Does the calculator include VAT and planning fees?

No. The headline figure shows the build contract cost only (ex-VAT). Add 20% VAT and allow 10–15% for professional fees (architect, structural engineer, planning application). Some projects may qualify for zero-rate VAT — your builder or accountant can advise.

Why do London extension costs vary by borough?

Labour availability, access constraints, higher site overhead and the premium nature of the work in central London all push costs higher in inner London boroughs. Kensington & Chelsea and Westminster can be 15–20% more expensive than outer London boroughs such as Bromley or Havering.

How accurate is this estimate?

This is a broad planning guide only, not a quote. Actual costs can vary by 30–50% or more depending on site-specific conditions, the existing state of the property, ground conditions, structural complexity, access constraints and the detail of the design. Only a formal tender following an on-site assessment gives a reliable figure.

House Extension Cost Calculator

Get an instant build-cost estimate for your London extension. Enter your extension type, floor area, spec level and borough — and get a realistic cost range with a full breakdown in under two minutes.

Calculate your extension cost

Answer four questions to get your personalised cost estimate. All figures are London market rates for 2026 and exclude VAT and professional fees.

Step 1 of 4

What type of extension are you planning?Choose the option that best describes your project.

Step 2 of 4

What is the approximate floor area of the extension?Measure the internal floor area in square metres. Not sure? Use our guide below.

Floor area (m²)

Typical sizes: single-storey rear 15–40 m² · wraparound 25–55 m² · two-storey 30–80 m²

Step 3 of 4

What level of finish are you looking for?This covers the quality of materials, fittings and detailing throughout the extension.

Step 4 of 4

A few final detailsThese affect the total cost significantly — take a moment to be as accurate as possible.

Main glazing option Your London borough Add-ons (optional)

Kitchen fit-out Price depends on spec chosen

Knock-through to existing house +£2,500–£5,000 (structural opening)

Underfloor heating +£80–£120 per m²

Budget guide range (ex-VAT)

— Indicative only · London market rates 2026 · wide range reflects real-world variation

⚠️ Important — please read before using this figure. This is a broad budget guide only, not a quote. Actual costs are determined after an on-site assessment and can vary significantly — often by 30–50% or more — depending on the condition of your existing property, ground conditions, access constraints, structural discoveries, and the specific requirements of your design. Do not use this figure to compare against or negotiate a builder's tender price.

Item	Low estimate	High estimate

VAT (20%): Not included above. Standard-rated construction work is subject to 20% VAT. Some projects may qualify for reduced or zero-rate VAT — your builder or accountant can advise.

This tool uses typical London market rates for 2026 as a broad planning guide. It cannot account for site-specific conditions, structural complications, drainage issues, neighbour constraints, conservation area requirements, or the quality and detail of your architect's design — all of which materially affect the final cost. Only a formal, itemised tender following a site visit gives you a reliable figure.

Get a free site visit & quote →

What does a London house extension cost in 2026?

London build costs are consistently higher than the national average — typically 20–40% above the rest of England. Labour is more expensive, access is often constrained, and the premium nature of the work in a competitive market all contribute. That said, costs vary significantly even within London: a project in Kensington & Chelsea will typically cost 15–20% more than the same project in Bromley. Bear in mind that site-specific factors — ground conditions, drainage, structural complexity, access and the condition of your existing property — can move any budget figure by 30–50% or more. This calculator gives you a planning-stage guide; only a formal tender after a site visit gives you a reliable number to work from.

The figures in this calculator reflect our experience pricing and delivering extensions across London in 2025–2026. They represent the build-contract cost only — the amount you pay your main contractor. On top of this, you should budget for:

Professional fees: Architect, structural engineer and planning consultant — typically 10–15% of the build cost.
VAT (20%): Standard-rated on most building work. Some projects qualify for reduced or zero-rate VAT (listed buildings, conversions changing use).
Planning application fees: Currently £258 for a householder application in England.
Party wall surveyor fees: £800–£2,000 if Party Wall Act notices are required.
Contingency: We recommend 10–15% of the build cost as a contingency fund, especially on older properties.

How build costs vary by extension type

Extension type	Why it costs more or less	Typical cost premium
Single-storey rear	Straightforward structure; lowest cost per m²	Baseline
Side-return	Narrow sites, complex drainage, often in conservation areas	+10%
Wraparound	Complex corner junction, steelwork, two roof planes	+12%
Two-storey rear	Party wall, structural complexity, second-floor works	+30%
Double side	Significant structural works, often requires planning permission	+35%

What does spec level actually mean?

Standard spec means good, honest building work using mainstream contractor-grade materials. UPVC or standard aluminium frames, mid-market tiling, laminate or engineered wood flooring, standard plasterwork. Perfectly suited to buy-to-let extensions or straightforward family-home improvements where budget efficiency matters.

Mid-range spec is where most owner-occupier projects land. Slim-frame aluminium windows, better-quality floor finishes, underfloor heating, thoughtful detailing. This is the sweet spot for most London homeowners — noticeably better quality without crossing into premium territory.

High-end spec covers architect-led projects with premium materials: structural steel and glass elements, polished concrete or stone flooring, bespoke joinery, smart home integration. Common in prime postcode areas where the added value justifies the investment.

Ready for a proper quote?

Our estimate gives you a realistic budget range. For a detailed, itemised quote based on your actual plans — contact us today. We'll visit the site, review your drawings and give you a transparent, fixed-price tender.

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Frequently Asked Questions

How much does a house extension cost in London in 2026?

A standard single-storey rear extension in London typically costs £2,550–£3,450 per m² at standard spec, £3,400–£4,600 per m² at mid spec, and £4,460–£6,040 per m² at high-end spec. Total costs depend on size, type, borough and finishes chosen. Use the calculator above for a personalised estimate.

Does the estimate include VAT?

No. The figures shown are all ex-VAT. Standard building work attracts 20% VAT. Add this on top of the estimate, plus professional fees of 10–15%. Some projects — such as conversions changing the number of dwellings, or works to listed buildings — may qualify for reduced or zero VAT; your builder or accountant can confirm.

Why are London extension costs higher than the rest of the UK?

London builder day rates are 20–40% above the national average, reflecting higher living costs. Access is often harder — narrow streets, parking restrictions, no space for skips or deliveries. Material delivery and plant hire cost more. Planning and building-control compliance is also more complex, particularly in conservation areas.

What is included in the build cost?

The build cost covers groundworks and foundations, structural frame and walls, roof structure and covering, external windows and doors, internal plastering, first-fix electrics and plumbing, floor finishes, and the builder's preliminaries (site set-up, scaffolding, waste removal). It does not include furniture, appliances (unless the kitchen fit-out add-on is selected), landscaping or decoration.

How accurate is this calculator?

This is a broad planning guide, not a quote. The wide range shown reflects genuine real-world variation in London tender prices. Site-specific factors — ground conditions, drainage, structural complications, the existing condition of your property, access constraints and the detail of your design — can shift any figure by 30–50% or more in either direction. This calculator gives you a sense of scale for budgeting conversations with your architect. A formal, itemised tender following an on-site assessment is the only reliable basis for committing to a project.

By My Local London Builder Team · Last updated May 2026 · All figures ex-VAT · Guidance only — obtain formal quotes before committing.