House Extensions New Barnet
Single storey rear and double storey extensions in New Barnet, 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 New Barnet, rear and kitchen extensions on its interwar semis and post-war houses, and side return infills on Victorian terraces where the plot is narrow but deeper than it looks. We handle the groundworks, drainage, steel, roof and fit out as one job, with the Barnet planning route and building control coordinated alongside.
In New Barnet, 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 New Barnet: 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.
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Side returns and rear extensions in New Barnet
The side return of an EN4 terrace is a strip of ground beside the back addition that usually holds a drain, a couple of bins and nothing else. Infilling it widens the rear of the house across its full depth, and on the Victorian terraces near the station it is the best value extension going, because the walls are largely there already. The roof is the detail that matters: a flat roof with a lantern or rooflights over the deepest part, laid to a proper fall, because the darkest point of the plan ends up furthest from the garden.
Foundations are the part that costs what it costs. You are digging within a metre or two of a neighbour's wall on both sides, often against a boundary wall of unknown depth, and there may be a mature street tree on the road frontage that pushes the depth further. We dig trial holes rather than assume. Where the existing footings are shallow, and in a New Barnet terrace they frequently are, the engineer will specify stepping down or an underpinning sequence rather than trenching alongside and hoping.
Housing in New Barnet
Around 10,670 homes make up the New Barnet and Hadley Wood statistical areas on the valuation list; 50% are flats and 50% houses, and 17% date from before 1919, 32% from between the wars and 52% from after 1945. Since many homes here are flats, a flat in New Barnet has no householder permitted development rights, so extending one needs planning permission and the freeholder's consent.
Checking permitted development rights before a New Barnet extension
Two groups of New Barnet homes start without the usual householder rights. Half of all homes here are flats, and flats have no permitted development rights for extensions. Even a small rear extension to a flat therefore starts as a planning application. The other group is newer housing. In New Barnet 11% of homes were built in 2000 or later, and a planning permission for new houses can remove permitted development rights by condition.
Before any drawing, look up the original permission for a newer house on Barnet Council's planning register and read its conditions. If rights were removed, an extension that would be allowed elsewhere needs full permission here. An application is judged against the current Barnet Local Plan, whose Planning Data record gives a start date of 2025-03-04, so a similar extension approved on a nearby road before that date is a weaker guide than it looks. The design brief for a New Barnet extension starts from those two checks.
Planning, drains and party walls in EN4
Many single storey rear extensions on a terrace fall within permitted development, but a terraced house, like a semi, is limited to 3 metres of depth (6 metres through prior approval) rather than the 4 metres a detached house gets, and any earlier extension counts against your allowance. Barnet Council decides, and we advise applying for a lawful development certificate where the scheme goes ahead without a full application, because a buyer's conveyancer will want that evidence. Anything double storey, or a wrap around, or a property that has already been extended, usually needs a planning application. We check the property's history before drawings are commissioned.
Two consents catch people out on New Barnet terraces. The first is drainage: the rear of these houses often has a shared foul drain running along the back or a manhole in the middle of where you want to build, so a build over agreement with Thames Water is needed before work starts. The second is the Party Wall etc. Act 1996, since excavating foundations within three metres of a neighbour's structure, and deeper than its footings, triggers a notice with one month to run, and you have neighbours on both sides.
Kitchen extensions in New Barnet
64% of the houses in New Barnet 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 New Barnet; the build figures are in the table above.
Double storey extensions in New Barnet
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 New Barnet often needs planning permission from Barnet Council. With 32% of homes here semi-detached or detached, the side of the house is often an option too. The national Planning Data service returns 16 conservation area results for the London Borough of Barnet, including Railway Terraces (council source). Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Side-return and rear extensions
- Trial holes to set foundation depth near drains and trees
- Bi-fold, sliding and French door openings to the garden
VOA Council Tax stock of properties, March 2025, for the New Barnet and Hadley Wood statistical areas, which also take in East Barnet and Hadley Wood. Counts rounded by the VOA.
Why Choose GS Renovation in New Barnet
New Barnet extensions are won or lost underground, on drains, foundations and boundaries, which is where a structural contractor earns its keep. GS Renovation digs trial holes, appoints the engineer, arranges the Thames Water build over agreement, prepares the information for your party wall notices to both neighbours and runs Barnet building control through to certificate. Founder led, with 30+ years of hands-on experience.
- Weathertight shell reached before the fit-out begins
- Building control applications and inspections coordinated for you
- Building control inspected at foundation, damp course and steel stages
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
The kitchen has been fitted to the high standard. Saaid was polite, professional, tidy, he pays attention to a little details that make huge…
Saiid did a complete refurbishment of our small bathroom. This involved ripping out the existing bath/shower, basin, toilet, pipework, and tiles and…
FAQ
Frequently Asked Questions about House Extensions New Barnet
What is included in your house extensions price in New Barnet?
The house extensions price for a New Barnet home is itemised as labour, materials, approvals and reinstatement, typically totalling £2,000 to £2,800 per m² for a rear extension (indicative). Where the design needs an engineer, GS Renovation appoints and coordinates the engineer and builds to their specification at EN5. The Barnet Council building control fee has its own line, and the lines follow the order of works: the planning route is confirmed before drawings, and footings inspected before concrete.
There is a manhole where I want to extend my New Barnet terrace. What now?
You need a build over agreement from Thames Water before you start, and it is not a formality. If the drain is shared with neighbouring houses, and on an EN4 terrace it very often is, they will want the pipe surveyed, the cover relocated or replaced with a double sealed cover, and the foundation designed so the load is carried past the pipe rather than onto it. Sometimes the sensible answer is to divert the run. We arrange the survey and the application, and the engineer details the foundation around whatever the survey finds.
Can my New Barnet extension be built right up to the boundary?
Often, but next door gets a say through the Party Wall etc. Act 1996 whenever the new foundations go deeper than theirs within 3 metres of their building. That requires one month's notice. On a semi, that almost always applies. Permitted development limits and building regulations are separate questions, so a boundary-line wall can still need planning if it exceeds the permitted development depth.
Should my New Barnet extension have a roof lantern or rooflights?
That depends on how deep the room is and where the kitchen sits. The middle of a deep extension is the darkest part, so a lantern or a row of rooflights above it matters more than a big rear door. Part L limits the total glazing unless it is compensated elsewhere, so the design balances the two. Flat roofs are laid to a fall and inspected before they are finished.
Can we stay in our New Barnet house during the extension build?
Usually, if you plan around the disruptive stages. The early groundworks happen outside. The difficult weeks are the drain diversion, when the WC or kitchen may be off, and the knock-through into the house, which comes late once the extension is weathertight. A temporary kitchen elsewhere keeps the household going. Only a few days are really hard to live through.
What does building control inspect on a New Barnet extension?
Several stages. The foundation trench is seen before concrete, the drains before they are backfilled, the damp proofing and floor insulation before the slab, and any steel before it is cased. The inspector checks insulation before it is hidden, then the finished work, before issuing the completion certificate. Barnet Council or a registered building control approver is booked for each stage.
Does the half-width rule for rear extensions apply to a New Barnet house?
That rule comes from Barnet's Hampstead Garden Suburb Design Guidance, which says rear extensions should generally not be wider than half the width of the rear elevation. It is written for the Suburb, where an extension also needs consent from both Barnet Council and the Hampstead Garden Suburb Trust. A New Barnet house outside the Suburb is not judged against that document, so do not shrink a design because of advice written for somewhere else.
Does a New Barnet extension pay Barnet's Community Infrastructure Levy?
A modest extension falls below the levy's minimum threshold, and a larger one can usually claim the exemption for residential extensions. Barnet adopted its revised charging schedule on 1 March 2022, and it came into effect on 1 April 2022. The exemption has to be claimed before work starts, so on a large New Barnet extension the form goes in with the approvals, not after the foundations.
Can the ground floor flat of a converted New Barnet house build a rear extension?
It can, but through a planning application, because flats have no householder permitted development rights. The upstairs flat's windows and its use of the garden are part of the assessment, and the freeholder's consent is needed for the structural connection. Half of New Barnet's homes are flats, and a lease variation may be needed if the extension adds to your demise. Check whether the garden is in your lease before paying for any drawings.
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