House Extension Cost London 2026

House Extension Costs at a Glance — London 2026

The table below gives a realistic range for each extension type across three specification levels. All figures include VAT at 20% and a typical allowance for professional fees. They do not include furniture, landscaping or party wall surveyor costs.

Type	Typical Size	Low Spec	Mid Spec	High Spec
Single-storey rear	20–40m²	£50,000–£70,000	£80,000–£110,000	£120,000–£160,000
Side return	15–25m²	£40,000–£55,000	£65,000–£85,000	£90,000–£130,000
Double-storey rear	40–70m²	£120,000–£160,000	£170,000–£230,000	£240,000–£320,000
Wrap-around	40–80m²	£130,000–£180,000	£190,000–£260,000	£270,000–£380,000

Cost Per Square Metre for House Extensions in London

Understanding cost per m² helps you cross-check any quote you receive. These are London-specific figures based on 2026 material and labour costs, not national averages.

Specification	Cost per m²	What's included
Basic	£2,000–£2,500/m²	Standard finishes, basic kitchen/bathroom if applicable
Mid-range	£2,500–£3,200/m²	Quality fixtures, underfloor heating option, better glazing
High-spec	£3,200–£4,500/m²	Premium materials, structural glazing, smart home, bespoke joinery
Ultra-premium	£4,500–£6,000+/m²	Architect-designed, luxury finishes, premium bespoke kitchens, composite glazing

Single-Storey Rear Extension Cost London

The single-storey rear extension remains the most popular project we undertake across London — and for good reason. It is typically the most cost-effective way to add meaningful living space to a terraced or semi-detached house, and it is the extension type most likely to qualify for permitted development without the time and cost of a full planning application.

For a standard 30m² single-storey rear extension in London, you should budget £75,000–£120,000 including VAT and professional fees at mid-to-high specification. At the lower end of that range, you're looking at a functional kitchen-diner with standard bi-fold doors and concrete flooring with ceramic tiles. At the upper end, you are getting structural glazing, a polished concrete or engineered oak floor, underfloor heating, and a fully fitted kitchen.

Several factors push the cost above our baseline figures. Deep excavation is required if you are extending significantly below existing floor level, which adds £5,000–£15,000 depending on ground conditions. Larger openings into the existing house — particularly removing a full rear wall — require a steel frame and temporary propping, typically adding £8,000–£20,000 depending on span. Conservation area properties and those with complex rooflines (such as mansard or hip roofs) also carry a premium.

On the lower end, costs can be reduced by keeping the kitchen simple, choosing off-the-shelf rather than bespoke joinery, opting for standard bifold doors rather than structural glazing, and selecting a lean-to roof rather than a flat roof with lantern. Even at the entry level, a quality London contractor will not drop below around £2,000/m² for a finished extension — anything significantly below that should prompt careful scrutiny of the specification and contractor credentials.

One important note: quoted costs vary enormously in London depending on the postcode. A contractor based in Kensington or Hampstead will typically price 15–25% higher than one operating in outer East London, reflecting both local wage expectations and the logistical costs of working in dense, traffic-restricted streets.

See our completed extension projects including Whitehall Park, N19 and Evelyn Road, Cockfosters EN4 for a sense of the specification levels we deliver.

Side Return Extension Cost London

Side return extensions are particular to London's Victorian terrace stock — that narrow strip of land running alongside the ground floor between the main house and the rear boundary wall. In most Victorian terraced houses in areas like Chiswick, Islington, Clapham and Hackney, this space is typically 1.5–2.5 metres wide and runs 5–8 metres to the rear. Extending into this space, combined with a rear extension, creates the fully open-plan kitchen-diner that has become the defining interior renovation of London residential property over the past two decades.

A pure side return infill — filling in just the alley without adding rear projection — typically costs £40,000–£65,000 for a 15–20m² addition at mid-spec. Combined with a rear extension, the combined project (a wrap-around at smaller scale) typically costs £90,000–£160,000 for a 30–40m² total gain in floor area.

The structural complexity of side return extensions should not be underestimated. The party wall between your home and the neighbouring property runs directly along this strip, meaning party wall notices will almost always be required. The existing rear ground floor typically requires demolition of a section of the external wall, a new steel frame, and careful integration with the existing structure. The roof is typically a flat roof with internal drainage or a glazed link, and the junction with the original house wall is a common point of water ingress if not properly detailed.

Rooflights are almost always essential in a side return extension — the narrow width means natural light from the side is limited, and a skylight above the new space is one of the most transformative design decisions you can make. Budget £2,500–£8,000 for a quality fixed or opening rooflight depending on size and specification.

Planning permission is not always needed for a side return, but the rules are less straightforward than for a rear extension. Side extensions 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. Many London boroughs have also removed side PD rights under Article 4 directions — check with your local planning authority or ask us to advise before assuming no planning is needed.

We recently completed a side return and rear extension at Evelyn Road, Cockfosters EN4 that demonstrates what can be achieved on a realistic budget — worth a look if you're in the early stages of planning your project.

Double-Storey Extension Cost London

A double-storey extension adds floor space on two levels simultaneously — typically a kitchen-diner or reception room at ground floor, and one or two bedrooms and a bathroom at first floor. This makes it the most efficient type of extension in terms of value per pound spent, since foundations, roof, and many structural elements are shared across both levels.

In London, a double-storey rear extension of 40–60m² total floor area (20–30m² per floor) typically costs £150,000–£280,000 at mid-to-high specification, or approximately £2,800–£3,800/m² of added floor space. The cost per m² is generally lower than a single-storey extension of equivalent total area, because the structural cost per m² of gained space is shared across both floors.

Unlike single-storey extensions, double-storey rear extensions almost always require a full householder planning application — they exceed the height limits for permitted development. This adds 8–12 weeks to the programme before work can start on site, plus a planning fee of £258 and architect fees for the planning drawings. In conservation areas or on listed buildings, additional constraints and specialist consultants may also be required.

The structural complexity is higher than a single-storey build. First floor structure, staircase integration, additional loadbearing elements and careful coordination with the existing roof all add to the programme and cost. On the other hand, the additional bedroom or bathroom added at first floor level can transform a family home and add significant value — in prime London postcodes, a well-designed double-storey extension can add £100,000–£200,000 to a property's value.

We have delivered double-storey extensions across London including in Wimbledon, Richmond and Cockfosters. If you are at the early assessment stage, we are happy to advise on feasibility and provide a preliminary budget before committing to design fees.

Wrap-Around Extension Cost London

A wrap-around extension combines a rear extension with a side return infill, wrapping around the back corner of the house to maximise ground floor area in a single project. It is the most ambitious — and expensive — single-storey extension type, but it produces the most dramatic spatial transformation: a fully open-plan ground floor with a total new footprint typically ranging from 40 to 80m².

In London, a wrap-around extension typically costs £130,000–£380,000 depending on size and specification. At mid-spec — bifolds or sliding doors, a good but not bespoke kitchen, solid flooring, underfloor heating, and a quality flat roof with rooflight — expect to pay £180,000–£260,000 for a typical 50–60m² addition. At high specification with structural glazing, custom joinery and premium finishes, £300,000+ is realistic for a large scheme in inner London.

The planning position for wrap-arounds is mixed. The rear element may qualify as permitted development, while the side element may require a planning application — or the combined project may trigger a full application as an aggregate. We always advise clients to take a pre-application view from the local planning authority before committing to a wrap-around design, particularly in boroughs with active Article 4 directions on residential side extensions.

Party wall considerations are also significant: a wrap-around typically involves works adjacent to one or both neighbours, meaning party wall notices are almost certainly required. Budget £800–£3,500 per neighbour for a party wall surveyor.

What Does the Cost Per Square Metre Include?

When a London contractor quotes a cost per m², it should — if properly specified — encompass all of the following work stages:

• Demolition and clearance — removing existing structures, disposing of waste to a licensed facility
• Excavation and foundations — strip or raft foundations depending on ground conditions and structural engineer's specification
• Structural frame — steelwork, concrete padstones, temporary propping of existing structure during opening-up works
• External walls — brick or block cavity wall construction to match or complement the existing house
• Roof structure — flat roof build-up, warm roof insulation, waterproof membrane; or pitched roof timbers, felt, battens and tiles
• Windows and external doors — bi-fold doors, sliding doors, or fixed glazing depending on design
• Insulation — to current Building Regulations Part L standards, floor, walls and roof
• Mechanical and electrical first fix — conduit, socket positions, lighting provisions, underfloor heating pipework if specified
• Plastering — skim to all internal walls and ceilings
• Second fix M&E — sockets, switches, lighting fittings, UFH manifold connection
• Flooring — screed and floor finish as specified
• Painting and decoration — mist coat and two full coats throughout
• External works — making good around the extension perimeter, drainage connections

What is typically not included: fitted kitchen, bathroom suites and tiling (these are usually quoted separately or as a variation), furniture, blinds and curtains, and landscaping.

Additional Costs to Budget For

Beyond the main build contract, there are several professional and regulatory costs that every extension project in London must account for. Omitting these from your initial budget is one of the most common causes of project overspend.

Item	Typical London Cost	Notes
Architect / design fees	£3,000–£8,000	5–8% of build cost for full design and planning service
Structural engineer	£800–£2,000	Required for all extensions; provides calculations for steelwork and foundations
Planning application fee	£258	Householder application fee in England (correct as of 2026)
Building regulations	£1,000–£3,000	Includes plan check and site inspection fees; charged by local authority or approved inspector
Party wall surveyor	£800–£3,500 per neighbour	Required where works affect a shared boundary or party wall; each neighbour may appoint their own surveyor
VAT	20% on all costs	Extensions are standard-rated for VAT; new builds are zero-rated but this does not apply to extensions
Temporary accommodation	Variable	Not always needed; most clients remain in their home throughout, though kitchen use may be disrupted for several weeks

What Factors Affect the Cost of a House Extension in London?

No two extension projects are identical, and a wide range of variables influence the final cost. Understanding these allows you to have an informed conversation with any builder and to interpret quotes accurately.

• Size and footprint — the single biggest cost driver; more m² means more of everything
• Specification and finish quality — the difference between a £2,200/m² and a £4,500/m² extension is almost entirely specification
• Site access — tight rear-access terraces in inner London add time and cost to deliveries and waste removal; scaffolding requirements increase in dense urban settings
• Ground conditions — London Clay is generally buildable, but poor drainage, high water table, or tree proximity (common in leafy boroughs) can require deeper foundations or specialist solutions
• Conservation area status — restricts materials and design, requiring planners' input and sometimes specialist consultants such as heritage officers or ecologists
• Listed building consent — if your home is listed, all works require listed building consent in addition to planning permission, and specialist contractors may be required for stonework, lime mortar or period details
• Structural complexity — the larger the opening between the new extension and the existing house, the heavier the structural frame required; multi-bay openings need significant steelwork
• Glazing specification — standard bi-fold doors sit at one end of the cost spectrum; structural frameless glazing at the other, with a factor of 5–8x difference in cost per metre run
• Kitchen and bathroom quality — the fitting out of new kitchen-diners and bathrooms within extensions varies enormously; a German kitchen with appliances ranges from £8,000 to £50,000+
• Programme phasing — extending an occupied home takes longer and costs slightly more than working in a vacant property

How Alstruct Prices House Extensions

At Alstruct, we do not provide ballpark figures and then adjust the price upward once work is underway. Every project begins with a detailed pre-contract process: we meet on site, review your drawings or commission concept sketches, obtain structural engineer input on the key elements, and produce a fully itemised quotation broken down by trade. You know exactly what you are getting and what each element costs before you sign anything.

All our contracts are based on the JCT Minor Works Building Contract — the industry-standard form of contract for residential construction in the UK. This means you have the legal protections of a recognised contract: a fixed price for the agreed scope, a defined programme, clear variation procedures, and dispute resolution provisions. We do not work on informal arrangements or verbal agreements, because in our experience this is the single biggest predictor of project disputes.

Where the scope changes — as it sometimes does when structural works reveal unexpected conditions, or when clients decide mid-project to upgrade a kitchen or add an additional rooflight — we issue a written variation instruction with the additional cost confirmed before any extra work is done. There are no surprises on your final account. If you are comparing quotes for your London extension, we are always willing to review a competitor's quote and explain any differences in scope or specification before you make a decision.

To get a detailed, no-obligation quotation for your extension, get in touch with our team.

Real Alstruct House Extension Projects

The best way to understand what a budget translates to in practice is to see finished work. Below are three recently completed London extension projects across different scales and specifications.

Frequently Asked Questions

Prices shown are indicative ranges based on London market data for 2026. Actual costs depend on site conditions, specification and programme. All figures include VAT at 20% unless otherwise stated. Contact Alstruct for a detailed quotation based on your specific project.