House Extensions Harlesden
Single storey rear and double storey extensions in Harlesden, 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 on the Victorian and Edwardian terraces of Harlesden, from rear and side-return additions to full wrap-arounds and double storeys. We handle the groundworks, steelwork, roof and fit-out as one accountable project.
In Harlesden, 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 Harlesden: 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 Harlesden
Harlesden range: choose above
Side-return and rear extensions in Harlesden
The bay-fronted terraces around Harlesden almost always have a side return, that narrow strip of yard alongside the original rear closet wing. Filling it in is the classic NW10 project, widening a cramped galley kitchen into a bright open-plan kitchen-diner. We dig and pour the new foundations, build the flank wall, and install steel beams to carry the existing structure where the old external wall comes out.
A rear extension pushes the ground floor further into the garden, and a wrap-around combines both moves for a generous L-shaped living space. We match brickwork and detailing to the Victorian and Edwardian character where it shows, add rooflights or a lantern for daylight, and manage the drainage runs and plan around the party wall notices that come with building tight against the neighbours on either side.
Housing in Harlesden
Roughly 6,350 dwellings sit on the valuation list for Harlesden. 17% are houses and 83% flats, and by age 78% predate 1919, 4% are interwar and 18% post-war. Most homes here are flats, and a flat in Harlesden has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent.
Brent's depth limits and drains for a Harlesden rear extension
Brent's Residential Extensions and Alterations SPD says single storey rear extensions to attached homes could be up to 6 metres deep from the original rear wall, and says two storey rear extensions must not be more than 3 metres deep. On a Harlesden terrace with a rear addition there is more than one rear wall, so agree with the designer which line the depth is measured from before the drawings are fixed. On a narrow plot the first-floor depth is the tighter constraint, so settle it before the ground floor layout is drawn around it.
The drains decide the building control route. Brent requires a full plans application for an extension built over, or within, three metres of a public sewer, so the quicker building notice route is not available on those plots. Brent aims to respond to a full plans application within three weeks, and that can run in parallel with the water company's build over agreement. If the extension is liable for CIL, relief for a home you occupy as your main residence is not automatic and must be applied for before commencement; start digging first and the relief can be lost.
Planning and permitted development in Brent
Whether your Harlesden extension is permitted development or needs a full planning application depends on the property and the size of the build. Many single-storey rear extensions on these NW10 terraces fall within permitted development, but double storeys, larger footprints, and any home in a conservation area or that has already been extended usually need permission from Brent Council. We assess this at the outset so there are no surprises.
Whichever route applies, the structural side is controlled under building regulations, so foundations, beams and the roof are all inspected and signed off by building control. Because these terraces share party walls, Party Wall Act notices are needed for the excavation and any work against the boundary; you serve them as building owner, and we give you the drawings and programme they need. One team ties the planning position, the structure and the finished rooms together.
Inspections and drains on an NW10 extension
An extension is inspected at set stages rather than once at the end, and knowing the list helps you follow progress. Building control wants to see the trench before concrete goes in, since foundation depth on London clay depends on what is actually found and on any tree within influencing distance. Then the damp-proof course and oversite, the cavity trays and lintels, the insulation before it is covered, and the drainage before it is backfilled.
Drains catch people out. Many Harlesden terraces have a public sewer running across the back gardens, and building over or within three metres of it needs a build over agreement from Thames Water, applied for well before the digger arrives. Where an existing inspection chamber ends up inside the new footprint, it has to be sealed and made accessible to a proper detail. We deal with the applications rather than leaving them to you to chase while the site waits.
Kitchen extensions in Harlesden
83% of homes in Harlesden are flats, and a flat has no householder permitted development, so a ground-floor flat's kitchen extension needs planning permission from Brent Council and the freeholder's consent. Most kitchen extensions here are on the 17% of homes that are houses. The fit-out is priced on its own page, kitchen renovation in Harlesden; the build figures are in the table above.
Double storey extensions in Harlesden
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 Harlesden often needs planning permission from Brent Council. It is a job for the 17% of homes that are houses; a flat cannot add a storey this way. Brent's Harlesden Conservation Area was designated in March 1994 (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Trial holes to set foundation depth near drains and trees
- Flat and pitched roofs with rooflights or lanterns
- Drawings and the structural engineer's design coordinated for you
VOA Council Tax stock of properties, March 2025, for Harlesden. Counts rounded by the VOA.
The GS Renovation Difference in Harlesden
Harlesden owners choose GS Renovation because we manage the whole extension, from the first foundation to the final coat of paint. We advise honestly on whether your NW10 build is permitted development or needs Brent Council planning, install the steel to the engineer's design, respect the shared party walls and keep an occupied home liveable. Our founder's 30+ years on the tools stands behind every project.
- Paperwork and certificates handed over for when you sell
- Brick and mortar matched to the existing elevation
- Drainage diverted or built over with the right approval in place
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 Harlesden
How much should a Harlesden homeowner set aside for house extensions?
A Harlesden homeowner should set aside the scoped figure for house extensions, typically within £2,000 to £2,800 per m² for a rear extension (indicative), plus an allowance for the unseen. GS Renovation prices what the NW10 survey shows and names what could change once the structure of a late Victorian or Edwardian home is opened. The cost of building regulations approval from foundations to completion belongs in the same budget.
Will my Harlesden side-return extension need planning permission?
Often a single-storey side-return or rear extension on an NW10 terrace falls under permitted development, so no planning application is needed. But if your home is in a conservation area, has already been extended, or you want a larger or double-storey build, Brent Council permission is usually required. We check the position for your specific property before we start and coordinate the application with your architect if needed. A listed building is the exception: work that affects its character, inside or out, including an extension or taking out a wall or chimney breast, needs listed building consent from Brent Council, and doing it without consent is a criminal offence.
Can I extend over the public sewer behind my Harlesden house?
Possibly, but only with the water company on board. Where a house backs onto others in Harlesden, a shared public sewer may cross the gardens, and footings over it or close beside it require Thames Water's written approval before digging. That can alter the foundation design, bridging the pipe or moving a wall line. A camera survey pins down its route and depth first.
Can the ground-floor flat in a Harlesden conversion build a rear extension?
Only through a full planning application to Brent Council, since flats have no permitted development rights for extensions. The freeholder must also consent, and upper flats may be affected by changes to light, drainage and the shared structure. Building regulations apply throughout. Check whether the garden is part of your lease before you start.
What happens to the soil stack on the rear wall when a Harlesden side return is built?
It usually has to move. The stack from the bathroom often runs down the outside of the house where the side return will go, so it is rerouted, boxed inside or extended through the new roof. The connection to the drain is changed at the same time under Part H. Plan it early, as it affects the layout of the kitchen inside.
How is the roof of a Harlesden side-return extension designed so the middle room still gets light?
Usually with glazing. A fully glazed or part-glazed roof lets daylight into the old middle room of the house, which otherwise becomes dark. Part L limits how much glazing is allowed without extra insulation elsewhere, so the design balances light and heat loss. The roof also needs a steel beam to carry the side wall of the house above.
How deep can a two storey rear extension to a Harlesden terrace be?
Brent's Residential Extensions and Alterations SPD says two storey rear extensions to attached homes must not be more than 3 metres deep from the original rear wall, against up to 6 metres for single storey. Brent's PREAPP 1 householder service gives a written response on extensions, and Brent aims to reply within 20 working days of receiving the enquiry and fee. That is a sensible check on a two storey scheme before the structure is drawn.
Is a building notice possible for a Harlesden extension if a sewer runs nearby?
Not where the extension is built over, or within, three metres of a public sewer, because Brent requires a full plans application in that case. Elsewhere, Brent's building notice does cover small extensions in residential use. Find the sewer first. A camera survey of the drains shows where the pipe runs, and that decides both the building control route and whether a build over agreement is needed.
Does CIL relief need claiming before extending a Harlesden house?
Where the extension is liable for CIL, yes. The Brent CIL Charging Schedule came into effect on 1 July 2013, and relief for an extension to a home occupied by the applicant as their main residence is not automatic: it must be applied for before commencement. Pouring the foundations before the claim is made can forfeit it. Ask the designer to confirm whether the extension is liable while the planning drawings are prepared, so the claim is made in time.
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