TL;DR — Key Facts
- Dormer loft conversion: £40,000 – £60,000 (outer London borough)
- Mansard conversion: £60,000 – £95,000
- Velux conversion: £25,000 – £40,000
- Planning permission: Generally available for rear dormers
- Conservation areas: 29 in Hillingdon
What does a Hillingdon loft conversion cost, and will it be approved?
Very probably, and more probably than in most of London. Hillingdon determined 476 larger rear extensions under prior approval across the three years ended March 2026, refusing 45 of them — 9.5%, the second lowest refusal share of the 27 boroughs with enough cases to measure, against 24.9% pooled. It granted 84.2% of 3,978 householder applications and recorded 1,756 lawful development certificates. The one number that should give you pause is the appeal record: inspectors decided 1,021 householder appeals from this borough and allowed 475 of them, 46.5% against 35.6% for England. Only Hounslow is overturned more often.
| Measure | Hillingdon | Outcome |
|---|---|---|
| Larger rear extensions determined under prior approval | 476 | 354 not required, 77 granted, 45 refused |
| … refusal share | 9.5% | 24.9% across the 33 boroughs |
| Householder decisions | 3,978 | 84.2% granted, against 83.0% for London |
| Lawful development certificates recorded | 1,756 | — |
| Article 4 areas in the register | 150 | 71 withdraw commercial-to-residential, 1 the householder extension right |
| Householder appeals decided | 1,021 | 46.5% allowed, against 35.6% for England |
| Rear dormer applications in the Datahub | 4,030 | 78% approved |
Sources: rows one and two from the Planning London Datahub, retrieved 15 August 2026 — phrase-matched descriptions, comparable within Hillingdon only. Rows three and four from MHCLG PS1 and PS2, twelve quarters to March 2026. Row five from the MHCLG Planning Data Platform register, retrieved 1 September 2026. Row six from Planning Inspectorate casework decided 2016–2026. Build costs on this page are our own 2026 London range.
What does a Hillingdon loft cost by roof type?
Once the budget is settled, the remaining questions are the drawing set and the consent route. Both are covered in detail on loft conversions in Hillingdon.
Two things set the number on a Hillingdon roof: whether the hip has to come off, and how close you are to the runway. Just under a third of the borough's 118,250 dwellings — 29.8% — were built in the 1930s and another 25.3% between 1945 and 1972, so hipped roofs are the default and a hip-to-gable is usually the first line of the quotation rather than an upgrade to it. Almost nothing here is old: 1.8% of the stock predates 1900, a share only Barking and Dagenham and Havering beat.
Loft conversion build costs in Hillingdon
Those brackets cover construction alone. Design fees, the structural engineer, a Party Wall surveyor and the council's own charges sit outside them, and so does anything the acoustic specification adds. The council charges themselves are small and nationally fixed: £249 to notify a larger extension, £274 for a lawful development certificate, £548 for a householder application. The variables that actually move the build figure are loft size, the fit-out specification, how much structural work the roof needs and how easily a lorry can reach the back of the house.
Hillingdon's interwar semis are dormer territory, which means the 20-30 square metres that form returns rather than a mansard's 30-50, reaching the middle of that range once the hip has been converted. Under the Heathrow noise contours there is a further consideration: the acoustic build-up a habitable room needs takes a little off the finished height, and occasionally off the usable area too. That is a building regulations question rather than a planning one, and it is settled at survey, not at quotation.
Which loft type suits Hillingdon properties?
Hip-to-gable with a rear dormer on the interwar semis that dominate the borough. The Victorian cottages in the Uxbridge and Ruislip village cores are lower and tighter, often rooflight-only, and sit inside the designated areas. Near Heathrow the choice of conversion may be driven by acoustic performance as much as by space, which is an unusual position and one worth designing for rather than retrofitting.
Hip-to-gable and dormer combinations
The hipped roof is the single biggest constraint on a loft here, and it follows directly from when the borough was built out: 36.8% of Hillingdon's dwellings date from between 1919 and 1939, in the suburban semi-detached form that cuts the end of the roof away and removes the standing height exactly where a bedroom wants it. A hip-to-gable squares that end back off, and combined with a rear dormer it usually yields 25 to 35 square metres of habitable floor. On a semi that combination is often still permitted development, provided the squared-off gable stays behind the existing front roof plane and the whole addition sits inside the 50 cubic metre allowance.
Velux (rooflight) conversions
A rooflight scheme (Velux) is the cheapest and least disruptive route, and it only works where the existing loft already has the standing height without raising anything. The roof shape stays exactly as built, which is precisely why it is the option that survives inside a designated conservation area, where an alteration to a visible roof plane is the thing most likely to be refused outright. The price of that is floor area: with no dormer, the usable width is bounded by the pitch, and on a shallow inter-war roof that can leave you with half the footprint below full height.
Does a Hillingdon loft need planning permission?
Hillingdon has a constraint that appears on no conservation area map. Around Harmondsworth and Sipson the Heathrow noise contours affect what can be built, and a loft conversion that creates a new bedroom is exactly the kind of proposal they bear on — expect to demonstrate sound insulation and glazing performance as part of the application. Elsewhere the borough is ordinary outer London, with 29 conservation areas and no Article 4 Direction.
Permitted Development for lofts
On the suburban stock that makes up most of Hillingdon, a rear dormer or a Velux conversion is normally permitted development: no application, no £548 fee, no eight-week wait. It stops being permitted development the moment any one of the following conditions is not met.
- The dormer must not exceed the highest part of the existing roof
- The dormer must be set back at least 200mm from the eaves
- Materials must be similar in appearance to the existing house
- No verandas, balconies, or raised platforms
- Side-facing windows must be obscure-glazed and non-opening below 1.7m
- Volume limit: 40 cubic metres for terraced houses, 50 cubic metres for others
Conservation areas
Hillingdon has 29 conservation areas, including Uxbridge, Ruislip Village, Northwood, Harefield, Ickenham. In conservation areas, Permitted Development rights for loft conversions are more restricted. Front-facing dormers and side-facing dormers visible from the highway generally require planning permission. Rear dormers may still qualify as PD, but the conservation officer may have views on materials and design. Mansard conversions in conservation areas are assessed against the area's character appraisal and management plan.
Mansard planning permission
A mansard alters the height and form of the roof and needs permission accordingly. On Hillingdon's interwar semis a hip-to-gable is the more economical route. The borough-specific point sits elsewhere: around Harmondsworth and Sipson the Heathrow noise contours mean a new habitable room may need a specified acoustic build-up, which is a building control requirement a planning permission does not address.
Hillingdon publishes its own pre-application scale. It is worth buying where the property lies under the Heathrow noise contours, because the acoustic expectations are specific and easier to establish early than to retrofit, and on Green Belt land in the north of the borough. For a standard Ruislip or Uxbridge dormer it is rarely necessary.
Our drawing fees for loft conversions in Hillingdon
We are MCIAT chartered architectural technologists, so a Hillingdon loft gets surveyed once and drawn twice from the same model: first the planning or certificate submission, then the building regulations package covering new floor structure, the protected escape route down from a third storey, and the fire separation the stair enclosure has to achieve.
Drawing fees for loft conversions
A Hillingdon loft package begins with a measured survey and delivers existing and proposed floor plans, a roof plan, long and cross sections cut through the new floor, every affected elevation, and a site plan. Where the scheme is a mansard the set also carries a streetscene elevation plus a design and access statement, since a mansard is judged on how the whole street's roof line reads, not on the merits of one house. The figure is fixed at instruction and lands well under what a London architect charges to draw the identical set.
We draw across the entire borough — HA4, HA5, HA6, UB3, UB4, UB7, UB8, UB9, UB10, UB11 — and the survey visit costs nothing at any of them. Hillingdon covers 115.7 square kilometres and only 57.2% of that is built up against 66.0% for London as a whole, so the honest difference between a Ruislip job and a Harmondsworth one is travel time rather than price.
Frequently asked questions
How much does a loft conversion cost in Hillingdon?
What a Hillingdon loft costs divides by roof type, not by postcode. A full-width rear dormer on an inter-war semi runs £40,000 to £60,000. A mansard, which rebuilds the roof rather than opening a hole in it, runs £60,000 to £95,000. A rooflight-only scheme (Velux) that leaves the roof shape untouched runs £25,000 to £40,000. Those are contractor build figures at 2026 rates and carry neither our drawings nor the council's fees. For scale against the asset: HM Land Registry recorded 2,458 Hillingdon sales during 2025 at a median of £510,000 (Price Paid data, retrieved 1 September 2026).
Do I need planning permission for a loft conversion in Hillingdon?
In most of Hillingdon, yes: no borough-wide Article 4 Direction, and 1,756 lawful development certificates issued in three years (MHCLG planning statistics, retrieved 1 September 2026). A rear dormer inside the volume allowance is the usual permitted form. The noise contours do not remove permitted development rights, but building regulations still apply to a habitable room, and sound insulation is where a Harmondsworth or Sipson conversion differs from one in Ruislip.
What is the minimum head height for a loft conversion in Hillingdon?
Hillingdon's interwar semis generally have it at the ridge; the Victorian cottages in the Uxbridge and Ruislip village cores frequently do not. Under the Heathrow noise contours there is a second measurement that matters as much — the acoustic build-up a new habitable room needs, which reduces the finished height further.
Do I need a Party Wall Agreement for a loft conversion in Hillingdon?
Yes on the semi-detached and terraced stock. Allow two months under the Party Wall etc. Act 1996 before cutting steelwork into the dividing wall, or taking it up to form a gable. The Act is entirely separate from the planning route and from building control, and having a lawful development certificate does not discharge it — a point that catches people who assume one consent covers everything.
Can I get a mansard loft conversion approved in Hillingdon?
Mansards are the hardest case to make in this borough, and the obstacle is the stock rather than the policy. Just 4.4% of Hillingdon's dwellings were built before 1919 (VOA CTSOP4.1, 31 March 2025), so there is very little of the flat-fronted urban terrace on which a mansard reads as normal. Put one on a 1930s semi between a hipped neighbour and a gable-fronted one and it reads as an added storey, which is how the case officer will describe it in the report. The exception is the older village cores, where the form has precedent on the street and precedent is what carries the argument.
