House Extensions East Barnet
Single storey rear and double storey extensions in East 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 East Barnet, on the Victorian terraces near the village and the interwar semis around them. Ground conditions in EN4 change with the fall towards the Pymmes Brook, so the foundation design and the drainage strategy are settled before anything is dug, not discovered halfway through. We handle groundworks, steel, roof and fit out as one job.
In East 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 East 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.
Quick estimate for East Barnet
East Barnet range: choose above
Foundations and flood risk near the Pymmes Brook
Parts of East Barnet sit in a flood risk zone, and the low ground either side of the Pymmes Brook through Oak Hill Park and past Brookside is where that bites. It affects three things on an extension: whether a flood risk assessment has to go in with the planning application, where the surface water from the new roof is allowed to go, and how deep the foundations need to be in ground that may be alluvium and made ground rather than the clay you find higher up. We check the flood map for the actual address before drawing anything.
Where the brook is a designated main river, work close to the bank can need consent from the Environment Agency as well as planning permission from Barnet Council, and Barnet is also the lead local flood authority for surface water. None of that stops an extension going ahead. It does mean the drainage strategy has to be real: soakaways sized for the ground that is actually there, or a controlled discharge, rather than a note on a drawing saying the water goes to a soakaway. On heavier clay a soakaway often will not work at all.
Housing in East Barnet
Roughly 6,690 dwellings sit on the valuation list for East Barnet. 70% are houses and 30% flats, and by age 12% predate 1919, 59% are interwar and 28% post-war. A rear extension on a semi in East Barnet digs beside the attached neighbour, so one party wall notice is usual, and 38% of homes here are semi-detached.
Two-storey side extensions on East Barnet's semis
In East Barnet 44% of homes are semi-detached or detached, and 59% were built between 1919 and 1939, and an interwar semi with a side passage commonly has a hipped roof. A two-storey side extension is outside permitted development, because side extensions under those rights are single storey. That makes it a planning application from the start. The Barnet Local Plan's Planning Data record gives a start date of 2025-03-04, and an application is judged against the plan in force, not against what a neighbour was allowed years ago.
The roof is the design problem. A side extension on a hipped roof either continues the hip, which keeps the pair balanced, or ends in a gable, which unbalances it. The side passage is the practical one. It often carries the drains, the gas meter and the only route to the garden for bins and deliveries. Before drawing, locate every pipe and meter in the passage of your East Barnet house, and decide how the garden will be reached once the passage is built over.
Rear and side extensions in EN4
On a Victorian terrace near the village, the usual scheme is a side return infill alongside the rear addition, or a full width rear extension replacing it. The wider interwar plots further out take a deeper single storey rear or a double storey addition with a bedroom over a bigger kitchen. In both cases the existing drainage is the thing that shapes the plan. Old terraces here often have a shared drain and an inspection chamber sitting exactly where you wanted the new kitchen, and building over a public sewer needs an agreement with Thames Water.
Some of this is permitted development and some is not, and it depends on the individual house rather than the street. Whether the property has been extended before, how close it goes to the boundary, whether it is a terrace or a semi, and whether an Article 4 direction or a conservation area applies all change the answer, so we check with Barnet Council before design is fixed. Building regulations approval is separate and always needed. Where the foundation is dug within three metres of next door's wall, the Party Wall etc. Act 1996 applies too.
Kitchen extensions in East Barnet
63% of the houses in East 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 East Barnet; the build figures are in the table above.
- First fix and second fix plumbing and electrics throughout
- Steel goalpost frames where the rear wall opens up
- Bi-fold, sliding and French door openings to the garden
VOA Council Tax stock of properties, March 2025, for New Barnet Town & East Barnet and Osidge. Counts rounded by the VOA.
Why East Barnet Homeowners Choose GS Renovation
East Barnet homeowners choose GS Renovation because we do the unglamorous work first. Flood map, trial holes, existing drain runs and the foundation depth the ground actually needs, all settled before the first cut of the spade. Then groundworks, steel, roof, services and fit out under one team, with Barnet Council building control inspecting as we go and the certificate handed over at the end.
- Programme planned around party wall notice periods before excavation near a neighbour
- A written scope and programme agreed before work starts
- Foundation depth set after trial holes, drains and trees are checked
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 East Barnet
What drives the cost of house extensions in an East Barnet home?
Three things drive the cost of house extensions in an East Barnet home within £2,000 to £2,800 per m² for a rear extension (indicative): how much structure is involved, access at the EN4 address, and making good afterwards. The biggest variable is usually hidden: the drainage run under the footprint decides the foundations. GS Renovation prices each of the three separately rather than averaging across Barnet.
My East Barnet house is in a flood risk zone. Can I still build an extension?
In most cases yes, but the design has to account for it. A flood risk assessment may be required with the planning application to Barnet Council, and the surface water from the new roof has to be dealt with properly rather than pushed into an already loaded system, since Barnet is the lead local flood authority. Practical measures matter too: floor levels, where the electrical sockets sit, and materials in the lower part of the walls that will survive a wetting. Check the flood map for your address first, because the zone can change within a single street near the brook.
How deep can I extend an East Barnet semi without planning permission?
Under permitted development, an attached house such as a semi or terrace can go 3 m deep at the rear on a single storey, and a detached house 4 m, without a planning application. The prior approval route allows more. A conservation area removes side extension rights, and none of this applies to flats or maisonettes. Whatever the planning position, building regulations approval is always required, so that application runs alongside.
Will garden trees affect the foundations for my East Barnet extension?
They can decide them. On clay, tree roots draw moisture out and the ground shrinks, so a trench near a mature tree has to go deeper than one in open lawn, and the mature planting around Oak Hill Park means plenty of gardens are affected. The engineer sets the depth from the species and its distance to the new wall. Building control inspects the trench before any concrete is poured.
When does the back wall of my East Barnet house get knocked through for an extension?
Late. The new walls, roof and windows go up first while the existing back wall stays in place, so the house remains secure and weathertight for most of the build. Only when the extension is sealed does the opening get formed, with the wall propped and the engineer's steel installed on padstones. Building control then inspects the bearings before the new ceiling closes around them.
Does an East Barnet extension have to meet current insulation standards?
Yes. Part L applies to every new extension, so floor, walls and roof all have to meet current insulation values, and generous glazing or a roof lantern has to be balanced within those limits. The build is inspected at stages by Barnet Council or a registered building control approver, and insulation is checked before it is covered. The existing house does not have to be upgraded, although a cold old wall joining a warm new room is worth tackling.
Is an East Barnet rear extension limited to half the width of the house?
Not by that guidance. Barnet's Hampstead Garden Suburb Design Guidance says rear extensions should generally not be wider than half the width of the rear elevation, and extensions there need consent from both Barnet Council and the Hampstead Garden Suburb Trust. That document is written for the Suburb. An East Barnet house outside it is assessed against the borough's general plan, so the width is argued on its own merits.
When does an East Barnet extension need a Community Infrastructure Levy form?
Before work starts, where the extension is large enough to be liable. A small extension falls under the levy's minimum threshold, and a large residential extension can claim an exemption, but only by applying before commencement. Barnet adopted its revised charging schedule on 1 March 2022, effective from 1 April 2022. Ask about the levy when the planning drawings are complete, so the form is not missed.
Could a newer East Barnet house have lost its permitted development rights?
Yes. When newer houses are approved, the planning permission can include a condition removing permitted development rights, so any extension needs an application. In East Barnet 9% of homes were built in 2000 or later. Look up the original permission for your house on Barnet Council's planning register and read its conditions before paying for any extension drawings.
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