Basement Conversion Plans Hillingdon | HA4

What's included

Fully end-to-end in Hillingdon.

Feasibility & structural survey

A desktop study and a measured survey together: what the existing foundations are, what the ground is made of, where the water table sits, and whether underpinning is viable at that address — all before you spend anything on design.

Planning application (where required)

Digging a new basement, or forming a lightwell, needs planning permission. Converting a cellar that is already there is normally permitted development, since nothing outside the building changes. We establish which of the two you have, then submit accordingly.

Structural engineering package

Underpinning sequence drawings, the reinforced concrete design and a temporary works strategy, all to Eurocode and all signed by a chartered engineer before anybody breaks ground.

Waterproofing strategy

Specified to BS 8102:2022 — cavity drain, cementitious tanking or a membrane system, chosen against the ground and groundwater conditions actually found at your property rather than picked from a catalogue.

Building Regulations submission

Full Plans application to Hillingdon Council Building Control or an approved inspector covering structure (Part A), fire safety (Part B), damp-proofing (Part C), ventilation (Part F) and thermal performance (Part L).

Party Wall & Thames Water coordination

Party Wall notices go with almost every Hillingdon basement. Where the excavation runs within 3 m of a public sewer we also handle the Thames Water build-over agreement, which is a separate consent on its own timetable.

Which postcode districts around Hillingdon do we cover for basement conversion work?

Each Hillingdon postcode district we work in, the housing that dominates it, and the authority validating a basement conversion there. Charges in the fee column are the 2026 statutory rates payable to the council, not basement conversion estimates.

Postcode	Area	Dominant stock	What it means for a basement conversion
UB8	Uxbridge / Cowley	Georgian fragments, 1930s semi-detached, post-war flats	UB8 covers Uxbridge and Cowley only. Hillingdon handles the basement and its officers apply the UB8 designation map, which takes in the older core of the town.
UB9	Denham / Harefield	Tudor and Georgian fragments, 1930s semi-detached	UB9 is Denham and Harefield, within Hillingdon for planning purposes, and the part of the borough where Green Belt constraints bite hardest on anything visible above ground.
UB10	Hillingdon / Ickenham	1930s semi-detached, Tudor fragments	UB10 spans Hillingdon village and Ickenham. The council determines a basement here, and its officers work from the UB10 designation map when they read it.
UB11	Stockley Park	1930s semi-detached housing dominates. Metropolitan Green Belt runs across much of the north, the Uxbridge and Ruislip villages hold small pockets of Victorian cottages, and the stock nearest Heathrow is largely post-war. Houses from that one decade account for 29.8% of the borough total.	Stockley Park sits wholly inside UB11, so Hillingdon validates everything submitted from there, although the district is largely commercial rather than residential.
HA4	Ruislip	1930s semi-detached, Tudor cottages	Ruislip lies wholly inside HA4, so every basement application from it is validated by Hillingdon Council, and the Ruislip Village designation covers part of that district.
HA5	Pinner / Eastcote	Tudor fragments, Edwardian villas, 1930s semi-detached	HA5 at Pinner and Eastcote is split between Hillingdon and Harrow. Either may determine the application, so the address goes onto both authorities’ maps before design begins.
HA6	Northwood	1930s detached, Edwardian villas	Northwood is contained within HA6 and validated by Hillingdon alone. It is one of the 29 designated areas, so the street elevation gets read closely even when the work is below ground.
UB3	Hayes / Harlington	1930s semi-detached, post-war estates	Hayes and Harlington make up UB3, and applications from there are determined by Hillingdon Council alone. Housing is mostly post-war, which normally means shallow foundations and a full dig rather than a cellar to deepen.

Which Hillingdon neighbourhoods suit a basement conversion best?

For this work the ground matters more than the roof line, and it changes over a short distance. 11.08% of Hillingdon lies in Flood Zone 2 and 6.32% in Flood Zone 3 on Environment Agency mapping, and which zone an address falls in decides whether a basement conversion has to carry a Flood Risk Assessment with it.

Yiewsley (UB7) — a basement conversion in Yiewsley starts from 1930s semi-detached, and Yiewsley reads as 1930s semi-detached. 1930s houses were built without cellars in UB7. Hillingdon Council assesses Yiewsley basement conversions against its own Yiewsley character guidance, and the location plan we draw for Yiewsley references Yiewsley High Street.
Eastcote (HA5) — 1930s semi-detached defines Eastcote, where 1930s semi-detached set what a basement conversion can do. 1930s houses were built without cellars across HA5. Any Eastcote basement conversion is validated by Hillingdon Council, with Eastcote Tube the usual Eastcote reference point on the site plan.
Hillingdon (UB10) — a basement conversion in Hillingdon starts from 1930s semi-detached, and Hillingdon reads as 1930s semi-detached, post-war. 1930s houses were built without cellars in UB10. Hillingdon Council assesses Hillingdon basement conversions against its own Hillingdon character guidance, and the location plan we draw for Hillingdon references Hillingdon Tube.
Uxbridge (UB8) — Georgian centre, 1930s residential defines Uxbridge, where georgian fragments, 1930s semi-detached, post-war flats set what a basement conversion can do. Georgian houses here already have a vault and a lower-ground floor across UB8. Any Uxbridge basement conversion is validated by Hillingdon Council, with Uxbridge High Street the usual Uxbridge reference point on the site plan.
Ruislip (HA4) — a basement conversion in Ruislip starts from 1930s semi-detached, tudor cottages, and Ruislip reads as 1930s semi-detached, Tudor fragments. 1930s houses were built without cellars in HA4. Hillingdon Council assesses Ruislip basement conversions against its own Ruislip character guidance, and the location plan we draw for Ruislip references Ruislip Lido.
Northwood (HA6) — a basement conversion in Northwood starts from 1930s detached, edwardian villas, and Northwood reads as 1930s detached and semi-detached. Edwardian houses rarely have a usable cellar in HA6. Hillingdon Council assesses Northwood basement conversions against its own Northwood character guidance, and the location plan we draw for Northwood references Northwood Hills.
Hayes (UB3, UB4) — 1930s semi-detached, post-war defines Hayes, where 1930s semi-detached, post-war estates set what a basement conversion can do. 1930s houses were built without cellars across UB3. Any Hayes basement conversion is validated by Hillingdon Council, with Hayes & Harlington the usual Hayes reference point on the site plan.

How do conservation areas and Article 4 affect a basement conversion in Hillingdon?

Hillingdon conservation designations: 29 in the borough, with Uxbridge, Ruislip Village and Northwood the ones met most frequently. Inside any of them the permitted-development shortcut disappears and the work goes in as a £548 householder application on an eight-week determination, rather than by the £249 prior approval route and its six. Harefield is the designation that most often changes how a basement scheme has to be drawn.

No blanket Article 4 over Hillingdon: Hillingdon Council has not withdrawn permitted development across Hillingdon, so a basement conversion outside a Hillingdon conservation area usually keeps it. The £274 Lawful Development Certificate is still the document a HA4 owner wants on file.

The building fabric around Hillingdon: 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. A basement conversion in Hillingdon is scoped against that stock first, because it decides whether the scheme runs as permitted development, a £274 Lawful Development Certificate, or a full £548 application to Hillingdon Council.

What happens if a Hillingdon property sits on a boundary?

Harrow — cross the Hillingdon boundary and Harrow Council takes the basement conversion instead, applying 29 conservation areas of its own around HA1. We re-price basement conversions at the Hillingdon edge because Harrow policy and Hillingdon policy rarely align on basement conversion depth.
Ealing — adjoining Hillingdon, with 30 conservation areas centred on W3. A basement conversion on the far side of the Hillingdon line is determined by Ealing Council, so the Hillingdon quote for basement conversions is checked against Ealing guidance before it is issued.
Hounslow — the Hounslow boundary runs against Hillingdon, so a basement conversion a street from Hillingdon is decided by Hounslow Council instead. Hounslow runs 30 conservation areas across W3 and its neighbours, which is why basement conversions priced for Hillingdon are re-checked against Hounslow policy whenever the Hillingdon address sits on that edge.

Sources for this Hillingdon basement conversion 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.

Hillingdon FAQs

Basement Conversions in Hillingdon — your questions answered.

What Hillingdon owners ask before committing to a dig: whether the house can take one at all, what the waterproofing has to achieve, which consent applies, and how long the ground work runs.

Start a free quote →

Do I need planning permission for a basement conversion in Hillingdon?

Turning an existing cellar into habitable space in Hillingdon usually needs no planning permission, because it is an internal alteration. Excavating a new basement, adding a lightwell, or dropping floor levels to form a storey generally does. The borough has no basement policy of its own beyond the national framework, so the questions here are structural, drainage and flood risk rather than policy — and with 6.32% of Hillingdon inside Flood Zone 3, whether a Flood Risk Assessment is required is settled from the address itself. A Thames Water build-over agreement applies within 3 m of a public sewer.

How much does a basement conversion cost in Hillingdon?

Per square metre a basement is the dearest way to add space anywhere, and Hillingdon is no exception: underpinning, waterproofing and temporary works are sequential, engineered, and none of them can be left out. Our package covers planning where it is needed, building regulations, the structural design and the BS 8102 specification. The fees are published.

Is my Hillingdon property suitable for a basement conversion?

Most can, though fewer Hillingdon houses start with a cellar than owners assume. Only 1.8% of the borough’s 118,250 dwellings pre-date 1900, with another 2.6% built between 1900 and 1918 (VOA stock figures, 31 March 2025) — among the three lowest pre-1900 shares of the 33 boroughs. Most of Hillingdon is interwar and post-war housing on shallow foundations with no basement at all, so the job is usually a dig rather than a conversion. What settles it at your address is foundation depth, the water table, and whether tree roots or drainage cross the excavation. All four are assessed at the first survey.

What waterproofing does a Hillingdon basement need?

BS 8102:2022 asks for a waterproofing strategy matched to the site rather than a generic one. In Hillingdon that usually means a Type C cavity drain system, sometimes with Type A barrier protection behind it. It has to be specified by a qualified waterproofing designer, and that sits inside our package rather than being charged as an extra.

How long does a basement conversion take in Hillingdon?

Six to twelve months on site is normal for a Hillingdon basement once drawings are approved. Our own stage runs 5 to 8 weeks from instruction to Building Control submission. The long build is the underpinning: it proceeds in short bays, in sequence, and rushing that is how houses move.

Basement Conversion Drawings in Hillingdon

What is a basement conversion drawings package in Hillingdon?

Quick facts · Basement Conversions in Hillingdon

How does planning policy in Hillingdon affect your basement conversion drawings?