House extension cost: what you pay in 2026

A house extension in the UK typically costs between £1,500 and £3,000 per square metre for the build alone, before professional fees, finishes, and VAT. A standard single-storey rear extension of around 20 square metres usually lands between £30,000 and £60,000. A double-storey extension costs more per project but less per square metre, because you share one set of foundations and one roof across two floors.

Those figures are construction estimates only, and current figures vary; confirm them with a broker or your builder before you budget. The final number depends on your location, the size, the specification, and the ground you build on. A kitchen-diner with bifold doors and underfloor heating sits at the top of the range. A simple box extension with standard finishes sits near the bottom.

Vortex Finance is a whole-of-market property finance broker. We do not build extensions and we are not a lender. What we do is arrange the finance that pays for them, through a panel of 100+ lenders, when your own cash will not stretch to cover the works. This guide explains what an extension actually costs, then where the money comes from.

What a house extension costs by type

Cost scales with complexity, not just floor area. Here is how the common project types compare.

• Single-storey rear or side extension. The most common project. Budget £1,500 to £2,500 per square metre for the build. A 20 square metre extension runs roughly £30,000 to £50,000; a larger 30 square metre open-plan kitchen extension reaches £45,000 to £75,000 once you add glazing, heating, and a fitted kitchen.
• Double-storey extension. Adding two floors at once. The per-square-metre rate often drops to £1,400 to £2,200 because the foundations and roof serve double the space. Expect £60,000 to £120,000 for a typical two-storey rear addition, depending on whether the upstairs is a bedroom or a full bathroom.
• Loft conversion. A different project, but the question that follows the same search. A basic rooflight conversion starts around £25,000, a dormer runs £35,000 to £60,000, and a hip-to-gable or mansard can exceed £70,000.
• Wrap-around or large open-plan extension. Combining rear and side returns. These bespoke projects sit at the top of the range, and the upper figures rise sharply with high specification. Current figures vary; confirm them with a broker or your builder.

Add the costs in the next section to reach your true total.

The costs people forget to budget for

The build quote is not the whole bill. Three categories catch homeowners out, and together they can add 20% to 30% on top of the headline construction figure.

Professional fees come first. An architect, a structural engineer to sign off the calculations, and sometimes a party wall surveyor if you build close to a neighbour. These commonly total 10% to 15% of the build cost.

Planning and regulatory costs come next. A householder planning application carries a fee, and many extensions also need Building Regulations approval. If your extension falls under permitted development you may avoid full planning, but Building Regulations still apply.

Then there are the soft costs. VAT usually applies to domestic extension work, though current figures and treatment vary; confirm the position with a broker or your accountant. Add a contingency of at least 10% for the things no survey predicts: poor foundations, drains in the wrong place, asbestos in an old garage. There is almost always one.

How to pay for a house extension

Most homeowners fund an extension one of four ways. The right choice depends on how much equity you hold, how fast you need the money, and whether the property is an investment.

Savings are the cheapest option when you have them. No interest, no fees, no lender. The limit is obvious: few people hold £60,000 in cash earmarked for building work.

Remortgaging or a further advance releases equity at standard mortgage rates. It is slow, it depends on your income and affordability, and your lender may decline if the property is mid-build and temporarily unmortgageable. A secured loan or second charge is faster and does not disturb a low fixed rate you want to keep.

For investment property, heavy refurbishment, or work that makes a home temporarily unmortgageable, short-term property finance often fits best. This is where a specialist broker earns their place.

When bridging or refurbishment finance fits

A house extension is structural work. That single fact changes which lenders will help. Mainstream lenders dislike releasing money against a property that is part-built or that fails a habitability check, and a standard remortgage cannot fund a room that does not yet exist.

Refurbishment finance is built for exactly this. It funds the works against the projected end value of the property, not its value today. On heavy refurbishment, which includes most extensions, you can typically borrow up to 70% of the current value plus up to 100% of the works cost, released in tranches as each stage completes. Indicative monthly interest on heavy refurb runs 0.85% to 1.10%, with terms of 6 to 18 months and completion in 10 to 21 days.

This route suits landlords and investors extending a buy-to-let, anyone running a buy-refurbish-refinance project, and homeowners whose property is unmortgageable during the build. You complete the works, the property revalues higher, and you exit onto a term mortgage or a sale.

If you want the full picture on funding the works against end value, including light versus heavy projects and staged drawdowns, read our refurbishment finance page. It covers the product in depth and explains how the application runs from indicative terms to drawdown.

A quick caveat on every figure above. Rates, fees, and loan-to-values are indicative. A lender confirms the final terms once they have seen the property, the works schedule, and your file. We are a broker arranging finance, not a lender, and this guide is information rather than advice.

Book a call to fund your extension

If your extension is for an investment property, or the build will leave your home temporarily unmortgageable, the fastest way to know what is realistic is a short call. One of our brokers will look at the property, the works budget, and your exit, then come back with indicative terms from suitable lenders inside 24 hours. There is no charge to get indicative terms, and nothing is payable until you give us the go-ahead to submit.

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