Hillingdon's basement policy and what's approvable today.
Hillingdon has no basement-specific policy of its own, so a scheme is judged against national policy and the borough's general development management approach: structural adequacy, drainage, and flood risk. Flood risk is the one worth checking early - Environment Agency mapping puts 11.1% of the borough inside Flood Zone 2 and 6.3% inside Flood Zone 3. A zone is drawn ignoring every defence that exists, so it is not a forecast that water will arrive. Its effect is narrower and more useful than that: a Flood Risk Assessment becomes part of the submission, or it does not. Note also what a basement is not: it is not permitted development, so the certificate route is closed to it. Hillingdon has 1,756 certificates of lawful development on its return for 2023-24 to 2025-26, and not one of them is a basement's route to consent. Source: MHCLG live planning statistics.
What generally passes is one storey, under roughly half the garden footprint, with light wells and rooflights judged on how they read from the street. Two-storey basements are heavily restricted or simply refused across most of central London - Westminster, Kensington and Chelsea and Camden all take that line - though Hillingdon's pressures are different, and the constraint here is more often ground and Green Belt than townscape.
Construction, which is separate from anything we charge for drawings, has run £4,500 to £6,500 per square metre on single-storey rear basements under a garden in our recent work. Light wells, drainage runs and underpinning are what move a project within that band and above it.
Expect to submit a structural engineer's report, a ground conditions report, a hydrology and drainage assessment, party wall notices and a construction management plan. Inside a conservation area, add a heritage statement to that list.
Almost certainly. Dig within three metres of a neighbour's foundations, or within six metres where your excavation goes below the line struck down at 45 degrees from theirs, and the Party Wall Act is engaged. Where the neighbour dissents, surveyor fees have run £1,000 to £3,000 per adjoining owner on the projects we have handled - that is our observed range, not a published scale.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
District by district across the parts of Hillingdon we draw in: what the housing is, and which authority determines a submission from it. Every pound figure below is a nationally set planning application fee for 2026. Digging the basement costs what it costs, and that is not in this table.
| Postcode | Area | Dominant stock | What it means for a basement extension |
|---|---|---|---|
| UB10 | Hillingdon / Ickenham | 1930s semi-detached, Tudor fragments | UB10 takes in Hillingdon village and Ickenham. The council determines it under its own Local Plan, and the inter-war plots here usually have enough garden to make a rear basement geometrically possible. |
| UB11 | Stockley Park | 1930s semi-detached housing dominates the borough - it accounts for 29.8% of Hillingdon dwellings on the VOA figures at 31 March 2025 - alongside post-war estates near Heathrow, Metropolitan Green Belt over much of the north, and a scattering of older cottages surviving in the historic village centres at Uxbridge and Ruislip. Genuinely pre-1900 stock is thin here at 1.8%, so an existing cellar to deepen is the exception rather than the starting point. | UB11 is Stockley Park, and it lies entirely in Hillingdon, so the council determines any application from it. The district is overwhelmingly commercial, which makes a residential basement here an unusual proposition. |
| HA4 | Ruislip | 1930s semi-detached, Tudor cottages | HA4 is Ruislip and falls to Hillingdon Council throughout. Several of the village-core streets are designated, which adds a heritage statement to the already long list of basement documents. |
| HA5 | Pinner / Eastcote | Tudor fragments, Edwardian villas, 1930s semi-detached | HA5, covering Pinner and Eastcote, is split between Hillingdon and Harrow. Establish which authority holds the address before a basement scheme is drawn, because the two apply different policy to excavation and to the effect on neighbouring amenity. |
| HA6 | Northwood | 1930s detached, Edwardian villas | HA6 is Northwood, determined by Hillingdon Council. Plots here run large and detached, so the party wall exposure is often lower than elsewhere in the borough - though the six-metre excavation test still catches more schemes than owners expect. |
| UB3 | Hayes / Harlington | 1930s semi-detached, post-war estates | UB3 covers Hayes and Harlington. Hillingdon Council decides applications here. Ground conditions towards the Crane valley are the practical constraint, and a hydrology assessment is not a formality in this part of the borough. |
| UB4 | Hayes / Yeading | 1930s semi-detached, post-war estates | UB4 covers Hayes / Yeading only; Hillingdon Council handles the basement extension. |
| UB7 | West Drayton / Sipson / Harmondsworth | 1930s semi-detached, post-war | UB7 covers West Drayton / Sipson / Harmondsworth only; Hillingdon Council handles the basement extension. |
What lies under the garden, how deep the neighbouring foundations go and how much of the plot is left to dig in all change from street to street across Hillingdon, and all three decide what a basement can actually be at your address.
How designation changes a Hillingdon basement extension: Hillingdon Council maintains 29 conservation areas, including Ickenham, Uxbridge, Ruislip Village. Inside any of them a Hillingdon basement extension cannot rely on permitted development, and the route becomes a £548 householder application determined in 8 weeks rather than a £249 prior approval in 6 weeks. Ruislip Village sets the tone for Hillingdon casework.
Hillingdon permitted development status: there is no borough-wide direction from Hillingdon Council, so a basement extension at a HA4 address is normally still permitted development. We certify that with a £274 Lawful Development Certificate rather than leaving a Hillingdon sale to argue it later.
Hillingdon housing stock: 1930s semi-detached dominates; Metropolitan Green Belt covers much of north Hillingdon; Victorian cottages in Uxbridge and Ruislip villages; post-war housing near Heathrow. Largest outer west London borough; significant Green Belt; Heathrow noise contours affect planning decisions in Harmondsworth and Sipson. Read that before choosing a basement extension route in Hillingdon — construction, not postcode, sets what is achievable.
Sources for this Hillingdon basement extension page: borough policies maps and conservation-area registers (2025–26), Royal Mail postcode district boundaries, the Town and Country Planning (Fees for Applications) (England) Regulations, the Town and Country Planning (General Permitted Development) (England) Order 2015 as amended, and the Party Wall etc. Act 1996.
Much of north Hillingdon is Metropolitan Green Belt, where the volume limits on extensions can make a basement the only way to add space without changing the building's footprint.
| Measure | Hillingdon | Context |
|---|---|---|
| Decided under delegated powers | 97.0% | 10th lowest of 33; 97.0% London average |
| Applications withdrawn | 5.5% | 14th lowest of 33; 7.4% London average |
| Given an extension of time | 40.9% | 34.5% London average |
Counts on this page are drawn from Hillingdon's PS1 and PS2 returns to MHCLG and from the MHCLG Planning Data Platform registers, spanning the quarters from spring 2023 to spring 2026. Contains public sector information licensed under the Open Government Licence v3.0.
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