House Extension Cost London 2026: Complete Price Guide
Updated: 15 August 2026 | By QuoteMyRenovation
House extension cost in London in 2026 range from £60,000 for a small single-storey rear extension to £380,000+ for a large wrap-around at premium specification. London extension costs run 20-40% above the UK national average, driven by higher labour rates, constrained sites, planning complexity, and the complications of period London properties. This guide covers every extension type, real per-m² figures, architect fees, hidden costs, VAT, what an extension adds to your property value, planning permission rules, and how to compare quotes without overpaying.
SINGLE STOREY REAR
£60k – £160k
20–40m², incl. VAT & fees
DOUBLE STOREY
£120k – £320k
40–70m², incl. VAT & fees
WRAP-AROUND
£130k – £380k
40–80m², incl. VAT & fees
How much does a house extension cost in London in 2026?
House extension cost in London in 2026 averages £3,200–£4,600 per m² for construction alone, before professional fees, kitchen or bathroom fit-out, and VAT. For a complete all-in budget for construction, architect, structural engineer, party wall, building regulations, VAT, and a 10% contingency, add 35-45% on top of the raw construction cost.
| Extension Type | Size | All-In Cost (inc. VAT & fees) |
|---|---|---|
| Single-storey rear | 20–40m² | £60,000 – £160,000 |
| Side return infill | 15–25m² | £40,000 – £130,000 |
| Double-storey rear | 40–70m² | £120,000 – £320,000 |
| Wrap-around (rear + side return) | 40–80m² | £130,000 – £380,000 |
| Basement extension | 20–50m² | £150,000 – £400,000+ |
LONDON VS UK NATIONAL AVERAGE Outside London, a mid-range single-storey rear extension (30m²) typically costs £50,000–£80,000 all-in. The same project in London costs £85,000–£130,000, a 20-40% premium attributable to higher labour rates, constrained site access, planning complexity, and London Clay ground conditions.
How much does a house extension cost per m² in London in 2026?
House extension cost per m² in London in 2026 ranges from £2,800/m² for a basic single-storey build to £5,500+/m² for a premium architect-designed extension. These figures cover construction only; they do not include professional fees (add 15–20%), kitchen or bathroom fit-out, or VAT (add 20%).
| Specification | Construction Cost per m² | Typical Features |
|---|---|---|
| Basic | £2,800 – £3,200/m² | Standard bi-fold doors, concrete screed floor, simple lean-to roof |
| Mid-Range | £3,200 – £4,200/m² | Quality glazing, engineered timber or porcelain floor, flat roof with rooflights |
| High-End | £4,200 – £5,500+/m² | Structural glazing, underfloor heating, bespoke joinery, polished concrete |
How much does a single-storey rear extension cost in London?
A single-storey rear extension is the most common extension type in London, and typically the most cost-effective per m² of additional space gained. For a standard 30m² project at mid-to-high specification, budget £85,000-£130,000 all-in, inclusive of VAT and professional fees.
| Specification | 20m² | 30m² | 40m² |
|---|---|---|---|
| Basic | £55,000 – £70,000 | £75,000 – £95,000 | £95,000 – £125,000 |
| Mid-Range | £70,000 – £90,000 | £95,000 – £120,000 | £120,000 – £155,000 |
| High-End | £90,000 – £120,000 | £120,000 – £160,000 | £155,000 – £210,000 |
What drives cost at the upper end: removing the full rear wall (requiring a steel frame, £8,000–£20,000), structural glazing or large sliding doors, deep excavation below the existing floor level (£5,000–£15,000 extra), underfloor heating, and a fully fitted kitchen as part of the extension.
A 30m² single-storey rear extension in London typically takes 12–16 weeks on site and qualifies for permitted development in most cases; no planning application is needed (see planning permission section below).
How much does a side return extension cost in London?
A side return extension is unique to London’s Victorian and Edwardian terrace housing stock, filling the narrow strip of land running alongside the ground floor between the main house and the rear boundary wall. At a pure infill of 15-20 m², costs run £40,000–£65,000. Combined with a rear extension as a wrap-around, the total rises to £90,000–£160,000 for 30-40m² of new floor area.
| Project Type | Size | All-In Cost |
|---|---|---|
| Pure side return infill (mid-spec) | 15–20m² | £40,000 – £65,000 |
| Side return + rear extension combined | 30–40m² | £90,000 – £160,000 |
| Large side return + rear (premium spec) | 40–55m² | £160,000 – £260,000 |
Side returns almost always involve a party wall, except party wall notices to at least one neighbour, adding £800–£3,500 per surveyor. Rooflights are essential to bring natural light into what would otherwise be a dark internal space: budget £2,500–£8,000 for quality fixed or opening rooflights.
How much does a double-storey extension cost in London?
A double-storey extension adds floor space on two levels simultaneously typically a kitchen-diner on the ground floor and one or two bedrooms with a bathroom above. In London, a double-storey rear extension of 40–60m² total floor area costs £120,000–£280,000 at mid-to-high specification. The cost per m² is generally lower than a single-storey extension of equivalent floor area because foundations, roof, and structural elements are shared across both levels.
| Specification | 40m² (20m² per floor) | 60m² (30m² per floor) |
|---|---|---|
| Basic | £110,000 – £140,000 | £155,000 – £200,000 |
| Mid-Range | £140,000 – £190,000 | £200,000 – £265,000 |
| High-End | £190,000 – £250,000 | £265,000 – £320,000 |
Double-storey extensions almost always require a full planning application; they typically exceed permitted development height limits. Add 8–12 weeks to the project timeline and £548 in planning fees, plus architect time to prepare the planning submission.
How much does a wrap-around extension cost in London?
A wrap-around extension combines a full-width rear extension with a side return infill to create the largest possible ground-floor footprint. It is the most expensive single-storey extension type but creates transformative open-plan living space. In London, a wrap-around of 40–80m² costs £130,000-£380,000, depending on size and specification.
| Size | Mid-Spec Cost | High-Spec Cost |
|---|---|---|
| 40–50m² (smaller wrap-around) | £130,000 – £180,000 | £180,000 – £240,000 |
| 50–65m² (typical London wrap-around) | £180,000 – £240,000 | £240,000 – £320,000 |
| 65–80m² (large wrap-around) | £240,000 – £310,000 | £310,000 – £380,000+ |
How much does a basement extension cost in London?
A basement extension (also called a basement conversion or dig-out) is the most expensive type of London extension per m², but creates significant floor area without reducing garden space, making it popular in high-value inner London properties where outdoor space is at a premium.
| Basement Type | Size | All-In Cost |
|---|---|---|
| Single-level basement conversion (existing cellar) | 20–40m² | £80,000 – £150,000 |
| New-build basement (full dig-out) | 20–40m² | £150,000 – £280,000 |
| Large new-build basement with lightwell | 40–70m² | £250,000 – £400,000+ |
Basement extensions in London require specialist contractors and structural engineers experienced in London Clay and groundwater management. They are generally not covered by permitted development and require a full planning application plus a structural method statement. They are also the extension type most likely to affect neighbouring properties; party wall agreements are almost always required.
How much do architect fees cost for a house extension in London?
Architect fees for a house extension in London typically cost £3,000-£8,000 for a standard project, or 5-8% of the total build cost for full-service design, planning drawings, construction drawings, and contract administration. For a £100,000 extension, expect to pay £5,000–£8,000 in architect fees.
| Service Level | What’s Included | Typical Cost |
|---|---|---|
| Planning drawings only | Concept design, planning application drawings, submission | £1,500 – £3,500 |
| Planning + construction drawings | Above plus detailed technical drawings for building regulations | £3,000 – £6,000 |
| Full service (design to completion) | Above plus contract administration, site visits, builder coordination | £5,000 – £12,000+ (or 5–8% of build cost) |
Some architects charge a fixed fee per stage rather than a percentage. For smaller extensions (under £80,000 construction cost), a fixed fee arrangement typically gives better value. For larger or more complex projects, a percentage fee aligns the architect’s interests with keeping the project on budget.
IS AN ARCHITECT NECESSARY FOR A HOUSE EXTENSION IN LONDON? Technically no, but practically yes for any extension of meaningful complexity. An architect-designed extension in London adds measurable value: better use of light, smarter layouts, and materials that age well. It also reduces planning risk: a well-prepared planning application has a significantly higher approval rate than a DIY submission, particularly in conservation areas and Article 4 boroughs.
What other professional fees should I budget for a house extension?
Architect fees are only one component of the professional fees for a London house extension. Budget for the following in addition:
| Professional Fee | Typical Cost (London) |
|---|---|
| Structural engineer | £800 – £2,000 |
| Planning application fee (England) | £548 |
| Building regulations (plan check + inspections) | £1,000 – £3,000 |
| Party wall surveyor (per neighbour) | £800 – £3,500 |
| Ground investigation / soil survey | £1,000 – £3,000 |
| Tree survey / arborist report (if trees nearby) | £500 – £1,500 |
| Energy performance / SAP assessment | £300 – £700 |
| Total professional fees (all-in) | 15–20% of construction cost |
What affects the cost of a house extension in London?
Seven factors account for most of the cost variance in London house extension quotes.
1. Size and footprint
The single biggest cost driver. Every additional m² adds construction cost, professional fee percentages, and VAT. Define your minimum viable floor area early; adding 5 m² to a 25 m² extension adds £14,000-£23,000 to the total budget.
2. Specification level
The difference between a basic and premium specification on the same footprint is almost entirely in glazing, flooring, and fit-out. Standard aluminium bi-fold doors cost £3,000–£6,000. Structural frameless glazing costs £15,000-£40,000 for a similar opening. The spec decision is the lever with the most impact on the final number.
3. Ground conditions
London Clay is the dominant soil type across most of inner London. It shrinks in dry summers and swells in wet winters, which means foundations must be deeper than in most UK regions, typically 1.5-2.5 m rather than 0.9-1.2 m. This adds £5,000–£15,000 to foundation costs. Proximity to mature trees can require specialist underpinning, adding more.
4. Site access
London terraces often have no side or rear vehicle access. Materials must be carried through the house or craned over the roof, both of which add labour time and cost. Restricted parking for builder vans in CPZ zones adds daily permit costs throughout the build.
5. Conservation area or listed building status
Approximately 25% of London properties fall within conservation areas. Extensions in these areas face restrictions on materials, roof form, and window design, often requiring more expensive natural materials and more architect time for a compliant design. Listed buildings require listed building consent and specialist contractors for any work affecting historic fabric.
6. Structural complexity
Removing a full rear wall to create an open-plan extension requires a steel beam (RSJ) to carry the load above. Steel size, temporary propping, and making good after installation typically add £8,000-£20,000 to the project cost.
7. Kitchen or bathroom fit-out
The per-m² construction cost does not include kitchen or bathroom fit-out. A mid-range kitchen adds £15,000–£30,000; a premium kitchen £30,000–£60,000+. A new bathroom in the extension adds £8,000–£15,000. These costs must be budgeted separately.
How does house extension cost vary by London borough?
| London Area | Mid-Range Single-Storey (30m²) All-In | Notes |
|---|---|---|
| Prime Central (Chelsea, Kensington, Mayfair) | £130,000 – £180,000 | Highest labour rates; listed buildings and conservation area constraints common |
| Inner South West (Wandsworth, Clapham, Battersea) | £110,000 – £155,000 | Strong demand; Article 4 directions in several wards remove side PD rights |
| Inner North (Islington, Camden, Hackney) | £105,000 – £150,000 | Victorian terrace stock; deep London Clay foundations typical |
| South London (Croydon, Bromley, Lewisham) | £80,000 – £120,000 | 10–15% below Wandsworth; good builder availability |
| East London (Stratford, Walthamstow, Barking) | £80,000 – £115,000 | Growing market; competitive pricing; period terrace stock increasing |
| Outer London (Sutton, Enfield, Havering) | £65,000 – £100,000 | Lowest rates; 20–30% below central London for equivalent specification |
What are the hidden costs of a house extension in London?
Hidden costs are discovered once work begins and ground or structure is opened up. They are more common in London than elsewhere in the UK due to the age of the housing stock and ground conditions. Budget a 10–15% contingency for most London extensions, or 15–20% for Victorian or Edwardian properties.
| Hidden Cost | Typical Cost (London) |
|---|---|
| Deep or specialist foundations (London Clay, tree proximity) | £5,000 – £15,000 |
| Steel frame for full rear wall removal | £8,000 – £20,000 |
| Temporary propping during structural work | £2,000 – £5,000 |
| Ground investigation / unexpected drainage issues | £1,000 – £5,000 |
| Asbestos survey or removal | £500 – £3,000 |
| Underpinning (rare but possible near trees) | £10,000 – £30,000+ |
| Skip permits and hoarding (London borough requirement) | £500 – £2,000 |
| Service diversions (gas, electrics, drainage rerouting) | £1,500 – £6,000 |
| Party wall award dispute resolution | £1,500 – £5,000 |
| Temporary kitchen or bathroom provision during build | £500 – £2,000 |
Is VAT charged on house extensions in London?
Yes. The standard VAT rate of 20% applies to house extension labour and materials in London. There is no reduced rate for residential extensions. Always confirm whether quotes are inclusive or exclusive of VAT a £100,000 ex-VAT construction quote becomes £120,000 including VAT, before professional fees.
One exception: a newly created self-contained annex or dwelling that meets HMRC’s zero-rating criteria may qualify for 0% VAT on construction. This does not apply to standard residential extensions. If you are converting a garage or outbuilding into a separate dwelling, seek specialist tax advice before assuming zero-rating applies.
Does a house extension add value to your home in London?
Yes — and in most cases by more than the extension costs. A well-designed London house extension typically adds 10–20% to property value, with prime London boroughs occasionally achieving uplift of up to 30%.
10–20%
Typical property value uplift from a well-designed extension
Up to 30%
Value uplift potential in prime London boroughs
£50k–£150k
Typical stamp duty saving vs buying a larger London property
Is extending cheaper than moving in London?
In almost every case, yes. Moving to a larger London property triggers stamp duty, estate agent fees, conveyancing costs, and removal expenses, typically £50,000–£150,000 in total transaction costs on a London move. A well-specified extension delivers equivalent additional space at a fraction of the total cost, while keeping you in a neighbourhood and school catchment you already know.
Which extension type has the best ROI in London?
The single-storey rear extension consistently delivers the strongest return on cost it creates the most usable, light-filled space at the lowest cost per m². Double-storey extensions offer the best value per m² of added floor area. Wrap-around extensions transform the ground floor entirely but carry higher costs and longer planning timelines. Basement extensions have the highest cost and longest return timeline but are uniquely attractive in high-value inner London locations where outdoor space cannot be sacrificed.
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Do you need planning permission for a house extension in London?
Not always but the rules are more complex in London than elsewhere in the UK, with many boroughs having removed or restricted permitted development rights.
Single-storey rear extensions permitted development rules
Under standard permitted development rights, a single-storey rear extension does not need planning permission if it extends no more than 3m beyond the rear wall of a terraced or semi-detached house, or 4m for a detached house. The maximum height is 4m.
Under the Neighbour Consultation Scheme (Prior Approval), these limits extend to 6m for terraced or semi-detached properties and 8m for detached subject to neighbour objections and council approval.
When planning permission is always required
Planning permission is always required for: double-storey extensions, extensions that exceed permitted development dimensions, side extensions that cover more than half the width of the original house, extensions in conservation areas where Article 4 directions have removed PD rights, and any work on listed buildings (which also requires listed building consent).
| Extension Type | Planning Required? |
|---|---|
| Single-storey rear (within PD limits) | No – permitted development |
| Single-storey rear (Neighbour Consultation Scheme) | Prior approval only (not full planning) |
| Double-storey rear | Yes – full planning application |
| Side return (standard borough) | Often no – check borough PD rules |
| Side return (conservation area or Article 4) | Yes – full planning application |
| Wrap-around extension | Depends on dimensions – check with architect |
| Basement extension | Usually yes – full planning application |
| Any extension on listed building | Yes – plus listed building consent |
Always check your specific London borough’s planning portal before assuming permitted development applies. Approximately 25% of London properties fall within conservation areas where PD rights are restricted or removed entirely.
How long does a house extension take in London?
House extension timelines in London have two phases: design and planning (before work starts) and construction (on site). Total project duration from initial decision to moving into the new space runs 9-18 months for most London extensions.
| Phase | Single-Storey | Double-Storey / Wrap-Around |
|---|---|---|
| Design and drawings | 6 – 10 weeks | 8 – 14 weeks |
| Planning (if required) | 8 – 13 weeks | 8 – 13 weeks |
| Party wall process | 2 – 8 weeks (concurrent) | 2 – 8 weeks (concurrent) |
| Procurement (tendering, awarding) | 4 – 8 weeks | 6 – 10 weeks |
| Construction on site | 12 – 16 weeks | 16 – 24 weeks |
| Total from decision to completion | 9 – 12 months | 12 – 18 months |
The planning process is the least controllable variable. A permitted development extension with no planning application skips 8–13 weeks from the timeline. A double-storey extension in a conservation area where planning permission is required and neighbours object can stretch to 16+ weeks for determination alone.
Why do house extension quotes vary by 20–30% in London?
It is common to receive three quotes for the same London house extension and find a £20,000–£40,000 range between the lowest and highest. Here is exactly why and how to navigate it.
Different scope assumptions
The most common cause of quote variation is scope. One builder’s quote may include demolition of the existing rear extension; another may not. One may include the kitchen fit-out; another may exclude all fit-out entirely. Without an itemised quote based on identical drawings and specifications, you are comparing different projects, not different prices for the same project.
Different glazing and joinery assumptions
A vague brief that says “bifold doors to rear” leaves enormous room for assumption. Standard aluminium bifolds cost £3,000–£6,000. Large sliding aluminium doors cost £6,000–£12,000. Structural frameless glazing costs £15,000–£40,000. If each builder prices different glazing, the quotes are incomparable.
Risk and contingency priced differently
An experienced London extension builder working in Victorian terrace stock will price contingency for unexpected foundations, tree-related complications, and service diversions. A cheaper quote from a less experienced builder may exclude all contingency, making them appear competitive at tender but expensive once variations are added during construction.
Professional fees included or excluded
Some builders quote construction only and expect you to engage the architect, structural engineer, and party wall surveyor separately. Others offer a turnkey price that includes coordination of all consultants. Both approaches are legitimate but they produce very different headline numbers.
How to get comparable quotes
Commission your architect to produce planning and construction drawings before going to tender. Send identical drawings, a written specification, and a scope of works document to every builder you approach. Ask each to return a fully itemised quote. This is the only reliable way to compare London extension quotes on a like-for-like basis.
How can I reduce house extension costs in London without compromising quality?
Simplify the form
A square or rectangular footprint with a flat roof is the cheapest structural form for a single-storey London extension. Complex rooflines, angled walls, and curved glazing all add structural cost disproportionate to the space they create. Every design complication is a budget complication.
Choose aluminium glazing over structural glass
Aluminium bi-fold or sliding doors deliver excellent indoor-outdoor connection at £3,000–£8,000. Structural frameless glass looks like no frame at all, but costs five to eight times more for a similar opening. The visual difference is significant, but the functional difference is minimal. Choosing aluminium over frameless glass can save £10,000–£30,000 on a single rear wall opening.
Use engineered timber rather than solid hardwood for floors
Engineered oak flooring delivers the same visual result as solid hardwood at 30–50% less cost, with superior dimensional stability in the fluctuating humidity of a new extension.
Self-manage the project
Engaging your own architect, structural engineer, and main contractor separately rather than using a design-and-build firm that marks up all consultants can reduce professional fees by 15–25%. This requires more of your time and attention but is feasible for homeowners who can commit to weekly site visits and decision-making.
Get at least 3 competing quotes
London extension builder pricing for the same project can vary by 20–30% depending on the builder’s current workload, team composition, and appetite for the job. Getting three itemised quotes based on identical drawings is the most reliable way to establish true market rate and negotiate from a position of knowledge.
How can you finance a house extension in London?
Remortgage or further advance
The most common financing route for extensions above £60,000. Releasing equity through a further advance on your existing mortgage or a full remortgage typically offers the lowest available interest rate. The extension adds value to the property, which can support the increased borrowing. Allow 4–8 weeks for mortgage processing; do not start on site before funds are confirmed.
Secured home improvement loan
A second charge loan secured against your property. Rates are higher than a remortgage (typically 6–12% APR) but faster to arrange and does not disturb your existing mortgage deal. Suitable for extensions of £30,000–£80,000 where remortgaging is not cost-effective due to early repayment charges.
Personal (unsecured) loan
Suitable for smaller projects or to top up equity release. Rates in 2026 run 5–15% APR for well-qualified borrowers. Available up to £25,000–£50,000 depending on the lender. Fast to arrange, typically 24–72 hours, but monthly payments are higher than secured borrowing.
Staged payments to your builder
All reputable London extension builders will accept a milestone payment schedule: a deposit on contract signing (10–15%), payments tied to foundations, superstructure, weathertight stage, first fix, second fix, and a final payment on practical completion. Never pay more than 15–20% upfront. A builder requesting 40–50% before work begins is a significant warning sign.
How to compare house extension quotes in London
Getting at least three competing quotes based on identical drawings and specifications is the most effective way to establish fair market price and avoid overpaying for your London house extension.
Step 1 – Commission drawings before going to tender
Ask your architect to produce construction-level drawings and a full specification document before approaching builders. Builders pricing from identical drawings produce comparable quotes. Builders pricing from a verbal brief or outline sketch produces incomparable ones, and the cheapest initial quote often becomes the most expensive final account.
Step 2 – Request fully itemised quotes
Every quote must be itemised by trade: groundworks and foundations, superstructure, roofing, glazing and external doors, first fix (electrics, plumbing, carpentry), plastering, second fix, flooring, decoration, and external works. Any contingency or project management fee must be stated separately. Reject any lump-sum quote; it provides no basis for comparison or negotiation.
Step 3 – Verify extension-specific experience
Ask for references and completed project photos from at least two London house extensions of similar type and size. Check that the builder holds public liability insurance of at least £2 million. Confirm Gas Safe and NICEIC credentials for any gas or electrical work. Ask specifically how they handle unexpected ground conditions or structural discoveries; their answer reveals how they manage risk and price variations.
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House extension cost London – frequently asked questions
Q – How much does a house extension cost in London in 2026?
Answer: House extension cost in London ranges from £60,000 for a small single-storey rear extension at basic spec to £380,000+ for a large wrap-around at premium spec. The average construction cost is £3,200–£4,600/m² before professional fees, fit-out, and VAT. London costs run 20-40% above the UK national average.
Q – How much does a single-storey rear extension cost in London?
Answer: A 30m² single-storey rear extension in London costs £85,000–£130,000 all-in at mid-to-high specification, including VAT and professional fees. A basic specification 20m² extension starts around £55,000–£70,000.
Q – How much do architect fees cost for a house extension in London?
Answer: Architect fees for a London house extension typically run £3,000–£8,000 for a standard project, or 5–8% of the build cost for full design-to-completion service. Planning drawings only cost £1,500–£3,500.
Q – Is VAT charged on house extensions in London?
Answer: Yes. Standard 20% VAT applies to all house extension labour and materials. A £100,000 ex-VAT construction quote becomes £120,000 including VAT. Always confirm whether a quote is inclusive or exclusive of VAT.
Q – Does a house extension add value in London?
Answer: Yes. A well-designed London house extension adds 10–20% to property value. Prime borough extensions can achieve up to 30% uplift. Extending is almost always more cost-effective than moving; stamp duty savings alone on a comparable London property typically run £50,000–£150,000.
Q – Do you need planning permission for a house extension in London?
Answer: Not always. Single-storey rear extensions within 3m (terraced/semi) or 4m (detached) qualify for permitted development. Under the Neighbour Consultation Scheme, this extends to 6m and 8m with prior approval. Double-storey extensions, conservation area extensions, and listed buildings require full planning permission.
Q – How long does a house extension take in London?
Answer: A single-storey extension takes 9–12 months from decision to completion, including design, planning, and 12–16 weeks on site. A double-storey or wrap-around extension takes 12–18 months total, with 16–24 weeks of construction.
Q – Why do house extension quotes vary by 20–30% in London?
Answer: Because different builders make different scope, specification, and risk assumptions from a vague brief. Always provide identical drawings and a written specification to every builder you approach, then request itemised quotes so you can compare like-for-like.
Q – How do I get free house extension quotes in London?
Answer: Submit your project brief on QuoteMyRenovation and receive up to 3 competing quotes from builders vetted specifically for extension work in London. Free and no-obligation for homeowners. House Extension Cost London
