How accurate is this extension calculator?

It is designed for early budgeting, not a final quote. The tool gives a low / expected / high range and widens that range when you don't yet have plans or confirmed planning status. Accuracy improves significantly once you have measured or architect drawings and a site survey.

Why do you show a range instead of one exact price?

Exact pricing before surveys and drawings is usually misleading. Build cost depends on structural details, access, specification, and hidden site conditions. A transparent range is more trustworthy and more useful for budgeting decisions.

Does the estimate include VAT and contingency?

Yes. The result screen clearly separates base build cost, extras, professional/statutory fees, contingency, and VAT. VAT is applied to taxable costs such as build work, extras, professional fees and contingency; statutory fees are shown separately.

What if I don't know planning or structural details yet?

That's normal. The calculator is built for homeowners at an early stage. If you leave planning/professional fees unselected, it automatically includes sensible allowances based on your answers.

Do you only work in London?

Better Homes primarily serves London, so London is the main pricing focus of this tool, including a zone option. We also include other UK regions so homeowners can still use the calculator for budgeting comparisons.

What should I do after getting my estimate?

Download the PDF, compare the expected and high ranges, then speak to an architect/designer or builder with your rough brief. Once you have plans, update the calculator and request a survey-based quote.

How much does a house extension cost per square metre in the UK?

In London, extension construction costs typically run £2,500 to £4,500 per square metre in 2026 depending on specification, with inner boroughs at a 15-30% premium. Outside London, rates are generally lower. Remember that per-square-metre figures usually exclude VAT, professional fees and fit-out - this calculator includes them so your budget is realistic from the start.

Is this calculator free, and do I have to give my details?

The calculator is free and you can see your low/expected/high estimate on screen without entering any contact details. If you'd like the full PDF breakdown with the budget checklist, we ask for your name and email so we can send it to you.

Extension Cost Calculator UK & London (2026)

Budget guidance

Make sense of your extension estimate

Use the calculator first, then use these notes to understand the range, the assumptions and the next step before asking for quotes.

How this extension cost calculator works

Most online extension calculators give you a single number, which is exactly why most extension budgets go wrong. Build cost depends on structure, access, specification and site conditions that no early-stage tool can see, so an honest calculator gives you a range, not a promise.

This tool asks for the essentials: property type, location, extension type, size and complexity. It returns a low / expected / high budget with professional and statutory fees, contingency, and VAT at 20% broken out separately. If you don't yet have drawings or confirmed planning status, it widens the range and applies sensible fee allowances automatically, then gives you a confidence score.

Low / expected / high

Fees + VAT separated

PDF budget checklist

Why ranges matter

Plan with the high end before you commit.

The expected figure is useful for orientation. The high figure is the number to use when checking finance, contingency and whether the project still makes sense if hidden conditions appear.

Best use of this estimate

Brief architects, compare builder quotes and refine the range once measured drawings and a site survey are ready.

2026 benchmarks

Typical London extension budgets

Use these planning-stage ranges to sanity-check your result. Figures include build, fees and VAT at a mid-range specification.

Extension type	Typical all-in budget
Side return extension	£35,000 - £60,000
Single-storey rear extension	£50,000 - £95,000
Wraparound (L-shaped) extension	£75,000 - £140,000
Kitchen extension with fit-out	£80,000 - £160,000
Double-storey extension	£100,000 - £200,000+

For per-square-metre rates, hidden costs, borough premiums and what's driving 2026 prices, read the full extension cost breakdown for London in our complete 2026 House Extension Guide.

Cost drivers

What moves an extension estimate up or down

Size and type

Cost doesn't scale evenly with floor area. A double-storey extension shares foundations and roof between two floors, so its per-square-metre rate is lower than a single-storey, but the total project is bigger. Wraparounds carry the most structural work per square metre because two external walls come out.

Specification

The same shell can vary by £1,000+ per square metre depending on finish: standard plaster-and-paint at the low end; underfloor heating, bi-folds and engineered flooring mid-range; bespoke joinery, Crittall-style glazing and stone worktops at the top. Our calculator's complexity setting is a proxy for this, so be honest with it.

Access and party walls

Terraced houses, which make up much of London's stock, usually mean tighter access, more hand-balling of materials, scaffold licences and party wall agreements with one or both neighbours. These are real costs that flat-fee calculators ignore; this tool allows for them based on your property type.

Where you are in London

Inner-London boroughs typically run 15-30% above outer London for the same build, driven by labour rates, access logistics and parking/scaffold licensing. The calculator's London zone option accounts for this.

Planning route and linked project costs

Many single-storey rear and side return extensions qualify for permitted development, while larger and double-storey schemes usually need full planning. The planning permission rules for extensions are covered in detail in our guide. If you're extending as part of a wider refurbishment, our renovation cost calculator budgets the whole project.

If your project is really a kitchen transformation with an extension attached, run the numbers in our kitchen cost calculator too. The kitchen fit-out is often the biggest single line after the build itself.

Next steps

What to do with your estimate

Speak to the design-and-build extension team

Download your PDF

It includes the full low/expected/high breakdown and a budget checklist, useful for finance conversations and for comparing builder quotes line by line.

Read the guide

Use the guide to understand types, planning, building regs, party walls and value: everything between rough budget and ready to build.

Talk it through

When you want a real answer for your specific property, book a feasibility consultation and pressure-test the budget before anything gathers momentum.

Our complete 2026 House Extension Guide covers the detailed research between rough budget and ready to build.

Extension Cost Calculator

Tell us about the property

Property Type

Region