TL;DR — Key Facts
- Build cost per sqm: £2,200 – £3,200 (outer London borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 29 designated areas in Hillingdon
- Permitted Development: Generally available outside conservation areas
What does Hillingdon do with an extension application?
It waves a lot of them through. On the 6m prior approval route Hillingdon handled 476 notifications and told 354 that no prior approval was required, granting 77 and refusing 45. It approved 84.2% of the 3,978 householder applications it decided.
| Measure | Hillingdon | London |
|---|---|---|
| Larger-extension notifications | 476 | 354 not required, 77 granted, 45 refused |
| Householder approval rate | 84.2% | 83.0% |
| Householder decisions made | 3,978 | 77,680 |
| Extension applications in the Datahub | 11,496 | 74% approved |
| Lawful development certificates issued | 1,756 | — |
| Householder planning application fee | £548 | statutory, England 2026 |
Where the numbers come from: decision counts are the PS1 and PS2 returns Hillingdon files with MHCLG, covering decisions taken across 2023-24 to 2025-26. One row is Datahub-derived: we read the Planning London Datahub on 15 August 2026 and matched records by phrase, so it is comparable inside Hillingdon and nowhere else. Statutory application charges come from the Fees for Applications Regulations 2012, as amended since. Build costs are our own 2026 London range and no statistical office publishes them.
What does a Hillingdon extension cost in 2026?
With the numbers in front of you the next decision is the drawing set, and that is a separate page: house extensions in Hillingdon.
Build costs in Hillingdon run £2,200 to £3,200 per square metre, an outer-London range rather than a central one. That is construction alone. It excludes our fees, it excludes the planning fee Hillingdon charges, and it excludes VAT.
The Hillingdon variable that does not appear on other pages is acoustic. Under the Heathrow noise contours a new habitable room needs specified glazing and insulation build-up, and that is a real line in the quote rather than an upgrade. Away from the contours the borough prices like any outer-London suburb, on span and glazing. Establish which side of the contour the property sits on before budgeting.
Typical extension costs in Hillingdon (build only)
The build figure is not the whole bill. Drawings are an additional line, and we set ours at roughly 30% below the going rate for a London architect.
What moves the price of a Hillingdon extension?
Hillingdon has a cost factor that exists in no other borough here. Around Harmondsworth and Sipson the Heathrow noise contours affect what can be built and what it must achieve acoustically, so an extension creating a habitable room may need specified glazing and insulation. Elsewhere the borough is ordinary outer London: interwar semis with side access, 29 conservation areas, no Article 4 Direction.
Conservation areas
Twenty-nine conservation areas are designated across Hillingdon, Uxbridge, Ruislip Village, Northwood, Harefield and Ickenham among them. A house inside one is on article 2(3) land, so side extensions, roof alterations and cladding drop out of permitted development and the work goes in as a full application. The householder application fee in England is £548. Expect the material specification to be scrutinised as well: brick, mortar and window detail all get read against the conservation area appraisal, and specification of that standard is not the cheapest way to build.
Property type
The housing stock in Hillingdon is predominantly 1930s semi-detached dominates; metropolitan green belt covers much of north hillingdon; victorian cottages in uxbridge and ruislip villages; post-war housing near heathrow. Victorian and Edwardian terraces typically suit side-return and rear extensions, while 1930s semi-detached properties are well suited to two-storey rear extensions. Detached houses offer the most flexibility but often have larger footprints, meaning higher absolute costs even if the per-sqm rate is similar.
Access and logistics
The interwar stock that dominates Hillingdon generally has a side passage, so plant reaches the rear and the groundworks price conventionally. Uxbridge and Ruislip's village cores are tighter and sit inside the designated areas. Where a property is under the noise contours, the acoustic specification rather than the access route is what separates the budget from a comparable house elsewhere.
Our drawing fees for Hillingdon extensions
Architectural Drawings London quotes a figure and holds it. Nothing is billed by the hour. What we quote is everything Hillingdon Council needs to validate the application, plus the building regulations package behind it.
Drawing fees for extensions in Hillingdon
Whichever package you take, it includes a measured survey recording the property in its present state, floor plans and elevations both existing and proposed, a site plan, and wherever Hillingdon requires it, the design and access statement as well. MCIAT chartered technologists carry it from the survey through to submission, and the correspondence with the council is ours.
The borough's postcodes are all covered. HA4 and HA5 take in Ruislip and Eastcote, HA6 is Northwood, and to the west and south sit UB3, UB4, UB7 with UB8, then UB9, UB10 and finally UB11.
Planning or permitted development in Hillingdon?
Permitted development covers a great many Hillingdon extensions, and no planning application is needed for them. Without notifying anyone, a single-storey rear extension may project 3 metres beyond the original back wall of a terrace or semi, and 4 metres on a detached house. Double those numbers and you are into the larger householder route, which requires prior approval under the neighbour consultation scheme rather than a full application. Note that the allowance measured in cubic metres belongs to roof works, not to this. And the borough does check: Hillingdon issued 146 enforcement notices and served 83 planning contravention notices over the same three years, against 3,554 and 3,634 across the 33 boroughs together. That is a shade above its share of London's planning decisions, which is 3.5%. A contravention notice is the council asking, formally, what exactly you have built.
Hillingdon's 29 conservation areas sit mainly over the Uxbridge and Ruislip village cores rather than the interwar suburb. In those cores the materials restriction governs, over older and more varied stock. On a designated interwar street the side-extension loss is the one that bites. Neither has anything to do with the Heathrow noise contours, which are a separate map again.
Two-storey extensions take a full application in all but rare cases. What Hillingdon's officers look for is a two-storey addition sitting clearly below the ridge and behind the frontage of the original house, set in from the side boundary, and drawn so that the loss of light and the overlooking of neighbours can be assessed from the sections rather than argued about later.
Hillingdon granted 1,756 certificates of lawfulness in three years (MHCLG planning statistics, retrieved 1 September 2026). The certificate confirms the planning position and nothing else - it says the work was permitted development, not that it complies with building regulations. Under the Heathrow contours that distinction matters, because the acoustic performance is a building control question that a certificate does not touch.
Frequently asked questions
How much does a house extension cost in Hillingdon?
The build alone lands somewhere between £2,200 and £3,200 per square metre in Hillingdon, which is the outer-London bracket. At 15 square metres the construction bill starts around £33,000; at 20 square metres and the upper rate it reaches £64,000. Neither figure includes a drawing. Professional fees, the council's own charge and VAT all sit outside that range.
Can I extend under Permitted Development in Hillingdon?
A single-storey rear extension in Hillingdon often needs no planning application. Three metres of projection is the baseline for a terrace or semi and four for a detached house; going beyond that, to six and eight metres respectively, means notifying the council under the neighbour consultation scheme and waiting to see whether an adjoining owner objects. Hillingdon determined 476 of those larger schemes and refused only 45, which is 9.5% against 24.9% across London. Inside the borough's 29 conservation areas the rights are narrower. A Lawful Development Certificate is the way to put the question beyond argument for a future buyer. The fee for a lawful development certificate is £274.
How long does a house extension take in Hillingdon?
Five to eight months covers most Hillingdon extension projects from the first phone call to the last invoice. The measured survey and the drawn set account for the first two to three weeks; eight weeks is Hillingdon Council's determination period, if permission is needed at all; a fortnight to a month covers building regulations; and the build itself runs anywhere from eight to sixteen weeks depending on what you are putting up.
Do I need a Party Wall Agreement for an extension in Hillingdon?
One adjoining owner on the semi-detached stock, two on the village terraces. The excavation limb of the Party Wall etc. Act 1996 is the one that usually applies to a rear extension, at a month's notice, with two months where the work touches the shared wall. Acoustic construction under the noise contours does not change any of that; it changes the build-up, not the neighbour's rights.
What experience do you have with extensions in Hillingdon?
We prepare extension drawings for properties across all Hillingdon postcodes (HA4, HA5, HA6, UB3, UB4, UB7, UB8, UB9, UB10, UB11). Our MCIAT chartered architectural technologists understand Hillingdon Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
