Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
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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
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Side & rear extensions · Semi-detached and detached houses
Side extension for a semi-detached or detached house: cost, planning and build time
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A side extension builds into the passage or driveway beside a semi-detached or detached house — a very different job from the side return of a terrace. Single storey it gives a utility, a wider kitchen or a downstairs WC; two storey it adds a bedroom and bathroom above; combined with a rear extension it produces the full-width, open-plan ground floor most families are after. This page covers the 2026 numbers, the permitted development limits that apply to the side of a house, the 1 m rule most North London boroughs apply to first-floor side extensions, and how the job runs with one team.
Quick answer
From £42,000 inc. VAT (£35,000 + VAT) for a single-storey side extension; £72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT) for side and rear together; £90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT) for two storeys. Design and build inclusive, postcode dependent.
Single storey to the side is usually permitted development (half the width of the house, 4 m high). Two-storey side extensions always need a planning application.
10–14 weeks on site single storey; 14–18 weeks with a rear extension; 16–24 weeks for two storeys.
A free 30-minute phone call with our team, no obligation.
Guide price
£42,000 – £198,000 inc. VAT (£35,000 – £165,000 + VAT)
Typical build
10–24 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 side extension from LCCL includes
Design and build inclusive — the price covers the whole job, not just the bricks.
Measured survey of the house, the side passage and the garden
Architectural design and drawings, with revisions until you're happy
Lawful Development Certificate or planning application, 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
New side wall, steels, 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 side extension cost in London in 2026?
Guide ranges, design and build inclusive. Every project is priced from your postcode and house type.
Extension
Typical use
Guide range
Single-storey side (≈8–15 m²)
Utility, downstairs WC, kitchen widened into the side passage
£42,000 – £93,600 inc. VAT (£35,000 – £78,000 + VAT)
Side + rear together (≈25–50 m²)
Full-width, open-plan kitchen, dining and living — priced as a wraparound
£72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT)
Two-storey side (≈16–30 m²)
Kitchen or utility below, bedroom and bathroom above — always a planning application
£90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT)
Related extension
Guide range
On site
Rear extension only
Depth across the back, no side element
£42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT)
10–14 wks
Wraparound extension
Side and rear together on a terrace or semi
£72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT)
14–18 wks
Two-storey extension
Ground floor plus bedrooms above, rear or side
£90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT)
16–24 wks
What the price includes: survey, architectural drawings, lawful development certificate or planning application, 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: whether the side wall sits on the boundary (a party wall process and a fair-faced wall) or a metre inside it; a two-storey scheme, which needs a new roof tied into the existing one; moving the drain run that usually sits in the side passage; the length of the steel where the side and rear wings meet; glazing; and the finish inside. A garage conversion beside the house costs less than a new-build side extension because the foundations and walls already exist.
Side extensions by house type
1930s semi-detached
The most common side extension in North London. The side passage (typically 2–3 m) is built over for a utility and a wider kitchen; with a 3–4 m rear extension across the back the ground floor becomes one open-plan space. Single storey is usually permitted development; the attached neighbour gets a party wall notice.
Extensions in Barnet · Sturgess Avenue NW4 wraparound and loft
Two-storey side on a semi
A bedroom and bathroom over the new side wing. Every borough treats this as a planning application, and most North London councils ask for the first floor to be set back about 1 m from the boundary and set down from the main ridge so two semis do not read as a terrace.
Detached house
The most freedom: 4 m of rear permitted development plus a side extension up to half the width of the original house. Where the side is wanted for a garage conversion, a utility or a home office, we design it as a separate PD element and keep the rear within its own limits.
Planning
Do you need planning permission for a side extension?
Single storey, usually not. Two storeys, always. The side of a house has its own permitted development limits — about width and height, not depth.
Route
Limits
Time
What we do
1. Permitted development
Single storey, no wider than half the width of the original house, max 4 m high (3 m eaves within 2 m of a boundary), not forward of the front wall
No application
Lawful Development Certificate, ~8 weeks, included
2. Full planning
Any two-storey side extension; conservation and Article 4 areas; flats; side + rear beyond PD limits
8 weeks
Drawings, design statement, precedent search and application by our team
3. Prior approval
Only for the rear element where it goes beyond 3 m (up to 6 m on a semi, 8 m detached)
42 days
Larger home extension submission with neighbour notification, side element kept within PD
Building regulations & party walls
The other two approvals
Every 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.
A side extension on a semi almost always sits on or against the boundary, so the Party Wall Act applies to the attached neighbour and often to the neighbour on the open side too. Notices go out at least two months before we start; most neighbours consent, and where a surveyor is appointed we manage it.
The side of a house cannot use the larger home extension (prior approval) route — that scheme is for depth, not width. Where a side extension is combined with a rear extension beyond 3 m, the rear part goes through prior approval and the side must still meet the permitted development limits, or the whole scheme goes to full planning. Two-storey side extensions are never permitted development.
Permitted development rights for house extensions · Side extension guide 2026 See our permitted development projects page for every loft and extension you can build without a planning application, with from-prices at economy specification.
Recent side and 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: check the drains before you fall in love with the plan
On most semis the foul drain and the manhole run down the side passage — exactly where the new extension goes. Building over it needs a build-over agreement with Thames Water and sometimes a rerouted run, and that is a cost and a fortnight that should be in the estimate from day one, not discovered when the digger arrives. We lift the manhole cover on the survey visit for that reason.
The second thing: decide whether you want to keep side access to the garden. A 1 m passage costs you a metre of kitchen but saves carrying every bin and bicycle through the house.
How a side extension runs with LCCL
From the first call to the completion certificate. A single-storey side extension usually needs only a Lawful Development Certificate; a two-storey scheme needs a planning application, so allow 12–16 weeks for design and approvals before work starts.
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, Thames Water build-over agreement where the drain runs down the side. Nothing starts on site until it is signed off.
BUILD
On site
10–14 weeks single storey, 14–18 weeks with a rear extension, 16–24 weeks for two storeys. Weekly updates from your project manager, one point of contact throughout.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, guarantees in writing. 10-year LABC structural warranty.
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Side extension questions we get asked most
From £42,000 inc. VAT (£35,000 + VAT) for a single-storey side extension in 2026, design and build inclusive. Side and rear together are priced as a wraparound at £72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT); a two-storey side extension sits in the £90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT) two-storey range. The kitchen and second-fix items you choose are on top.
A side return fills the narrow strip beside the outrigger of a terrace. A side extension builds into the passage or driveway beside a semi-detached or detached house, and can go to two storeys. Same idea, different house, different planning rules.
Single storey, usually not: a side extension no wider than half the original house and no higher than 4 m is permitted development, and we obtain a Lawful Development Certificate. Two-storey side extensions always need a planning application, as do flats and homes in conservation or Article 4 areas.
Most North London boroughs ask for the first floor of a side extension to be set back about 1 m from the side boundary and set down from the main ridge, so that two neighbouring semis do not join up into a terrace. It is council design guidance rather than national law, so we check your borough's policy before the first drawing.
At ground floor, usually yes, subject to the 3 m eaves limit within 2 m of the boundary and the Party Wall Act. Gutters and foundations cannot overhang the neighbour's land, so the wall is normally set a few centimetres in.
On most semis the foul drain runs down the side passage. Building over it needs a build-over agreement with Thames Water and sometimes a rerouted run; we check on the survey and price it into the estimate.
Ten to fourteen weeks on site for a single storey, fourteen to eighteen with a rear extension, sixteen to twenty-four for two storeys, plus 8–16 weeks of design and approvals before that.
Yes, and it costs less than doing them a year apart because the scaffold, the structural design and the party wall process are shared. Sturgess Avenue NW4 is a semi where we did a wraparound and a loft in one programme.
Where we build side 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. The 1930s semis of Barnet, Hendon, Finchley, Mill Hill, Edgware and Borehamwood are where most of our side extensions are built.