Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Call 020 8037 7332 or get an instant estimate online.
Rear extensions · London & Hertfordshire
Rear extension London: cost by size, planning rules and build time — from £42,000 inc. VAT (£35,000 + VAT)
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A rear extension is the most direct way to get the open-plan kitchen and dining space a London house was never built with. This page gives you the 2026 numbers by size, the three planning routes, what changes on a terrace, semi or detached house, and how the job runs when one team designs, approves, builds and signs it off.
Quick answer
£35,000–£75,000 for a single-storey rear extension, design and build inclusive (kitchen on top). Postcode dependent.
Usually none: permitted development to 3 m (4 m detached); 6 m / 8 m by prior approval. Lawful Development Certificate included.
10–14 weeks on site; 8–16 weeks of design and approvals before that.
A free 30-minute phone call with our team, no obligation.
Guide price
£42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT)
Typical build
10–14 weeks on site
Guarantee
12-month workmanship guarantee; 10-year LABC structural warranty
Awards
LABC 2024 & 2026 winners
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4: rear extension, hip-to-gable loft conversion and whole-house renovation
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road, Hampstead NW6: penthouse loft conversion and full renovation
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee; 10-year LABC structural warranty
What a rear extension from LCCL includes
Design and build inclusive — the price covers the whole job, not just the bricks.
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 own 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; 10-year LABC structural warranty
How much does a rear extension cost in London in 2026?
Guide ranges, 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
£42,000 – £60,000 inc. VAT (£35,000 – £50,000 + VAT)
4–5 m full width (≈18–25 m²)
Open-plan kitchen, dining and seating
£60,000 – £78,000 inc. VAT (£50,000 – £65,000 + VAT)
6 m full width (≈25–30 m²)
Kitchen, dining, living and utility
£72,000 – £90,000 inc. VAT (£60,000 – £75,000 + VAT)
Related extension
Guide range
On site
Rear extension to the outrigger
Extending a Victorian/Edwardian outrigger, side return kept
£42,000 – £78,000 inc. VAT (£35,000 – £65,000 + VAT)
8–12 wks
Side + rear extension
Side return filled in at the same time
£72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT)
14–18 wks
Double-storey rear
Kitchen below, bedroom or bathroom above
£90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT)
16–24 wks
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: the kitchen itself, supplied and fitted (£9,000 – £87,360 inc. VAT (£7,500 – £72,800 + VAT) by size and specification; appliances extra), 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.
Rear extensions by house type
Victorian or Edwardian terrace
Most London rear extensions. The question is whether to extend the outrigger, fill the side return, or do both as a wraparound. Permitted development allows 3 m beyond the original rear wall (6 m by prior approval); the outrigger usually counts as the original wall, so measure from there. Party wall notices to both neighbours are almost always needed.
1930s semi-detached
Typically a full-width 3–4 m extension across the back with a flat or shallow-pitch roof and a lantern. Permitted development to 3 m, or 6 m by prior approval; Barnet's 2026 guidance accepts 3.5 m on a semi under a planning application. One party wall notice, to the attached neighbour, plus notice to the other side if you dig within 3 m of their foundations.
Detached house
Permitted development allows 4 m, or 8 m by prior approval, so most detached rear extensions never need a planning application. The limits that bite are height (4 m to the ridge, 3 m at the eaves within 2 m of a boundary) and the half-garden rule. Larger glazing spans mean heavier steel — budget for it early.
Planning
Do you need planning permission for a rear extension?
Usually not. There are three routes, and which one you need depends almost entirely on depth.
Route
Depth
Time
What we do
1. Permitted development
To 3 m (terrace/semi) or 4 m (detached)
No application
Lawful Development Certificate, ~8 weeks, included
2. Prior approval (larger home extension)
To 6 m (terrace/semi) or 8 m (detached)
42 days
Council notifies neighbours; we prepare the submission
3. Full planning
Beyond those limits, flats, conservation and Article 4 areas
8 weeks
Drawings, design statement and application by our team
Building regulations & party walls
The other two approvals
Every rear extension needs building-regulations approval whether or not it needs planning: foundations, structure, insulation, glazing area, drainage and fire. We do this through LABC with full-plans drawings and our own structural calculations, so the inspector is checking a design he has already approved, not improvising on site.
If the extension is on or near the boundary of a semi or terrace, the Party Wall Act applies. Notices go out at least two months before we start; most neighbours consent, and where a surveyor is appointed we manage it.
The exceptions are flats and maisonettes, conservation areas, Article 4 areas and houses where permitted development rights were removed by an earlier consent. We check your address before the first drawing, and we obtain a Lawful Development Certificate as part of the design stage — it is the only document that proves the extension was lawful when you come to sell.
Planning permission for house extensions — the full guide See our permitted development projects page for every loft and extension you can build without a planning application, with from-prices at economy specification. Not sure whether your project needs a planning application? Planning check
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
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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 £4,800 – £19,200 inc. VAT (£4,000 – £16,000 + VAT) for the load-bearing opening, depending on its width — we price it on every proposal so there is no surprise.
The second thing I say on every rear-extension call: put the rooflights over the island. Rooflights over kitchen islands transform natural light more than any other single design decision, so we position them on the plan before the kitchen layout is fixed, not after.
How a rear extension runs with LCCL
From the first call to the completion certificate. Design, planning and building regulations typically take 8–16 weeks before work starts; party wall agreements on a semi or terrace can add up to three months if left late.
WEEK 0
Free consultation call
A 30-minute phone call with our team about the house, what you want from the space and a realistic budget. Your instant estimate is the starting point.
WEEKS 1–4
Survey & design
Measured survey, architectural drawings and your detailed itemised estimate — design, planning, structure, construction and warranty priced line by line.
WEEKS 4–12
Approvals
Lawful Development Certificate or planning application, building-regulations drawings, structural calculations, party wall notices. Nothing starts on site until it is signed off.
WEEKS 12–26
Build
10–14 weeks on site for a single-storey rear extension: groundworks, structure, weather-tight shell, first fix, plaster, second fix. Weekly updates from your project manager.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, guarantees in writing. 10-year LABC structural warranty.
“We engaged with LCCL Construction to get our loft conversion and house extension project going. The whole process was explained to us in a straight forward way including how the design stages work, timescales, what to expect during the build and the costs…”
Daniel Freedman · Trustpilot
“I highly recommend LCCL Construction. They completed our loft conversion to a very high standard, and the quality of workmanship throughout was excellent. The team was professional, reliable, and completed everything within a very reasonable timeframe.…”
Alina A · Google Reviews
“I couldn't be happier with my loft conversion and extension done by LCCL Building Construction. Real craftsmanship, professional management of the entire build from start to finish, seamless integration of new space within the existing one. The work was…”
Plamen Petrov · Trustpilot
Rear extension questions we get asked most
£42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT) for a single-storey rear extension in 2026, design and build inclusive — survey, drawings, planning or lawful development certificate, structural calculations, construction to plaster finish and building-control sign-off. A 3 m extension sits at the bottom of that range and a 6 m full-width one at the top; the kitchen itself is on top. The instant estimator gives you a figure for your postcode in thirty seconds.
On most houses, yes. A single-storey rear extension up to 3 m deep (4 m on a detached house), no higher than 4 m and covering no more than half the garden is permitted development. You still need building-regulations approval, and we obtain a Lawful Development Certificate so the extension is documented as lawful when you sell. Flats, conservation areas and Article 4 areas are the exceptions.
3 m under permitted development, 6 m under prior approval (the larger home extension scheme), and more with full planning if the design respects the neighbours' light and outlook. Barnet's 2026 guidance, for example, accepts 3.5 m on a semi under a planning application. We check your borough's rules before the first drawing.
Ten to fourteen weeks on site for a single-storey rear extension, plus 8–16 weeks before that for design and approvals. If a full planning application rather than a lawful development certificate is needed, allow eight weeks for the council's decision. Side-and-rear and double-storey extensions run 14–24 weeks on site.
Most clients do. The extension is built as a shell before the back wall of the house comes out, so you keep a working kitchen for most of the programme. We agree the date the old kitchen comes out with you in advance and set up a temporary kitchen where the house allows.
On a semi-detached or terraced house, usually yes — a rear extension almost always involves excavating within 3 m of the neighbour's foundations or building on the line of junction. We serve the notices for you at cost (£300 each), and most neighbours consent without appointing a surveyor.
On a terrace, a side return gives you width in the existing kitchen for the least money and usually no planning application; a rear extension gives you depth and a proper dining space. If the budget stretches, a wraparound does both under one contract and one scaffold. Our free consultation call is the quickest way to settle it for your house.
An extra reception or a bigger kitchen-diner is the improvement most London buyers pay for. Our house-extension value guide sets out what the data shows by area. We are not valuers, so treat any figure as indicative and talk to a local agent before you commit.
Where we build rear extensions
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 rear extensions: East Barnet EN4, Mill Hill NW7 and Hampstead NW6.