House Extensions London
Design and build high-quality house extensions that add space, value and functionality to your London home.
House extension builders in London
Alstruct designs and builds house extensions across London: single-storey rear, side return, wrap-around and double-storey, whether you are opening up a kitchen-diner or adding bedrooms above. We work for homeowners who want a mid to high specification build handled by one contractor, from the first drawings to building control sign-off.
Every extension is priced as a fixed-price, itemised written quote after a site visit, built under a JCT contract and covered by our 10-year workmanship warranty. We are CHAS Elite, Constructionline Gold and Federation of Master Builders (FMB) accredited, with in-house architectural support for design, planning and building regulations.
At a glance: at mid to high specification, a single-storey rear extension in London typically costs £80,000–£160,000, a side return £65,000–£130,000, a double-storey £170,000–£320,000 and a wrap-around £190,000–£380,000, including VAT. Allow 12–16 weeks on site for a single-storey extension and 18–24 weeks for a double-storey. Most rear extensions under 3m (terraced or semi-detached) or 4m (detached) are permitted development; double-storey and wrap-around schemes almost always need planning permission. Full breakdown: house extension cost guide.
Types of house extension we build
The right type depends on the house, the plot and what you need the space to do. The prices below are the mid and high specification ranges from our cost guides, including VAT, and the durations are time on site.
Single-storey extensions
Single-storey rear extension
Extends the back of the house into the garden, typically by 20–40m², for a kitchen-diner or family room. It suits most terraced and semi-detached houses and is the type most likely to qualify as permitted development. £80,000–£160,000; typically 12–16 weeks on site.
Side return extension
Fills the strip beside the rear of a Victorian or Edwardian terrace or semi, typically 1.5–2.5 metres wide, to widen a narrow kitchen. Rooflights are almost always essential and party wall notices almost always required. £65,000–£130,000 for 15–25m²; typically 12–16 weeks on site.
Wrap-around extension
Combines a rear extension with a side return infill to open up the whole ground floor, adding 40–80m². It suits period terraces and semis that have a side return. £190,000–£380,000. The side element may need planning permission, so we advise a pre-application view from the council before the design is fixed.
Double-storey and kitchen extensions
Double-storey extension
Adds space on two levels: typically a kitchen-diner or reception room below and one or two bedrooms and a bathroom above, sharing one set of foundations and one roof. We have built them on Victorian and 1930s semi-detached houses. £170,000–£320,000 for 40–70m²; typically 18–24 weeks on site.
Kitchen extension
Most kitchen extensions are one of the types above; on a Victorian terrace, a side return is usually the best value. The build price delivers a finished shell ready for the kitchen install. The kitchen itself (units, worktops and appliances) is a separate budget, typically £15,000–£60,000+, and we coordinate its delivery and installation within the build programme.
Planning a kitchen? Our kitchen extensions page covers layouts, glazing, costs and how a kitchen project runs.
What a house extension costs in London
The table shows the mid and high specification ranges from our 2026 cost guide, which also gives a basic specification range. Use it to set a budget; your quote will be a fixed price for your house and your drawings.
| Type | Typical size | Mid specification | High specification |
|---|---|---|---|
| Single-storey rear | 20–40m² | £80,000–£110,000 | £120,000–£160,000 |
| Side return | 15–25m² | £65,000–£85,000 | £90,000–£130,000 |
| Double-storey rear | 40–70m² | £170,000–£230,000 | £240,000–£320,000 |
| Wrap-around | 40–80m² | £190,000–£260,000 | £270,000–£380,000 |
Indicative ranges for London in 2026. Figures include VAT at 20% and a typical allowance for professional fees. They do not include furniture, landscaping or party wall surveyor costs.
After size, specification moves the price most. Our guide puts mid-range work at £2,500–£3,200/m² and high specification at £3,200–£4,500/m², and the gap is mostly glazing, kitchen and finishes. Structural frameless glazing can cost 5–8 times as much per metre run as standard bi-fold doors, and a quality rooflight is £2,500–£8,000.
Structure and site come next. Removing a full rear wall needs a steel frame and temporary propping, typically adding £8,000–£20,000 depending on span, and deep excavation adds £5,000–£15,000 depending on ground conditions. Tight inner-London access, conservation area requirements, listed building consent and building while you live in the house all add time and cost.
The house extension cost guide sets out every line, including professional fees. There are separate guides to Victorian side return extension costs and kitchen extension costs.
A fixed price, not a range
Get a fixed-price quote for your extension
We visit, measure and send an itemised quote on a JCT contract, with our 10-year workmanship warranty on the finished work.
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How a project runs with Alstruct
One team handles design, approvals and construction, so you have one programme and one point of responsibility. From first meeting to handover, a single-storey extension typically takes 6–10 months and a double-storey 8–14 months. Design, planning and building regulations typically account for 3–6 months of that before work starts on site.
- Site visit and brief. We meet you at the house, assess the structural considerations, discuss your planning position and review your brief and budget. If you already have drawings or photos, send them with your enquiry: the contact form takes PDF, JPG, PNG, HEIC and DWG files.
- Design and drawings. Our in-house architectural team carries out the measured survey and prepares the design and planning drawings, typically over 4–8 weeks for a single-storey extension and 6–10 weeks for a double-storey. If you already have an architect, we work from their drawings.
- Planning, permitted development and party wall. Where planning permission is needed it adds 8–12 weeks; the typical London householder decision in the year to August 2026 took 8.0 weeks from validation (every borough compared). Building regulations drawings run alongside on the Full Plans route, and we serve party wall notices at the same time so they do not hold up the start.
- Fixed-price quote and JCT contract. With the drawings and structural engineer's input in hand, we issue a fully itemised quotation broken down by trade. The contract is the JCT Minor Works Building Contract: a fixed price for the agreed scope, a defined programme and written variation instructions, with any extra cost confirmed before the work is done.
- Pre-construction. Typically 2–4 weeks. Final material selections are confirmed, long-lead items such as structural steelwork, windows and roof lanterns are ordered, and the construction programme is issued to you.
- Build. A single-storey rear extension runs in sequence: demolition and groundworks (weeks 1–3), structural frame and walls (weeks 3–7), roof and weathertight (weeks 6–9), first fix (weeks 8–11), plastering and second fix (weeks 10–14), then finishing and decoration (weeks 13–16). The break-through into the existing house is the last major structural operation and typically takes 1–3 days. We have delivered single-storey rear extensions within 14 weeks on site and complex double-storey and wrap-around projects within 22 weeks.
- Building control sign-off. We manage the inspections at each stage. After snagging, Building Control carries out the final inspection and issues the Completion Certificate, which we obtain for you.
- Handover and warranty. Our 10-year workmanship warranty starts at practical completion, the date the works are handed over, and passes to the new owner if you sell. The full wording is in our terms and warranty.
Planning, building regulations and neighbours
Most rear extensions under 3m on a terraced or semi-detached house, or 4m on a detached house, qualify as permitted development and do not need a planning application. Larger extensions need a householder application. Where a project is permitted development, we recommend confirming it with a Lawful Development Certificate before work starts.
Side extensions are more restricted: they are not permitted development if they would exceed half the width of the original house, are more than one storey, or are in a conservation area. Double-storey rear extensions almost always need a full householder application. A wrap-around is assessed as one extension that must meet both the rear and the side limits, so most need a householder planning application.
Many London boroughs have removed permitted development rights with Article 4 directions; our side return guide lists Islington, Camden, Hackney, Lambeth, Haringey and Southwark among those with widespread coverage. In a conservation area every side extension needs planning permission, and materials and design are restricted. A listed house needs listed building consent as well as planning permission. We advise on the position for your address before design starts.
A side return extension almost always triggers the Party Wall etc. Act 1996; a rear extension usually does only where it sits on the boundary or its foundations are within 3m of a neighbour's structure. Notices are served one or two months before the start, depending on the type, and the neighbour has 14 days to respond. Surveyor costs in London are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour where each side appoints its own, paid by the building owner. We prepare and serve the notices; the party wall agreement cost guide has the detail.
Building regulations are separate from planning and apply to every extension: structure, fire safety, insulation, drainage and ventilation. We use the Full Plans route as standard, so the technical drawings are approved before construction starts.
What is included
One contract covers the work from survey to sign-off. Fitted kitchens, bathroom suites and tiling are usually quoted separately or as a variation, so the specification stays your choice.
- Free site visit and consultation
- In-house architectural drawings and 3D visualisations
- Planning permission applications (where required)
- Building regulations approval
- Structural engineering calculations
- Party wall notices prepared and served
- Full groundworks and foundations
- Structural steelwork and masonry
- Roofing and waterproofing
- Electrics, plumbing and heating
- Plastering, decoration and finishing
- Kitchen and bathroom installation (if applicable)
- Full project management throughout
- 10-year workmanship warranty
Recent extension projects
Three published case studies show the range, from a Victorian terrace in Archway to a 1930s semi in Golders Green; more are in our projects portfolio.
Areas we cover
We build house extensions across London. Side return extensions work best on the Victorian terraces and semis of Chiswick, Clapham, Islington, Hackney and Ealing, and we have delivered double-storey extensions in Wimbledon, Richmond and Cockfosters. In East London we cover South Woodford and Wanstead, among others. Each area page has local planning notes; see all areas.
Frequently Asked Questions
How much does a house extension cost in London in 2026?
At mid to high specification, a single-storey rear extension in London typically costs £80,000–£160,000, a side return £65,000–£130,000, a double-storey rear extension £170,000–£320,000 and a wrap-around £190,000–£380,000. The ranges include VAT at 20% and a typical allowance for professional fees. The kitchen itself is a separate budget, typically £15,000–£60,000+. After a site visit we give you a fixed-price, itemised quote rather than a range.
Do I need planning permission for a house extension in London?
Most rear extensions under 3m on a terraced or semi-detached house, or 4m on a detached house, qualify as permitted development and do not need planning permission. Larger extensions need a householder planning application, and double-storey and wrap-around extensions almost always do. Conservation areas and Article 4 directions can remove permitted development rights, so we advise on the planning position for your address before design starts.
How long does a house extension take to build?
A single-storey rear extension typically takes 12–16 weeks on site and a double-storey extension 18–24 weeks. Before that, allow 4–8 weeks for design and drawings and, where planning permission is needed, a further 8–12 weeks (see how long planning permission takes in London). From first meeting to handover, a single-storey project typically runs 6–10 months and a double-storey 8–14 months. Projects that qualify as permitted development reach site sooner.
Can we live in the house while the extension is built?
In most cases, yes. The works are largely external until the wall between the house and the new extension is opened up near the end of the build. Expect 1–2 weeks of higher disruption during the structural openings, when a temporary hoarding protects the interior until the new structure is weathertight. Kitchen use may be disrupted for several weeks, and we maintain access to your home throughout.
Do you handle the drawings, planning application and building regulations?
Yes. Our in-house architectural team can take a project from initial design through planning permission where required, building regulations approval and structural engineering to construction, so you deal with one company throughout. We use the Full Plans route for building regulations as standard. If you already have an architect, we are happy to work directly with them and price from their drawings.
Will I need a party wall agreement for my extension?
A side return extension almost always triggers the Party Wall etc. Act 1996. A rear extension usually does only if it is built on the boundary or its foundations are within 3m of a neighbour's structure. Surveyor costs in London are typically £800–£1,500 with a single agreed surveyor, or £800–£3,500 per neighbour if each side appoints its own, and the building owner pays. We prepare and serve the notices for you.
Will a house extension add value to my London home?
Usually, although it depends on the borough and the quality of the finish. Our side return cost guide puts the typical uplift from a well-designed side return extension in London at £80,000–£150,000, and our extension cost guide notes that a well-designed double-storey extension can add £100,000–£200,000 in prime London postcodes. In inner-London boroughs, open-plan kitchen-diners are the single most sought-after feature among family buyers.
What contract and warranty do you work under?
Every extension is built under a JCT Minor Works Building Contract: a fixed price for the agreed scope, a defined programme and written variation instructions, with any extra cost confirmed before the work is done. Our 10-year workmanship warranty covers defects in our workmanship, starts at practical completion and passes to the new owner if you sell. We are CHAS Elite, Constructionline Gold and Federation of Master Builders accredited.