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: 30 in Hounslow
What does a Hounslow loft conversion cost to get through planning?
£1,225 for the drawing set and £548 to the council, or £274 for a certificate of lawfulness if the work stays permitted. Hounslow is the harder borough for it: 695 householder refusals against 1,615 grants, a 69.9% approval rate that only Barking and Dagenham undercuts.
| Measure | Hounslow | Outcome |
|---|---|---|
| Rear dormer applications in the Datahub | 1,129 | 68% approved |
| Loft conversion applications in the Datahub | 348 | 60% approved |
| Mansard roof applications in the Datahub | 245 | 53% approved |
| Householder decisions | 2,310 | 69.9% approved |
| Conservation areas | 43 | 2,064 hectares |
| Householder appeals allowed after refusal | 49.3% of 659 | 40.8% of 13,067 |
Provenance for the table above: rows one to three come from the Planning London Datahub as it stood on 15 August 2026, phrase-matched from application descriptions and therefore comparable only against other Hounslow rows. Row four is drawn from MHCLG PS1 and PS2 quarterly returns, April 2023 through March 2026. Row five was read from planning.data.gov.uk on 1 September 2026. Row six is a count of Planning Inspectorate cases decided over 2016 to 2026. Construction ranges quoted here are our own London estimates for 2026 and come from no published dataset. Contains public sector information licensed under the Open Government Licence v3.0
What does a Chiswick or Feltham loft cost?
With the numbers agreed, drawings come next: loft conversions in Hounslow.
Hounslow decides loft applications quickly and refuses a great many of them, which is an awkward combination to plan around. Of the 1,470 householder decisions timed against the statutory period across 2023-24 to 2025-26, 1,341 came back inside eight weeks — 91.2%, close to the London figure of 92.5% — and only 20 of the entire set took longer than 16 weeks. Yet 695 of 2,310 householder decisions were refusals, so barely 70% were grants, where the equivalent London share is 83.0%. A fast answer is not the same as the answer you wanted, and the drawings are what decide which of the two you get. Source: MHCLG live planning statistics.
Loft conversion build costs in Hounslow
Build cost is not project cost. On top of it come the drawings, a structural engineer's calculations for the beams, a party wall surveyor where a neighbour dissents, and the council's fee. The variables that genuinely swing the total are the finish you specify, how much of the existing roof has to be rebuilt to carry a new floor, and whether the site can take a skip and a delivery lorry.
Expect a conventional rear dormer to yield somewhere near 20 to 30 square metres of new floor. A mansard yields 30 to 50, and gives full standing height across the whole plan instead of only under the ridge. That gap in usable area is the honest reason to consider the more expensive roof. What a conversion does to a sale price varies by street and by finish, and no dataset we hold puts a number on it.
Which loft type suits Hounslow properties?
A Feltham or Hounslow semi is hipped and wants a hip-to-gable before the ridge is long enough to be worth converting. The Edwardian villas of Chiswick and Turnham Green have depth for a rear dormer alone, though the conservation officer's view will decide the detailing. Isleworth's Georgian frontages and the Brentford riverside flats are generally not loft candidates at all.
Hip-to-gable and dormer combinations
Hipped roofs are everywhere on Hounslow's interwar semis — 20.2% of the borough's 111,270 dwellings went up between 1930 and 1939, per the VOA's council-tax stock return dated 31 March 2025 — and a hipped end wastes the best corner of the loft. A hip-to-gable conversion squares that end into a full gable wall; put a rear dormer alongside it and the pair typically return between 25 and 35 square metres. Permitted development can cover the work on a semi, provided the gable does not project beyond the plane of the roof slope fronting the highway.
Velux (rooflight) conversions
Cheapest of the four, and the one that keeps you living in the house while it happens: a rooflight conversion works anywhere there is already enough head height under the existing ridge. Velux windows sit flush in the plane of the roof, so the shape of the roof does not change — which is precisely why they remain realistic inside Hounslow's 30 conservation areas, where altering a roof line is the thing officers refuse most readily. The price is floor area: less than a dormer gives you, and a good deal less than a mansard.
Does a Hounslow loft need planning permission?
Hounslow runs from Chiswick, as heritage-controlled as anywhere in west London, out to Feltham's interwar semis, and the loft answer changes completely along that line. What both halves share is a council that refused 695 householder applications in three years, an approval rate of 69.9% (MHCLG planning statistics, retrieved 1 September 2026). That is the second-lowest in London, and it makes guessing at the permitted development position an expensive habit here.
Permitted Development for lofts
The great majority of Hounslow lofts get built under permitted development, dormer and rooflight alike, with no planning application anywhere in the process. Velux schemes almost never need one. Those rights are conditional, and these are the conditions that decide it:
- 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
Hounslow has 30 conservation areas, including Chiswick, Bedford Park, Old Isleworth, Brentford. 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
Mansards need full permission wherever proposed. In Hounslow the reason to think carefully is the borough's record: it approves 69.9% of householder applications, second-lowest in London, refusing 695 in three years (MHCLG planning statistics, retrieved 1 September 2026). On a Chiswick or Turnham Green address, where the conservation coverage is heaviest, a mansard is a heritage argument first and a design one second.
Use the figure in Hounslow's published pre-application scale. Few boroughs make the case for buying an early officer view as plainly as this one does - with roughly one householder application in three refused, an early officer view converts an unknown into something you can design against before the fee and the drawings are committed.
Our drawing fees for loft conversions in Hounslow
Everything is drawn in-house by MCIAT chartered architectural technologists. That covers the planning application where a scheme needs one and, in every case, the building regulations drawings that the builder and the inspector will both work from.
Drawing fees for loft conversions
The drawing set opens with a measured survey and closes with a submission-ready pack: existing and proposed floor plans, elevations both ways, cross-sections, plus roof and site plans at whatever scale the council specifies. A mansard scheme also has the terrace drawn out in streetscene, with a design and access statement putting the argument into words. On price we come in roughly a third below what an architect in London bills for the same deliverable.
Coverage runs across the borough's postcodes — W4, TW3, TW4, TW5, TW7, TW8, TW13, TW14 — and into the Hounslow edge of W3, the bulk of which is Ealing rather than Hounslow.
Frequently asked questions
How much does a loft conversion cost in Hounslow?
A Hounslow rear dormer costs £40,000 to £60,000 to build. A mansard costs £60,000 to £95,000. A Velux conversion, rooflights and nothing else, costs £25,000 to £40,000. Those are contractor figures — no design fee, no engineer, no council charge inside them.
Do I need planning permission for a loft conversion in Hounslow?
Outside the 30 conservation areas, usually. Hounslow has no borough-wide Article 4 Direction, so a rear dormer inside the volume allowance and off the front roof slope is normally permitted, and the council issued 1,352 lawful development certificates over three years (MHCLG planning statistics, retrieved 1 September 2026). In Chiswick, assume you need permission. Mansards need it wherever you are.
What is the minimum head height for a loft conversion in Hounslow?
Hounslow's Feltham and Hounslow semis reach it at the apex only. The Edwardian villas of Chiswick and Turnham Green have considerably more. Isleworth's Georgian frontages are the exception where the roof form, rather than the measurement, is what rules a conversion out.
Do I need a Party Wall Agreement for a loft conversion in Hounslow?
Terraced and semi-detached Hounslow property, which covers most of the borough, brings the Party Wall etc. Act 1996 into play as soon as a beam is seated in the dividing wall. Notice runs two months, and a hip-to-gable adds a second notifiable item because the wall itself is being built up. Neither is affected by whether the council needs to see the scheme.
Can I get a mansard loft conversion approved in Hounslow?
Hounslow operates no blanket mansard policy, so asking whether mansards are allowed is the wrong question; the council decides each one on its own street. What officers weigh is the streetscene, whether the address sits inside one of the borough's conservation areas, the quality of the design, and whether anything comparable has already been approved nearby. A precedent two doors down is worth more to your case than any amount of argument.
