Home improvement calculators

Extension cost estimator

Get a ballpark cost for a single-storey or double-storey house extension based on your floor area and chosen finish level. Includes a cost breakdown by trade and a what-to-budget-for checklist. All figures are UK 2026 estimates.

Your extension details

Extension type

Single-storey extension

Extends the ground floor only. Kitchen-diners, living rooms, utility rooms. Lower cost and simpler planning.

Double-storey extension

Adds two floors at once. More cost-effective per square metre than building twice. Usually needs full planning permission.

Floor area e.g. 4m x 5m = 20m²

m²

Finish level

Rough / basic

Structurally complete, plastered, basic electrics and plumbing. You finish the decorating yourself.

Standard finish

Fully plastered, painted, standard flooring, fitted kitchen or bathroom if applicable. Move-in ready.

High spec

Premium materials, underfloor heating, bi-fold or sliding doors, high-end kitchen or bathroom, bespoke joinery.

Location uplift London and SE costs more

North / Midlands

-15% vs average

Rest of England

Average rates

London / South East

+25% vs average

Estimated total cost

Ballpark cost range

£0

20m² single-storey, standard finish

Low estimate

£0

High estimate

£0

Cost per m² (low)

£0

Cost per m² (high)

£0

Floor area

20 m²

Contingency (15%)

£0

Always add a contingency

Budget at least 15% above the high estimate for unexpected costs. Extensions almost always surface surprises once groundwork begins.

Estimated cost breakdown by trade

How finish level affects the cost

What to budget for beyond the build

Architect or designer feesTypically 5–10% of build cost. Worth every penny for complex projects or where planning permission is needed.

Planning permissionAround £206 for a householder application in England. Budget 8–12 weeks for the decision and the possibility of a refusal.

Building regulationsSeparate from planning. Typically £500–£1,500 depending on project size. Your builder should handle the submissions.

Party wall surveyorIf your extension is on or near a boundary shared with a neighbour. Fees typically £700–£2,000 per surveyor.

Structural engineerRequired for any work involving beams or load-bearing walls. Typically £500–£1,500 for drawings and calculations.

Temporary accommodationIf the build makes part of your home unusable you may need to factor in staying elsewhere for weeks at a time.

VATMost extension work by a VAT-registered builder is subject to 20% VAT. Check whether your quotes include or exclude it before comparing.

Home insuranceTell your insurer before work starts. Failure to notify can invalidate your policy. You may need specialist renovation cover during the build.

Peter's tip

Always get at least three quotes before appointing a builder. The spread between the cheapest and most expensive quote for the same job is often 30–40%. Do not automatically choose the cheapest — check references, look at their previous work, and make sure the quote is itemised so you can compare like for like. A fixed-price contract is worth pushing for.

Permitted development

Many single-storey rear extensions can be built under permitted development rights without a planning application. Rules depend on size, location and property type. Check with your local planning authority before assuming you are covered.

Double-storey value

A double-storey extension typically costs only 50–60% more than a single-storey of the same footprint — because the groundwork, roof and scaffolding costs are spread across twice the floor area. If you need more space, going up is usually better value per square metre.

When costs overrun

Cost overruns are extremely common on extensions. The most frequent causes are unforeseen groundwork issues, changes to the brief mid-build, and hidden structural problems discovered once walls are opened up. A 15% contingency is the minimum — 20% is safer.

Financing your extension

Options include remortgaging (often the lowest rate), a further advance from your current lender, a secured loan, or a personal loan for smaller projects. Use the home improvement loan calculator to model the repayment cost before committing.

Please note: All figures are ballpark estimates based on typical UK build costs in 2026 and are for budgeting guidance only. Actual costs will vary significantly based on site conditions, specification, contractor, location and market conditions. Always obtain a minimum of three itemised quotes from qualified builders before proceeding. This is not a professional cost estimate or financial advice.

What does the extension cost calculator cover?

The extension cost calculator estimates the total cost of a single-storey rear or side extension, based on the floor area, specification level and your region. It includes the build cost (the main contractor’s price), architect’s fees, structural engineer’s fees, planning application fee, building regulations, and a contingency allowance.

How to use this calculator

Floor area: The internal floor area of the extension in square metres. A typical kitchen extension is 15 to 25 sqm; a larger family room extension might be 25 to 40 sqm.

Specification: Select the specification level — basic (standard materials and fittings), mid-range (quality finishes, larger glazing), or high specification (bifold doors, underfloor heating, premium materials). Specifications significantly affect cost.

Region: Build costs vary significantly across the UK – London and the South East typically cost 20 to 30% more than the Midlands or North. Select your region for a regionally adjusted estimate.

Reading the result: The calculator shows a cost range (not a single figure – build costs genuinely vary based on site conditions, contractor, and detail) and a breakdown by cost category. Use it as a starting point before getting quotes from three or more builders.

Frequently asked questions

Do I need planning permission for an extension? Many single-storey rear extensions fall under permitted development (PD) rights and do not require planning permission – subject to size limits, proximity to boundaries, and other conditions. The rules are complex and depend on your specific property. Check with your local planning authority or an architect before assuming PD applies.

How long does an extension take to build? A typical single-storey extension takes eight to sixteen weeks on site once construction starts, depending on size and complexity. Allow additional time for design, planning (if required – approximately 8 weeks), and building regulations approval before construction begins.

What is a contingency, and how much should I allow? A contingency is a budget reserve for unexpected costs – such as ground conditions, structural issues uncovered during construction, and design changes. A standard contingency is 10-15% of the build cost. This is not money you expect to spend, but money that is available if needed.

The information on this site is for educational and informational purposes only. Nothing on Be More Peter constitutes financial advice, and nothing should be treated as such. The calculators, articles and tools on this site are designed to help you understand financial concepts and run indicative numbers - they are not a substitute for professional advice tailored to your specific circumstances. Before making any financial decision - including decisions about mortgages, pensions, investments, savings or insurance — you should seek independent financial advice from a qualified adviser who is authorised and regulated by the Financial Conduct Authority (FCA). Calculator results are estimates based on the information you enter and the assumptions built into the tool. They are illustrative only and should not be relied upon as accurate projections. Rates, thresholds and rules change regularly - always verify figures with an appropriate professional or official source before acting on them. Be More Peter is not authorised or regulated by the FCA. The content on this site does not constitute a financial promotion.