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Rear extension cost · London & Hertfordshire
Rear extension cost in London 2026: £35,000–£75,000 by depth, and what moves it
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A single-storey rear extension in London costs £35,000–£75,000 in 2026, design and build inclusive — the price moves with depth more than anything else. This page gives you the range by size from the price table we build to, what is in the figure, what pushes it up or down, and how to get an itemised estimate for your house.
Quick answer
£35,000–£75,000 for a single-storey rear extension of 3–6 m, design and build inclusive (kitchen on top). Postcode dependent.
Up to 3 m (4 m detached) is usually permitted development; 4–6 m (8 m detached) by prior approval with neighbour notification; beyond that, a planning application.
10–14 weeks on site; 8–12 weeks of design and approvals before that.
A free 30-minute phone call with our team, no obligation.
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road NW6
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee, structural guarantee to building-control sign-off, 10-year LABC Warranty available
How much does a rear extension cost in London in 2026?
Guide ranges by size, design and build inclusive. Every project is priced from your postcode and house type.
Size (depth × width)
Typical use
Guide range
3 m full width (≈12–15 m²)
Kitchen extended into a kitchen-diner
£35,000 – £50,000
4–5 m full width (≈18–25 m²)
Open-plan kitchen, dining and seating
£50,000 – £65,000
6 m full width (≈25–30 m²)
Kitchen, dining, living and utility
£60,000 – £75,000
Related extension
Guide range
On site
Rear extension to the outrigger
Extending a Victorian/Edwardian outrigger, side return kept
£30,000 – £65,000
8–12 wks
Side + rear extension
Side return filled in at the same time
£60,000 – £128,000
14–18 wks
Double-storey rear
Kitchen below, bedroom or bathroom above
£75,000 – £165,000
16–24 wks
Where the money goes
What a design and build price is made of, and which lines are priced separately.
Cost element
What it covers
How it is priced
Survey, design and approvals
Measured survey, architectural drawings, planning or lawful development certificate, building-regulations drawings, structural calculations
Included in the design and build price; council application fees at cost
Structural steelwork
Steels, padstones and the engineer's design where the back or outrigger walls come out
Typically £4,000–£12,000 on a Victorian terrace — in every estimate from day one
Shell
Foundations, drainage, walls, roof, rooflights, doors and windows to weather-tight
The largest share — priced from the survey and the drawings, not per m²
First and second fix
Electrics, plumbing, underfloor heating where specified, plaster, decoration, flooring fitted
Included; the items you choose (tiles, flooring, sanitaryware) are supplied by you or through us
Kitchen fit-out
Units, worktops, appliances
£15,000–£30,000 mid-range, £30,000–£75,000 high specification — priced separately from the build
Party wall
Notices to each adjoining owner; surveyor only if a neighbour dissents
£300 per notice at cost; surveyor from £750 per neighbour
Location
Access, parking, conservation conditions, logistics
Inner London +10–15%, Central London +20–25%, Hertfordshire about 5% less
Added afterwards
—
Nothing: the figure on your itemised estimate is the figure you pay
What the price includes: survey, architectural drawings, planning or lawful development certificate, building-regulations drawings, structural calculations, construction to plaster finish with first-fix electrics and plumbing, LABC sign-off and guarantees. On top: kitchen units, worktops and appliances (typically £15,000–£75,000), flooring and second-fix items you choose. The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Postcode dependent — the instant estimator adjusts the range for your area in thirty seconds, and a detailed itemised estimate follows one survey.
What moves the price: depth and width; the amount of glazing (sliding or bi-fold doors and rooflights are the biggest single variable); a flat roof with a lantern versus a pitched roof; how much of the back wall comes out and the steel needed to hold the house up; drainage and whether a Thames Water build-over agreement is needed; ground conditions and tree roots; and the finish level inside. Inner London (Camden, Islington, Hackney, Wandsworth, Hammersmith and similar) add 10–15%. Central London (Westminster, Kensington & Chelsea, the City) add 20–25%. Hertfordshire is about 5% less. These are estimated ranges — every home is surveyed and costed individually.
What the price includes
Design and build inclusive — one contract from the first sketch to the completion certificate.
Measured survey of the house and garden
Architectural design and drawings, with revisions until you're happy
Planning application or Lawful Development Certificate, prepared and submitted for you
Building regulations drawings and structural calculations by our chartered engineer
Party wall notices served on your behalf (£300 each, at cost)
Foundations, drainage and any build-over agreement with Thames Water
Structure, roof, rooflights, doors and windows to a weather-tight shell
Plaster finish, first-fix electrics and plumbing, underfloor heating where specified
Fitting of the kitchen, flooring and second-fix items you supply
LABC inspections and completion certificate
A dedicated project manager and a weekly progress update
12-month workmanship guarantee and structural guarantee; 10-year LABC Warranty available
Recent rear extension projects
Bohun Grove, East Barnet EN4 — rear extension, hip-to-gable loft and whole-house renovation
LABC 2024 Best Residential Extension in London
Semi-detached family home One programme loft + extension + renovation
A total renovation of a family semi: a rear extension opening into a full-width kitchen and dining space with floor-to-ceiling glazing and skylights, a hip-to-gable loft conversion with a rear dormer, two Lusso Stone bathrooms, enlarged bedrooms, new windows throughout, a regulated outbuilding and landscaping.
Because the loft and extension ran under one contract, the steelwork for the open-plan ground floor and the loft floor was designed together and lifted in on the same day — one crane hire, one scaffold, one building-control file.
See the Bohun Grove project · All projects
ANDERSON · FOUNDER, LCCL CONSTRUCTION
Anderson's advice: budget for the steelwork, not just the kitchen
Clients budget for kitchen units but forget the structural steelwork when opening up ground-floor walls. On a Victorian terrace that is typically £4,000–£12,000 of steel — we price it on every proposal so there is no surprise.
Get the structural design done early, from a measured survey, and the steel arrives on site the week it is needed rather than holding the programme.
“Everyone at LCCL Construction was skilled, trustworthy and worked to the highest standard. I am delighted with our loft conversion, it is exactly what we wanted and I highly recommend the company - they have transformed the quality of my home. Overall it…”
KashImaging · Google Reviews
“We are very pleased to have used Lccl Construction to do full house renovation and loft conversion to our edwardian terraced house. Project involved total strip out of interiors and remodelling repositioning the interior walls, electrical and plumbing…”
neil smith · Trustpilot
“We did a 3x10m single storey rear extension with LCCL recently. It was scheduled to take 12 weeks to complete but with Alex's experience and agile project management, we are ahead of schedule (finished in 10). Alex, and LCCL,is extremely knowledgeable and…”
Galichet Christophe · Google Reviews
Cost questions we get asked most
£35,000–£50,000 in 2026 for a 3 m full-width single-storey extension of about 12–15 m², design and build inclusive. It is usually permitted development on a terrace or semi, so there is no planning application to wait for.
£60,000–£75,000 for a 6 m full-width extension of 25–30 m² — kitchen, dining, living and a utility. On a terrace or semi 6 m goes through prior approval with neighbour notification rather than a full planning application.
Because the survey, design, approvals, steelwork and drainage cost roughly the same at 3 m as at 6 m. Those fixed costs are spread over more floor on a deeper extension.
Kitchen units, worktops and appliances (typically £15,000–£75,000), flooring and second-fix items you choose, bespoke joinery, landscaping, council fees, and party wall surveyor fees if a neighbour dissents (from £750). Survey, design, approvals, structure, build, LABC sign-off and guarantees are in the price.
Yes. Inner London (Camden, Islington, Hackney, Wandsworth, Hammersmith and similar) adds 10–15%, Central London (Westminster, Kensington & Chelsea, the City) adds 20–25% and Hertfordshire is about 5% less — the difference is parking, access, conservation-area conditions, party wall complexity and logistics. The instant estimator applies the right band to your postcode.
No. The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Items you choose, such as kitchen units or sanitaryware above the allowance, are listed separately before you sign.
Two steps. The instant estimator gives you a range for your postcode and project in thirty seconds. Then one measured survey gives you a detailed itemised estimate — design, planning, structure, construction and warranty priced line by line — with no obligation.
A 12-month workmanship guarantee and a structural guarantee on every project, with a 10-year LABC Warranty available. Building control sign-off is through LABC — we are an LABC Registered Partner and LABC Building Excellence Award winners in 2024 and 2026.
Where we build
From Finchley N3 we cover North London, North-West London, Central London, South-West and West London, and Hertfordshire — and we also work across the rest of London, Buckinghamshire, Surrey, Kent and Essex. Recent extensions: East Barnet EN4, Finchley N3, Putney SW15, Mill Hill NW7, Wood Green N22 and Borehamwood WD6.