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: 30 designated areas in Hounslow
- Permitted Development: Generally available outside conservation areas
What does Hounslow do with an extension application?
Refuses more of them than almost any borough: 69.9% approved, with only Barking and Dagenham lower, and 695 refusals from 2,310 decisions. The counterweight is the appeal record — 49.3% of Hounslow householder appeals succeed, the highest rate in London and well above the 40.8% London figure. Its 43 conservation areas cover 2,064 hectares.
| Measure | Hounslow | London |
|---|---|---|
| Householder approval rate | 69.9% | 83.0% |
| Refused | 695 of 2,310 | — |
| Householder appeals allowed after refusal | 49.3% of 659 | 40.8% of 13,067 |
| Extension applications in the Datahub | 33,282 | 67% approved |
| Larger-extension prior approvals granted | 78 | 26.6% of those determined |
| Householder planning application fee | £548 | statutory, England 2026 |
Sources: decision figures from MHCLG District Planning Application Statistics (PS1 and PS2), twelve quarters to March 2026; the Datahub row from the Planning London Datahub, retrieved 15 August 2026, phrase-matched and comparable within Hounslow only; appeals from Planning Inspectorate casework decided 2016–2026; the fee from the Fees for Applications Regulations 2012 as amended. Build costs on this page are our own 2026 London range.
What does a Hounslow extension cost in 2026?
Costs covered. For the drawings and the application itself, see house extensions in Hounslow.
Construction in Hounslow runs £2,200 to £3,200 the square metre, an outer-London range. Those numbers are the build and nothing more: professional fees, planning fees and VAT all lie outside them.
Chiswick is where Hounslow extensions get expensive, and the reason is detailing rather than size: matched brick, correct mortar and joinery that satisfies a conservation officer, on a stock where the rear elevations are visible and consistent. In Feltham and Hounslow the same floor area is a conventional job. The borough's refusal record also means a redesign is a realistic cost to carry.
Typical extension costs in Hounslow (build only)
Drawings sit above the build cost, and ours are priced 30% under what a London architectural practice would typically charge for the same set.
What moves the price of a Hounslow extension?
The dominant cost factor in Hounslow is the risk of a refusal. The council approved 69.9% of householder applications over three years, second-lowest in London, refusing 695 (MHCLG planning statistics, retrieved 1 September 2026). A refused scheme has spent the fee, the drawings and several months of programme. On the construction side the borough divides between Feltham's semis, which have side passages, and Chiswick's tightly packed villas, which mostly do not.
Conservation areas
There are 30 designated conservation areas in Hounslow, with Chiswick, Bedford Park, Old Isleworth and Brentford among them. Inside a conservation area permitted development rights are curtailed, so the work is likely to need a full application at £548. Materials and detailing generally have to be better than the minimum too, which costs money — how much depends on the scheme, and no published dataset puts a percentage on it.
Property type
The housing stock in Hounslow is predominantly edwardian villas in chiswick and turnham green; georgian in isleworth; 1930s semi-detached in hounslow and feltham; riverside new-build in brentford. 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
A Feltham or Hounslow semi will usually take a mini excavator down the side, keeping the groundworks conventional. Chiswick and Turnham Green are denser and frequently require everything to move through the house. Isleworth's Georgian frontages sit on narrow historic streets where delivery and skip placement need arranging in advance rather than assumed on the day.
Our drawing fees for Hounslow extensions
Fixed fees, quoted before anything starts, nothing billed by the hour. The quote covers a submission Hounslow will validate and the building regulations package sitting behind it.
Drawing fees for extensions in Hounslow
Every package opens with a measured survey on site. Out of it come floor plans and elevations for the house as built and as proposed, the site plan that locates them, and, where the application calls for it, a design and access statement. The same MCIAT chartered technologist stays with it from survey through validation, and it is us who correspond with Hounslow's planners.
We work across W3, W4, TW3, TW4, TW5, TW7, TW8, TW13 and TW14. W3 straddles the Ealing boundary, so an Acton address is not automatically a Hounslow one.
Planning or permitted development in Hounslow?
Not every Hounslow extension needs an application. The permitted development allowance for a single-storey rear extension is three metres on terraced and semi-detached houses, four on detached ones. Beyond that, six or eight metres is reachable by prior approval, where neighbours are consulted and the council has six weeks to act. Across the twelve quarters ending March 2026 Hounslow determined 741 such applications and refused 215, while telling 448 householders that prior approval was not required — those 448 were free to build.
Thirty conservation areas in Hounslow, most of their weight in Chiswick and Turnham Green. There the stock is closely spaced Edwardian and the materials rule carries the assessment. Out in Feltham and Hounslow, where designation is sparse and the houses are interwar semis, the side-extension restriction would be the meaningful one if it applied at all.
Two-storey extensions need full planning permission in all but rare cases. What Hounslow wants to see is a two-storey addition subordinate to the original house, set back from the side boundary, and shaped so neighbouring windows keep their light and their privacy. Householder decisions across the borough ran at 69.9% granted on 2,310 determinations, where London as a whole sat at 83.0%, so the design work here is not a formality. Source: MHCLG live planning statistics.
Hounslow granted 1,352 certificates of lawfulness across three years (MHCLG planning statistics, retrieved 1 September 2026). In the borough with the second-lowest householder approval rate in London, establishing that a scheme needs no permission at all is worth more than usual - it removes the refusal risk entirely rather than merely reducing it. The drawings for a certificate come from the same survey as the build set.
Frequently asked questions
How much does a house extension cost in Hounslow?
Hounslow build rates sit at £2,200–£3,200 per square metre, which is where outer-London boroughs generally land. Take a rear extension of 15 to 20 square metres: £33,000 at the bottom of that range, £64,000 at the top, for construction on its own. Drawings, structural design, the party wall process and the council's charge all sit outside that. The householder application fee in England is £548.
Can I extend under Permitted Development in Hounslow?
A single-storey rear extension in Hounslow is often permitted development: three metres deep on a terrace or semi, four on a detached house, with six and eight available instead by way of the neighbour consultation scheme. Two things narrow that. Hounslow's 30 conservation areas cut the rights back, and Flood Zone 2 covers 12.7% of the borough's land with Zone 3 at 9.7% — along the Thames at Chiswick, Brentford and Isleworth, and along the Crane. A site inside either zone needs a Flood Risk Assessment as soon as a planning application is involved. The fee for a lawful development certificate is £274, and a Lawful Development Certificate is how the permitted development position gets put in writing.
How long does a house extension take in Hounslow?
Instruction to finished build runs 5-8 months in Hounslow. Give the measured survey and drawings 2-3 weeks, a council determination 8 weeks where permission is needed, the building regulations approval another 2-4, then 8-16 weeks on site. Where a Flood Risk Assessment is required it gets written during the drawing stage and adds nothing to the planning clock.
Do I need a Party Wall Agreement for an extension in Hounslow?
Semi-detached Hounslow property involves one neighbour; the terraced stock in Chiswick involves two. Sink a footing closer than three metres to the neighbour's, and deeper, and a month's notice becomes compulsory; construct on the boundary line and the period doubles. None of that changes if the scheme turns out to be permitted development.
What experience do you have with extensions in Hounslow?
We prepare extension drawings for properties across all Hounslow postcodes (W3, W4, TW3, TW4, TW5, TW7, TW8, TW13, TW14). Our MCIAT chartered architectural technologists understand Hounslow Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
