House Extensions Harrow
Single storey rear and double storey extensions in Harrow, 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 Harrow, working with the generous plots that the borough's 1930s semis and interwar suburban houses were built on. We handle groundworks, steelwork, roofing and fit-out as a single project from Pinner to Stanmore.
In Harrow, 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 Harrow: 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 Harrow
Harrow range: choose above
Rear and side extensions in Harrow
The wide plots typical of Harrow's interwar streets suit rear and wrap-around extensions particularly well, and the side access on many semis in HA2 and HA5 makes a side extension straightforward. We begin with proper foundations and groundworks sized to the ground conditions, then install the steel beams needed to open the new space into the existing house, all designed by an engineer and signed off by building control.
Many rear extensions here are kitchen extensions: a single or double storey addition that turns a compartmented 1930s layout into an open kitchen-diner facing the garden, with the old back wall carried on steel. We manage the roof works, whether a flat roof with lantern or a pitched tie-in to the existing roofline, and complete the full fit-out, so the new room is decorated, wired and plumbed rather than left as a shell.
Housing in Harrow
The stock in Harrow runs to roughly 96,870 dwellings, 38% of them flats and 62% houses, built 10% before 1919, 52% between 1919 and 1939 and 38% after 1945. A rear extension on a semi in Harrow digs beside the attached neighbour, so one party wall notice is usual, and 31% of homes here are semi-detached.
How floor area sets the building control charge on a Harrow extension
Harrow prices building control for extensions by floor area, and the bands have edges worth knowing. Table B has one total charge, inspection included, for an extension with floor area over 10m2 but not exceeding 40m2, by full plans or building notice. For an extension over 40m2 but not exceeding 60m2, full plans splits the charge into a plan charge and an inspection charge, while a building notice carries a single charge. Harrow's Scheme 15 sets those charges for applications received on or after 6 April 2026.
So measure the design before the drawings are finalised. A rear extension on a Harrow semi that comes out a little over 40m2 sits in the higher band, and a small change in depth can bring it back. Size matters for the levy as well. Harrow's own CIL applies borough-wide where there is over 100 square metre gross internal floor space, and its CIL page lists Form 9, the Residential Extension Exemption Claim, for an extension to a dwelling house. On a large wraparound scheme, make that claim before work starts on site.
Planning and permitted development in Harrow
Whether your project is permitted development or needs a full planning application depends on the property and the design, and we set this out honestly at the start. Many single storey rear extensions in Harrow proceed under permitted development, including in most conservation areas unless an Article 4 direction removes it. Side extensions, two storey rear extensions and roof extensions in a conservation area such as Harrow on the Hill or Pinner Village need permission from Harrow Council, and larger or double storey schemes usually need it anywhere.
We help you understand which route applies, work alongside the designer who prepares the drawings for any council submission, and plan the programme around the party wall notices you serve as building owner, which almost always follow because your neighbour shares a boundary. Building control then inspects the foundations, steelwork, drainage and insulation as the work proceeds. Planning the consents, party wall timing and building regs together keeps an HA3 or HA7 extension moving without avoidable delays.
Kitchen extensions in Harrow
63% of the houses in Harrow are semi-detached or detached, which usually leaves room for a kitchen extension across the full rear width, with a steel beam over the opening into the house. The fit-out is priced on its own page, kitchen renovation in Harrow; the build figures are in the table above.
Side return extensions in Harrow
Side returns belong to Victorian and Edwardian terraces, and only 10% of homes in Harrow predate 1919, so they are the exception here; most extensions run across the full rear width instead. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².
Double storey extensions in Harrow
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 Harrow often needs planning permission from Harrow Council. With 39% of homes here semi-detached or detached, the side of the house is often an option too. Harrow Council's conservation page states that Harrow has 30 Conservation Areas (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Planning applications and permitted development assessments
- Party wall process explained, and the extension built to the award
- Bi-fold, sliding and French door openings to the garden
VOA Council Tax stock of properties, March 2025, for all 30 statistical areas in Harrow. Counts rounded by the VOA.
Why Choose GS Renovation in Harrow
Harrow homeowners choose GS Renovation because we build the whole extension, not just the shell. From foundations and steel to roofing and finish, one accountable team manages the council route and building control on your interwar plot. Our founder's 30+ years on the tools means tidy working in an occupied family home and a result that adds real space.
- Brick and mortar matched to the existing elevation
- Building control inspected at foundation, damp course and steel stages
- Aftercare and snagging support once the job is finished
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
Saiid did a complete refurbishment of our small bathroom. This involved ripping out the existing bath/shower, basin, toilet, pipework, and tiles and…
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 Harrow
Does the cost of house extensions rise in a 1930s suburban Harrow property?
Age affects the cost of house extensions in Harrow less than people fear; the structural part is broadly similar, within £2,000 to £2,800 per m² for a rear extension (indicative). What differs in 1930s suburban housing is making good picture rails, plaster and floors around the HA1 works, priced as separate lines. The sequence also holds: the planning route is confirmed before drawings, and footings inspected before concrete.
Will my Harrow extension need planning permission or just building regs?
It depends on the design. Many single storey rear extensions on Harrow's generous interwar plots fall under permitted development and need only building regulations. Larger, double storey or two-sided schemes usually need a planning application to Harrow Council, as do side, two storey and roof extensions in a conservation area such as Harrow on the Hill. We assess your property first and tell you which route applies before any work begins.
Will tree roots or clay affect my Harrow extension foundations?
They can set how deep the footings go. Parts of the borough sit on shrinkable London clay, and a large tree nearby dries and swells that ground seasonally. The engineer and building control assess species and distance, and foundations may need to go deeper or use a different design. Knowing this before tendering stops a surprise when the trenches are dug.
How do we keep a Harrow extension from making the middle of the house dark?
By bringing light in from above. Extending the back of a 1930s semi pushes the old rear room further from daylight, so a roof lantern or rooflights over the join send light back into the house. They are designed alongside the steel that opens the wall, since the beam depth and roof build-up decide where the glazing can sit.
Is there a drain under the footprint of my Harrow extension?
Often there is an inspection chamber at the back of a semi. Where that run turns out to be a public sewer, the water company has to approve any building over or beside it before work starts, and Part H sets the falls for any diversion. A camera survey confirms where it runs, so the foundations can be designed around it rather than discovered mid-dig.
How does Part L change a Harrow extension design?
It sets the insulation standard the new walls, floor and roof must meet, and it limits glazing unless the rest compensates. That shapes wall thickness, the roof build-up for a flat roof, and how much of the rear can be glass. Harrow Council building control, or a registered building control approver, checks insulation on site before it is covered.
Does a rear extension on a Harrow semi need a party wall notice?
Where it is built on or near the boundary with the attached house, it usually does. 39% of homes in Harrow are semi-detached or detached, and on a semi the flank wall of a rear extension can be built on or astride the line of junction, with footings dug close to and deeper than next door's. The Party Wall Act covers both. As building owner you serve the notices, and the neighbour can consent in writing or appoint a surveyor. Serve them once the structural engineer we appoint has fixed the foundation depth, and allow for the notice periods before digging.
Is Harrow's pre-application advice worth paying for on an extension?
For anything beyond a plain rear extension, it usually is. Harrow's householder pre-application advice comes as a meeting with a written response, or as a written response only, and Harrow aims to reply to householders in writing within 15 working days. For a listed house, or one in a conservation area, the heritage category includes the Conservation Officer in the meeting. Give the officer's written response to the designer before the planning drawings are drawn, so the scheme starts from what has been said.
Can a basement go under a new Harrow rear extension?
Only within limits, and the fee changes too. Harrow Building Control states that where an extension includes a basement, the fee will be individually assessed rather than read from the table. Under Local Plan Policy GR4A a basement must stay ancillary to the house and must not reach into the rear garden beyond half the depth of the host dwelling. A deep rear extension with a basement under it can run into that limit, so have the depths checked before the design goes further.
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