House Extensions Isleworth
Single storey rear and double storey extensions in Isleworth, planning, structure and build under one contract.
- Rated 4.9 from 31 reviews on MyBuilder
- Single-storey rear extension from £40,000
- Free site survey, itemised quote
Isleworth's TW7 terraces, with their long rear gardens, side returns and closet wings, and its post-war houses on wider plots, are good candidates for extending. GS Renovation plans and builds single and double storey, rear, side-return and wrap-around extensions across the neighbourhood, bringing our founder's 30+ years of building experience. We manage foundations, steelwork, roofing and the full fit-out, and programme the build around the party wall notices you serve.
In Isleworth, a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m² (indicative London ranges, Architecture for London, Resi, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Isleworth: the structural engineer we appoint designs the structure, we build it, and building control signs it off.
Price guide
| Work | Indicative London range |
|---|---|
| Single-storey rear extension | from £40,000 |
| Rear extension, per m² | £2,000 to £2,800 |
| Side return, per m² | £2,400 to £3,200 |
| Wraparound, per m² | £2,400 to £3,200 |
| Double storey, per m² | £1,800 to £2,600 |
| Basement extension | see the basement page |
Indicative London ranges (Architecture for London, Resi, MyBuilder and Checkatrade, 2026), not a quotation. Prices checked September 2026. Full breakdown: Extension Costs & Planning.
Quick estimate for Isleworth
Isleworth range: choose above
Side-return extensions on Isleworth terraces
The side return, that narrow strip of land beside the rear closet wing, is often wasted on Isleworth terraces. Filling it in widens a cramped galley kitchen into a bright open-plan space, usually combined with removing the wall to the closet wing on a steel beam. We dig new foundations to the required depth, tie the new structure to the existing brickwork, and open the roof up with rooflights for extra daylight.
Many single storey rear and side-return extensions fall under permitted development, but this is not guaranteed. If your Isleworth home sits within a conservation area, is a flat, or has already been extended, you may need a full planning application to Hounslow Council. We assess your property first and advise on the likely route; your architect or designer prepares the drawings for a lawful development certificate or a householder planning application, and we can recommend one and coordinate the submission.
Housing in Isleworth
Isleworth's housing, as the Council Tax list records it, is about 13,670 homes: 44% houses, 56% flats, with 19% pre-1919, 21% interwar and 61% built since 1945. Since most homes here are flats, a flat in Isleworth has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent.
Isleworth terrace extensions: depths, infills and the riverside boundary
Terraced houses are 30% of Isleworth homes, and for a terrace Hounslow's residential extension guidelines set a maximum single storey rear extension depth of 3.05m, against 3.65m for semi-detached houses and 4.25m for detached houses. Where an infill extension beside an outrigger exceeds those depths, its eaves on the shared boundary should not exceed two metres and its roof pitch should not exceed 45 degrees. In practice that means a sloping roof down to the neighbour's side, so plan rooflights and the kitchen layout around the low edge.
Hounslow Council adopted the Character, Sustainability and Design Codes Supplementary Planning Document as of 14 May 2024, and it supersedes the Residential Extensions Guidelines SPD (2017), so confirm the designer is working to the current code. Then check the address. The Isleworth Riverside Conservation Area was designated on 11 November 1972, and in a Hounslow conservation area, two storey rear extensions and front or side extensions to single residential buildings need planning permission. A single storey rear extension within the national limits can still be permitted development there, unless an Article 4 direction covers the street. For an Isleworth house near the river, settle that question before the drawings go further.
Double storey extensions in Isleworth TW7
Where the plot allows, a double storey rear extension adds a larger kitchen-diner below and an extra bedroom or en-suite above, making real use of Isleworth's generous garden depths. These schemes are more likely to need full planning permission from Hounslow Council, and we factor that into the timeline from the outset. Deeper foundations, heavier steelwork and careful tying into the existing roof and party wall all demand experienced hands.
Because these extensions build up against or across the boundary shared with the adjoining terrace, the Party Wall Act nearly always applies, and you serve notices early to avoid delays. Building regulations govern the structure, insulation, fire safety and drainage regardless of the planning route. We coordinate the engineer, the party wall surveyor if needed and building control, then complete the full fit-out so you move straight into a finished, warm and watertight space.
Kitchen extensions in Isleworth
56% of homes in Isleworth are flats, and a flat has no householder permitted development, so a ground-floor flat's kitchen extension needs planning permission from Hounslow Council and the freeholder's consent. Most kitchen extensions here are on the 44% of homes that are houses. The fit-out is priced on its own page, kitchen renovation in Isleworth; the build figures are in the table above.
- Insulation, airtightness and thermal detailing to regulations
- Planning applications and permitted development assessments
- Roof works and structural alterations at roof level
VOA Council Tax stock of properties, March 2025, for Isleworth & Ivybridge, Syon Green and Spring Grove. Counts rounded by the VOA.
The GS Renovation Difference in Isleworth
With Osterley Park and Syon Park on the doorstep, Isleworth homeowners want extensions that respect the character of their homes. GS Renovation coordinates the design and the planning submission to Hounslow Council with your designer, then builds the structure and finish as one accountable job. Our founder's 30+ years on the tools means we can tell early which schemes fall within permitted development and which need planning, so there are no costly surprises.
- Weathertight shell reached before the fit-out begins
- Building control inspected at foundation, damp course and steel stages
- One point of contact from first survey to handover
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
George always kept to commitments and arrived on time. He offered helpful suggestions during his initial visit and promptly provided a reasonably…
The kitchen has been fitted to the high standard. Saaid was polite, professional, tidy, he pays attention to a little details that make huge…
FAQ
Frequently Asked Questions about House Extensions Isleworth
What should I budget for house extensions in Isleworth?
Budget for house extensions in Isleworth as three amounts inside £2,000 to £2,800 per m² for a rear extension (indicative): the structural element, the finishing trades and the approvals. Approvals belong in the budget wherever the job includes building regulations approval from foundations to completion. GS Renovation lays the TW7 quotation out the same way, so each element can be checked or deferred.
Do I need planning permission to extend my Isleworth house?
Many single storey rear and side-return extensions in Isleworth are permitted development, needing only building regulations approval. However, flats, homes in conservation areas, and properties that have already been extended often require a full planning application to Hounslow Council. Double storey extensions usually do too. We check your specific situation before any work is priced or drawn.
Can a ground-floor Isleworth flat be extended at the back?
Sometimes, though the route differs from a house. Permitted development rights apply to houses, not flats, so a planning application to Hounslow Council is needed, together with the freeholder's written consent under the lease, since the rear wall is part of the building. Owners upstairs may be affected by steel bearing into the block. Building regulations apply in full, as for any extension.
How is the back wall of an Isleworth house opened into a new extension?
With steel, once the extension shell is up. The engineer sizes a beam for the rear wall and whatever bears on it, often the first floor and part of the roof. The wall is propped, the opening cut, the beam set on padstones, and building control inspects before it is cased. Breaking through only once the shell is weathertight keeps the house sealed meanwhile.
Will a flat-roofed Isleworth extension be dark inside?
It can be if it is deep, which is why rooflights or a lantern are planned with the roof structure rather than added later. Each rooflight needs a trimmed opening between the roof joists, sized before the joists are ordered, and a lantern over the middle lights the part furthest from the doors. All that glazing counts in the Part L calculation, so the rest of the envelope may need to work harder.
Does an Isleworth extension over a manhole need special approval?
It depends who owns the drain beneath. For a private drain serving only your home, the inspection chamber can be moved outside the footprint or given a sealed, bolted cover inside, with building control agreeing the detail under Part H. A public sewer is different: the water company has to consent before anything is built over or close to it. Where a run is shared with neighbours, it is often public.
What changes if a public sewer runs within 3m of an Isleworth extension?
The building control route. Hounslow requires a full plans application, not a building notice, where an extension is to be built over or within 3m of a public drain or sewer. Behind a terrace the shared run can cross several gardens, so a camera survey comes before design. The foundations are then designed to bridge the pipe or keep clear of it, and the water company's agreement is sought for any work over the sewer.
Can an Isleworth rear extension have a balcony on its flat roof?
Expect not. Hounslow's residential extension guidelines say schemes using the flat roof of a single storey rear extension as a balcony or roof terrace will normally be refused. On a terrace with neighbours close on both sides, design the roof for rooflights and drainage, not for standing on. Leave out any door or window from the room above that could read as access, since that invites objection even when no terrace is proposed.
Does an Isleworth extension cost the same in building control fees under either route?
For a small one, yes. Hounslow's Table B charge for a domestic extension of under 40 square metres is the same for either a full plans or a building notice application, and the charges scheme effective from March 2026 includes VAT at 20%. With the charge equal, choose the route on risk. Full plans gets the foundations, the steel and the drainage checked on paper before the first trench, which suits a plot near the river.
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