Flood Risk Assessment in Hammersmith and Fulham: What Officers Ask For

Most of Hammersmith and Fulham lies in Flood Zones 2 and 3 behind the tidal Thames defences, so most planning applications there need a flood risk assessment. The Local Plan asks for every source of flooding to be assessed, for roof extensions as well as basements. New homes also attract a ventilation strategy condition, because the council's air quality targets are exceeded almost everywhere. This guide sets out what a flood risk assessment in Hammersmith and Fulham has to cover for officers to accept it, and how to discharge the ventilation condition, from Air & Flood Consultants (AFC) jobs in the borough and the council's policy.

Fee proposals: send the address and drawings through the enquiry form. For the ventilation condition, see AFC's ventilation strategy service.

Why most of the borough is a flood risk borough

The Hammersmith & Fulham Local Plan (adopted February 2018) states at paragraph 13.12 that "over 60% of the borough and about 75% of the population are in the Environment Agency's Flood Zones 2 and 3". The same paragraph requires FRAs "to assess the risk of flooding to the site e.g. in the event of a failure or breach of the defences". Figure 1 shows the pattern near Barons Court.

Figure 1. Flood Map for Planning around Barons Court, from AFC's FRA for a roof extension in Fulham. Almost every street shown is in Flood Zone 3 (present day).

The Local Plan also records elevated groundwater "mainly to the south of Goldhawk Road" (paragraph 13.16), and over 2,000 locations affected by sewer flooding in ten years on a combined sewer network (paragraph 13.18). The council has applied the Sequential Test across the whole borough at plan level (paragraph 13.14).

When a flood risk assessment in Hammersmith and Fulham is needed, and what "all sources" means

Policy CC3 requires a site-specific FRA for:

• all development in Flood Zones 2 and 3;

• new development over 1 hectare in Flood Zone 1;

• new development in a surface water flooding hotspot in the council's Surface Water Management Plan (SWMP);

• new development with a subterranean element where the SWMP shows elevated groundwater.

It adds that "the FRA must assess the risk of flooding from all relevant sources, in particular tidal, surface and groundwater, as well as sewer flooding". Policy F4 of the NPPF 2026 also asks for an FRA with all development in Flood Zones 2, 3a and 3b. The Planning Practice Guidance says minor extensions "should still meet the requirements to provide a site-specific flood risk assessment", and that "a shorter, simpler assessment" is likely to be enough (paragraph 051, Reference ID 7-051-20220825).

The council's Planning Guidance SPD (2018) adds the breach case at paragraph 15.7: FRAs should "cover this scenario and make it clear how occupants will stay safe in such a scenario". The council's SFRA maps a Rapid Inundation Zone, land that could flood within 30 minutes of a breach (Local Plan paragraph 13.15). CC3 does not permit new self-contained basement flats there, or in SWMP hotspots of significant or higher hazard, "unless a satisfactory means of escape can be provided".

Worked example: an all-sources FRA for a roof extension

A building of four flats in Fulham, in tidal Flood Zone 3, applied for a rear dormer, a second-floor rear extension, a roof terrace and lower-ground-floor door changes, with no new ground-floor footprint. The agent reported that the council asked for a flood risk assessment "including assessment of all potential sources of flooding: tidal, groundwater, surface water and sewer flooding".

AFC's FRA rated these sources of flooding:

• tidal and fluvial: high (Flood Zone 3);

• groundwater: high (very high susceptibility on the council's SFRA mapping);

• sewer: high (418 recorded incidents in the W14 postcode area, 2006 to 2016);

• reservoirs: medium;

• surface water: low.

The new floor area sits above the 2100 extreme water level of 5.92 mAOD used in the report, and a sedum roof was proposed on the flat roofs. The first issue was written from the title plan, and a second added the submitted drawings. The application was approved on 16 October 2026 (case study).

The approved works included doors at lower ground floor, at about 2.3 mAOD on the survey, and AFC's report did not set out resilience measures for them. CC3 requires "appropriate flood proofing measures" wherever there is a risk of flooding. An FRA here should therefore cover every element at or below the design flood level, with its resistance and resilience measures. Floor levels are covered in AFC's guide to finished floor levels in Flood Zones 2 and 3.

Basements: Policy DC11 and the construction method statement

Policy DC11 permits new or extended basements only where they meet twelve criteria, among them:

• no further into or under the garden than 50% of the depth of the host building, measured from the principal rear elevation (a), and no further than 50% of the depth of the garden (b): both limits apply;

• no more than one storey (exceptions on large sites);

• "a minimum of one metre of soil above any part of the basement beneath a garden";

• a design that minimises flood risk "from all sources of flooding" (f), with SuDS and "active drainage devices to minimise the risk of sewer flooding" (h);

• "a Construction Method Statement (CMS) (carried out by a qualified structural or civil engineer) to be submitted with planning applications for all basement projects" (k), including a construction traffic management plan (l).

CC3 adds structural waterproofing details for any subterranean element, and a non-return valve or equivalent for "more" or "highly" vulnerable uses at basement or lower ground floor. The council asks for a CMS rather than a formal basement impact assessment, and the flood, groundwater and drainage inputs sit with the FRA. See AFC's basement impact assessment service and the NPPF 2026 basement article.

Drainage: CC4, SuD2 and living roofs

Policy CC4 requires major developments to reduce peak runoff "to greenfield run off rates for storms up to the 1 in 100 year event (plus climate change allowance)". Smaller schemes are not exempt: "all other developments must maximise attenuation levels, achieving greenfield run off rates where possible, particularly where they are located in surface water flooding hotspots, or increase a site's impermeable area". The same policy says "all flat roofs in new developments should be living roofs".

For major schemes, Key Principle SuD2 of the SPD (paragraph 14.9) is a validation rule: "Major applications that do not include a Sustainable Drainage Strategy will not be accepted or validated by the planning authority until one is provided." The strategy is also tested against London Plan Policies SI 12 and SI 13 in chapter 9 of the London Plan.

Air quality: the ventilation strategy condition

The whole borough is an Air Quality Management Area. Policy CC10 requires "mitigation measures that reduce exposure to acceptable levels" for sensitive uses such as housing. The council adopted its Air Quality Action Plan 2025-2030 on 16 December 2024. Ventilation conditions on AFC's jobs apply the council's WHO-aligned 2030 targets: NO₂ 10, PM₁₀ 15 and PM₂.₅ 5 µg/m³. On a conversion in Hammersmith, modelled NO₂ of 19.6 µg/m³ was about half the national objective of 40 µg/m³ but above the borough target, so the condition applied to every floor.

The condition on that job required a ventilation strategy before above-ground works (Figure 2):

• intakes at rear roof level or on the rear elevation;

• extracts at least 2 m from intakes;

• an independently tested mechanical ventilation system with at least 90% NO₂ and particulate filtration, citing BS EN ISO 10121-1 and BS EN ISO 16890;

• habitable-room windows restricted to 200 mm;

• a design that avoids overheating.

A second condition required a post-installation compliance report by an MRICS chartered building surveyor before occupation.

Figure 2. The ventilation strategy condition on an AFC job in Hammersmith (schematic, not to scale).

On a lower-ground-floor flat near Shepherd's Bush, AFC's first ventilation strategy for the condition (P02, issued 19 August 2026) did not name a tested ventilation unit or filter system. At discharge, the council's air quality officers responded on 23 October 2026: "There are no required details included of the proposed NOx and PM2.5 filtration system for compliance with part '3' of condition 6. They do not recommend signing this off in the absence of these details." AFC's amended strategy (P03, 27 October 2026) names the MVHR unit, an activated-carbon NO₂ stage and an F9 supply filter, with the manufacturers' data sheets. The discharge was undecided at the end of October 2026.

A strategy should name the supply unit and each filter stage, and give each removal figure with the test method and test body behind it. It should also check the intake position against neighbouring kitchen extracts and flues. Extract-only systems provide no filtered supply air and do not meet the filtration part. The Shepherd's Bush and Hammersmith conversion case studies set out the two jobs, and AFC's air quality assessment page the wider service.

Figure 3. What the council asked for on three AFC jobs in the borough.

Of 2,950 approvals with published conditions between October 2023 and October 2026 (London Datahub condition titles, AFC analysis), 468 (15.9%) carried a condition relating to flood risk, 244 of them titled "Flood Risk Assessment", and 53 carried one titled "Ventilation Strategy".

What to submit in Hammersmith and Fulham

• An FRA covering tidal (including a breach of the defences), fluvial, surface water, groundwater, sewer and reservoir flooding, with the current Flood Map for Planning extract.

• Floor levels in mAOD, set against the design flood level, and resistance and resilience measures for every element at or below it, doors included.

• For a basement: the DC11 limits shown on the drawings, structural waterproofing details, a non-return valve or pumped device, and a CMS by a structural or civil engineer.

• A Rapid Inundation Zone check, and a means of escape for any self-contained basement flat in it.

• A SuDS strategy for major schemes (SuD2), and attenuation towards greenfield rate with living roofs on all flat roofs for every scheme.

• For new homes: a ventilation strategy that names the unit and filters with their test data, and a surveyor lined up for the compliance report.

Do I need a flood risk assessment for a loft or roof extension in Hammersmith and Fulham?

Yes, if the site is in Flood Zone 2 or 3. Policy CC3 asks for an FRA for all development in those zones, and on AFC's Fulham job the council asked for one covering tidal, groundwater, surface water and sewer flooding although no ground-floor footprint was added. A short assessment is usually enough.

Is Hammersmith and Fulham in Flood Zone 3?

Much of it is. The Local Plan states that over 60% of the borough and about 75% of its population are in Flood Zones 2 and 3, from the tidal Thames. The land is defended, but the zones are drawn without defences, so the Flood Map for Planning should be checked for each site.

What does an all-sources flood risk assessment cover?

Tidal and fluvial flooding (including a breach of the Thames defences), surface water, groundwater, sewer flooding and reservoirs, each with its evidence, a rating and the mitigation it needs. AFC's London FRA page lists the scope.

What does Hammersmith and Fulham require for a basement?

Policy DC11: no more than one storey; into the garden no further than 50% of the host building's depth (measured from the principal rear elevation) and no further than 50% of the garden's depth, both limits applying; at least one metre of soil above any part under a garden; SuDS and sewer flood protection; and a Construction Method Statement by a qualified structural or civil engineer with every application.

How do I discharge a ventilation strategy condition in Hammersmith and Fulham?

Answer each part of the condition in turn: intake position, extract separation, the named ventilation unit and filters with their test data, window restrictors and overheating. On AFC's Shepherd's Bush job, officers did not recommend sign-off of AFC's first strategy, which did not identify the filtration system. See AFC's ventilation strategy service.

What filters does the council ask for in a ventilation strategy?

The condition seen on AFC jobs asks for at least 90% removal of NO₂ and particulates and cites BS EN ISO 10121-1 (gas-phase filters) and BS EN ISO 16890 (particulate filters). In practice: an activated-carbon stage and a fine particulate filter on the supply air, each figure given with its test source.

Enquiries

Air & Flood Consultants prepares flood risk assessments, SuDS strategies and ventilation strategies for sites across Hammersmith and Fulham and London. Send the site address and drawings through the enquiry form.