How Much Does a House Extension Cost in London?

A useful extension budget needs to answer two questions: what will the building work cost, and what will you need to spend to finish the project? Those figures are rarely identical. Design fees, kitchen choices, work to existing rooms and temporary accommodation can all sit outside a headline construction price.

If you are planning a house extension in London, start with a broad allowance, then replace assumptions with a measured scope. The objective is a budget you can make decisions with, rather than an attractive number that grows once work begins.

What is a sensible starting point?

As an independent market reference, Architecture for London publishes a construction range of £3,000–£5,000 per square metre for architect-designed London extensions, excluding VAT and professional fees. This is a broad benchmark, not a Hutch quotation or a promise that every extension fits within it. Specification, access and structural complexity can move a project outside that range.

On that basis, a 25m² extension gives an illustrative construction allowance of £75,000–£125,000 before VAT and fees. The arithmetic is useful for an initial conversation, but it does not establish what your particular quote includes. A kitchen, landscaping or alterations elsewhere may need separate allowances.

For an estimate tailored to your chosen project type, use the Hutch project pricing tool. Its figures are indicative and exclude VAT; confirm the scope and exclusions before using them as a total budget.

Why extensions of the same size cost different amounts

Floor area is only one part of the picture. A straightforward addition with accessible foundations and standard openings is a different job from a narrow terrace requiring substantial steelwork and materials carried through the house.

Ask your project team to explain the main cost drivers:

• Ground conditions: foundation design, drainage routes and any investigative work.
• Structural alterations: the openings connecting the new space to the original house.
• Access: deliveries, storage, waste removal and protection of occupied rooms.
• Glazing: dimensions, opening arrangements, frame specification and installation.
• Services: heating, electrics, water supplies and any upgrades to existing systems.
• Finishes: flooring, fitted furniture, kitchen equipment and decoration.

A modest extension with extensive alterations to the existing ground floor can cost more overall than a larger addition with limited internal changes. Compare the complete project, not simply the new footprint.

Build the budget in separate layers

Your budget should distinguish construction, professional work, homeowner purchases and contingency. This makes omissions easier to spot and prevents the same item being counted twice.

Start with the building quotation. Then list design and engineering appointments, approval charges, any party wall advice and other project-specific professional input. Add anything you will purchase directly, such as appliances or light fittings. Finally, account for the practical costs of living through the work.

Ask whether kitchen fitting is included separately from kitchen supply. Check whether floor finishes include preparation and fitting, and whether decorating covers affected existing rooms. A statement such as “kitchen included” is not sufficiently precise to protect a budget.

Remember VAT

Ordinary alterations and extensions to an existing home are generally standard-rated at 20% when supplied by a VAT-registered contractor, although particular circumstances can have different treatment. Use the applicable VAT position confirmed for your project rather than assuming a reduced rate.

For illustration, £100,000 of work charged at 20% VAT becomes £120,000. If you have £120,000 available in total, that leaves nothing for separately charged fees, excluded purchases or contingency. Always compare quotations on the same VAT basis.

Keep a reserve for genuine unknowns

A contingency is money held for uncertainty, not a list of upgrades you already intend to buy. Set it with your project team after reviewing the property and the information available. An older house with limited investigation may need a different reserve from a well-understood, straightforward project.

As a simple budgeting exercise, a 10% reserve against a £100,000 allowance is £10,000. That is an illustration, not a guarantee of adequacy. Decide whether your reserve is calculated on costs including VAT, and keep the basis consistent throughout your spreadsheet.

Investigating a drain or opening up a small area before final pricing may be worthwhile if it resolves a major uncertainty. The aim is to make fewer expensive discoveries after the programme is committed.

Reduce cost before reducing quality

If the initial budget is too high, revisit the design while it is still on paper. Test whether a smaller structural opening, more standard glazing or a simpler roof arrangement delivers the room you actually need. Keep good construction and weather protection central to the specification.

Do not spread the available money so thinly that every element becomes compromised. Choose the features that improve daily life most, and identify finishes that can be upgraded later without reopening completed work.

What should you do next?

Bring your available drawings, photographs, target budget and a short description of how you want to use the space. Explain whether the budget includes VAT, fees and kitchen purchases. That one clarification makes the first discussion much more useful.

Explore Hutch extensions, then tell us about your project. A clear brief is the starting point for a meaningful price.

Sources and scope

Cost reference: Architecture for London. VAT: GOV.UK guidance for builders. Checked 15 October 2026. Examples are illustrative; obtain a property-specific quotation.