House Extensions Brentford
Single and double storey extensions in Brentford, 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
GS Renovation builds extensions across Brentford, working with the terraces and semis of TW8 and, where the lease and planning allow, its ground-floor flats. We handle groundworks, steelwork, roofing and fit-out as one project, and we tell you honestly whether your scheme is permitted development or needs a full application to Hounslow Council.
In Brentford, 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 Brentford: 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 Brentford
Brentford range: choose above
Ground-floor extensions in Brentford homes
Where Brentford has houses rather than flats, typically terraces and interwar semis set back from the river, a rear or side-return kitchen extension can turn a cramped ground floor into an open kitchen-diner. We begin with foundations and groundworks sized for the ground conditions near the Thames, then bring in steel beams where original walls are removed. Roof works, glazing and a full internal fit-out follow, all sequenced by one team so the trades never work against each other.
Whether your extension is permitted development or needs planning permission depends entirely on the property. Flats and maisonettes do not benefit from permitted development rights, and where a house sits in a conservation area, permitted development no longer covers side extensions, two-storey rear extensions or roof enlargements, so the address is checked against Hounslow Council's conservation area maps. We assess your specific case, advise on the likely route, and coordinate a full application to Hounslow Council with your architect or designer when one is required.
Housing in Brentford
On the valuation list Brentford counts about 13,230 homes, 65% flats and 35% houses; 15% predate 1919, 24% are interwar and 61% post-war. Where most homes are flats, a flat in Brentford has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent.
Hounslow's rear extension depths and which document sets them
Hounslow's residential extension guidelines set maximum single storey rear extension depths of 3.05m for terraced houses, 3.65m for semi-detached houses and 4.25m for detached houses. Those are the council's figures for a planning application; an extension built under permitted development follows the national limits instead, and flats have no such rights. For a Brentford terrace behind the High Street, 3.05m is the figure to design to, and a rear outrigger complicates where it is measured from. If the design needs more depth than the guidance allows, the application has to argue for it, so the drawings should show the neighbours' rear windows and the light they keep.
Check which document the designer is drawing to. 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). A scheme drawn against the older document can arrive with the wrong assumptions. On the building control side, Hounslow's Table B charge for a domestic extension of under 40 square metres is the same for a full plans or a building notice application. The choice of route in Brentford can therefore be made on risk rather than cost, and full plans checks the design before any trench is dug.
Extending near Brentford's riverside setting
Brentford's location on the Thames and the Grand Union Canal shapes many extension projects. Sites can be tight, access is sometimes shared, and ground conditions near the water demand careful foundation design. We survey the plot properly before quoting, so the groundworks and drainage are right for the location rather than a generic specification, and we plan deliveries and skips around narrow riverside access.
Regardless of whether planning permission is needed, every extension must meet building regulations for structure, insulation, drainage and fire safety, and building control inspects at key stages. Where an extension meets a wall shared with a neighbour, the Party Wall Act applies and you, as building owner, serve the correct notices in good time, using the drawings we supply. This combination of local knowledge and proper process keeps your Brentford project on track from foundation to finish.
Kitchen extensions in Brentford
65% of homes in Brentford 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 35% of homes that are houses. The fit-out is priced on its own page, kitchen renovation in Brentford; the build figures are in the table above.
Double storey extensions in Brentford
Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Brentford often needs planning permission from Hounslow Council. It is a job for the 35% of homes that are houses; a flat cannot add a storey this way. The St Paul's Brentford conservation area was designated on 28 February 1989 and extended on 17 May 2001 (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Insulation, airtightness and thermal detailing to regulations
- Brickwork and mortar matched to the existing elevation
- Building control inspections at foundation, damp course and steel stages
VOA Council Tax stock of properties, March 2025, for Brentford Central, Brentford West & Osterley Park and Brentford East. Counts rounded by the VOA.
Why Choose GS Renovation in Brentford
Brentford homeowners choose GS Renovation because we manage the whole extension under one roof, from foundations and steel to roofing and decoration. We give straight advice on whether you need planning permission or can build under permitted development, coordinate the Hounslow Council application with your designer when required, and keep to building regulations throughout. Our founder's 30+ years on the tools stands behind every project.
- Building control inspected at foundation, damp course and steel stages
- Weathertight shell reached before the fit-out begins
- Building control applications and inspections coordinated for you
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 Brentford
What happens to the house extensions price if the scope changes in Brentford?
The house extensions price in Brentford moves visibly when the scope changes, because the quotation is itemised within £2,000 to £2,800 per m² for a rear extension (indicative). A change at TW8 is priced against the same lines rather than renegotiated, and any change touching the approvals, such as a Hounslow Council building control inspection of the foundations before concrete is poured, is flagged. GS Renovation issues a written variation and waits for approval.
Can I extend my Brentford property under permitted development?
Houses often have permitted development rights for modest rear extensions, but flats and maisonettes, common in Brentford's conversions, do not. A house in a conservation area also loses some of those rights, such as side and two-storey extensions. We assess your specific home, advise on the likely route, and coordinate a full planning application to Hounslow Council with your architect or designer when one is needed.
Can a ground-floor flat in Brentford be extended into the garden?
Yes, but only through a planning application, because flats and maisonettes have no permitted development rights. The freeholder's consent is needed as well, since the extension attaches to the structure of the whole building and may cover land outside your lease plan. Building regulations apply as they would to any extension, and drainage from the flats above frequently runs under that ground, so it is traced before a design is fixed.
What happens if a public sewer runs under my planned extension in Brentford?
Building over or near a public sewer needs the water company's consent before the foundations go in, and behind a terrace the shared run can cross several plots. A drainage camera survey finds the pipe first. The foundation design then bridges it, diverts it or keeps the footprint clear. Falls for any new kitchen or bathroom drainage are set at the same stage, because those pipes go in before the slab is poured.
Do Brentford neighbours get notice before extension foundations are dug?
Yes, if the trench is dug within 3 metres of their building and below its foundations: the Party Wall etc. Act 1996 then requires a month's notice. Building a new wall on or against the boundary brings separate notices. Close to the Thames and the Grand Union Canal, ground conditions can call for deeper foundations, which is exactly what brings the 3 metre rule into play.
What insulation standard does a new extension in Brentford have to meet?
Current Part L standards for the walls, floor, roof and glazing, whatever the age of the house it joins. Large sliding doors or a big roof lantern can push heat loss beyond what the rules allow, so the glazed area is balanced against insulation elsewhere at design stage, not argued about later. Building control inspects the insulation in place before it is covered by plasterboard or screed.
How high can a Brentford side infill extension be on the boundary beside an outrigger?
Where an infill extension beside an outrigger exceeds Hounslow's maximum depths, its eaves on the shared boundary should not exceed two metres and its roof pitch should not exceed 45 degrees. That low eaves line sets the ceiling height along the neighbour's side, so a sloping ceiling with rooflights works better than a flat one. Put tall units and doors on the house side. Check the chosen tile or slate against its own minimum pitch, since the slope has to satisfy both that and the 45 degree cap.
Can the flat roof of a Brentford rear extension become a roof terrace?
Plan on the answer being no. 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. Design the roof as a roof: a warm deck with a proper fall, rooflights where the room needs light, and no door from the bedroom above opening onto it. A balustrade added later, or a door that looks like access, invites a complaint from a neighbour.
Is Hounslow pre-application advice worth paying for on a Brentford extension?
For a scheme beyond the published depths, or on a conservation area street, often yes. Hounslow's fee schedule offers householder pre-application advice on extensions with a written response only, or with a site visit at a higher fee. In a Hounslow conservation area, two storey rear extensions to single residential buildings need planning permission, as do front and side extensions of one storey or more. A written response suits a simple question, and a site visit suits a tight plot.
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