable uses go on the lowest-risk part of a site, even where the site itself is acceptable.
• Runoff held down. Greenfield runoff rates, or a maximum of 2 l/s unless that is shown to be unfeasible, with a drainage strategy built on London Plan Policy SI 13 and a completed SuDS proforma. Our note on the SuDS proforma explains what it asks for.

The evidence behind both versions is the West London Strategic Flood Risk Assessment, a joint Level 1 study shared by six boroughs, first published online in April 2018 and updated for Hounslow in 2024. Hounslow's own Level 2 SFRA of July 2024 looks at flood depth and hazard on 18 allocated sites, four of them in Brentford. For Hounslow, the West London SFRA adds that basements likely to flood need internal access to a higher floor and must not contain sleeping accommodation. Our explainer covers what an SFRA does.

Across the capital the London Plan of March 2021 applies through Policy SI 12 on flood risk management and Policy SI 13 on sustainable drainage; consultation on a draft replacement closed on 15 October 2026. Nationally, chapter 18 of the August 2026 NPPF holds the flood policies: F4 sets when an assessment is needed, F5 and F6 hold the sequential and exception tests, and F8 expects development to follow the 2025 National Standards for sustainable drainage.

Brentford's riverside allocations and what they will need

Hounslow's draft plan names Brentford as one of the parts of the borough with the most opportunity for change, with investment focused on the Great West Corridor and the town centre. Several of the sites it allocates set out their flood constraints in the allocation text itself.

Two schemes already on the ground show the scale. Ballymore's The Brentford Project, set where the Brent meets the Thames, is planned for 876 homes on 4.79 hectares and due to complete in 2027. In Brentford East, the 17,250-seat Gtech Community Stadium opened at Lionel Road in 2020, with more than 1,000 homes proposed across the wider masterplan.

Each allocation in the table asks for the sequential test and the exception test as well as an assessment. Basements, podium car parks and ground-floor flats close to the river add the questions covered in our guide to basements in a flood zone.

Does every Brentford site need a flood risk assessment?

Not every one, but a large share do, and often for a reason other than the river. These are the usual triggers in the town.

• Flood Zone 2 or 3. Land by the Thames, the tidal Brent or the canalised Brent needs an assessment under NPPF policy F4 and EQ3, defended or not.
• One hectare or more. Large sites need one wherever they sit, so large plots along the Golden Mile are caught by size alone even where they sit in Flood Zone 1.
• A mapped flow path or high groundwater. Policy F4 covers flooding from any source, so a Flood Zone 1 plot in Brentford End, on Boston Manor Road or in north Brentford can still need one. Our explainer on critical drainage areas shows how local drainage designations feed in.
• Basements and ground-floor bedrooms. Hounslow's SFRA rules on internal access and no basement bedrooms apply to extensions and conversions as well as new builds; see our note on extensions in a flood zone.
• Buying rather than building. The council plays no part, but lenders and insurers ask about the tide and surface water, and a pre-purchase flood survey answers for the property itself.

Reports for Brentford, from the Golden Mile down to the river wall

Which report a site needs depends on where it sits between the Great West Road and the Thames and on who is asking. All our consultants are chartered, Unda is a CIWEM Business Partner, and every report is checked and signed off by a senior consultant.

• Riverside and canal-side applications. Schemes on the tidal Brent, by the canal basin or behind the Thames wall need a flood risk assessment for planning that answers EQ3 and the West London SFRA.
• All the sources on one plot. In Brentford, a flood risk assessment weighs the defended tide, the Brent, the Brentford End flow path, groundwater in north Brentford and the sewers together.
• Before exchange in TW8. Flats at Brentford Dock and homes near the Brent raise lender and insurer questions that a flood risk survey for property purchase settles for the building.
• Homes on allocated riverside land. Albany Riverside, Brentside Park and Great West Plaza each name the sequential and exception test in their allocation wording.
• Runoff limits. The draft EQ3 holds discharge to greenfield rates or 2 l/s, which our drainage strategy work sets out for Hounslow alongside the SuDS proforma.
• Gravel over London Clay. Infiltration is worth testing in the gravels but rarely works into the clay, so a surface water drainage strategy here usually leans on storage, green roofs and a controlled outfall.
• After permission. Where Hounslow attaches a drainage condition to a consent, we prepare the evidence for the discharge of drainage conditions.
• Depths beside the canalised Brent. The Level 2 SFRA took depth and hazard for Brentside Park and Great West Plaza from the Environment Agency's Brent model, and flood modelling refines them where a site needs more than the strategic outputs.
• Three warning areas at one river mouth. Riverside homes in the Tidal River Brent and Tidal Thames warning areas are best served by a flood warning and evacuation plan that tells residents when to leave and where to go.
• Basements in north Brentford. Hounslow's flood strategy names north Brentford among the borough's areas most prone to groundwater flooding, which a groundwater flood risk assessment tests before anyone digs.
• After July 2021. TW8 logged more flood reports than any other Hounslow postcode district that month, and a flood risk assessment for insurance gives an underwriter evidence about the property itself.
• Office land on the Golden Mile. The former GSK site and Great West Plaza are allocated for housing-led change and lie partly in Flood Zones 2 and 3, so flood risk due diligence belongs before a purchase.
• Waterside trades. The draft plan backs Brentford's waterside industries around the docks and locks, and a business continuity flood assessment helps occupiers on the tidal frontage plan for an event.
• Sewer records in TW8. Thames Water records quoted in the 2021 plan show 10 sewer flooding incidents in TW8 0 and TW8 9, so a foul drainage strategy should confirm capacity early on larger schemes.
• New office floorspace. Brentside Park alone is expected to provide at least 11,120 m² of business space, and the draft plan asks non-residential new build for BREEAM Excellent, which a BREEAM flood risk assessment supports through the Pol 03 credit.

Flood warnings and who to contact in Brentford

Anyone in immediate danger from floodwater should dial 999, and 105 is the number to report a power cut. Day to day, the work is shared between the bodies below.

Three Environment Agency warning areas meet at the mouth of the Brent: Tidal River Brent at Brentford; Tidal Thames from the Hogarth Lane Roundabout to the River Brent, which names Brentford alongside Strand-on-the-Green; and Tidal Thames from the River Brent to the River Crane, which takes in Syon Park and East Isleworth. Inland, the River Brent from Hendon to Brentford flood alert area gives notice when the river is running high.

The Environment Agency's level gauge on the Brent at Brentford has a highest recorded reading of 2.06 m, just under the 2.14 m at which alerts for low-lying land may be issued; property flooding is expected from 2.39 m. It sits with the Richmond and Teddington Lock gauges on our live river levels map.

For the tideway as a whole, see our River Thames page. Chiswick shares Hounslow Council and begins the Hammersmith policy unit at Kew Bridge, Richmond upon Thames faces the town across the river, Twickenham sits upstream in the same Twickenham policy unit, and Fulham lies further down the tideway. Brentford lies in the historic county of Middlesex, and every other place we cover is on our list of areas.

Questions we are asked about Brentford sites

Unda Consulting Limited prepares flood risk assessments, sequential and exception tests and drainage strategies for homes, businesses and development land across Brentford, from Brentford End and Syon Park to Lionel Road. To discuss a site, call 01293 214444 or email enquiries@unda.co.uk.