How accurate is the estimate?

It is an indicative London market range for budgeting, not a quotation. Only a contractor pricing a defined drawing package can give a firm number.

Stamp-duty impact of an extension calculator

Q: What does the stamp duty and extension calculator include?

It estimates how extending affects stamp duty and related purchase costs. Typical London figures are stamp duty is charged on purchase price, not on later improvement — but buying a smaller property and extending is frequently cheaper than buying the larger one. Statutory fees and professional fees are separate.

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London terraced street — typical residential housing for house extensions in London

What this estimates

This tool estimates how extending affects stamp duty and related purchase costs. As a London market range, expect stamp duty is charged on purchase price, not on later improvement — but buying a smaller property and extending is frequently cheaper than buying the larger one. That is a budgeting range, not a quotation.

What moves the number?

Stamp duty land tax is payable on acquisition, so money spent extending after purchase attracts none.
The comparison that matters is purchase price plus build cost against the price of the larger property, including the SDLT difference.
Whether the smaller property can lawfully be extended — Article 4, conservation area status, and any condition removing permitted development rights.
Professional and statutory costs of extending: £548 planning, £500, £1,200 building control.
Local ceiling price, which caps what the finished property can be worth however much is spent.

What sits outside the build cost?

Statutory fees go to the council and are set nationally for England: a householder planning application is £548 determined in 8 weeks, larger home extension prior approval is £249 over 6 weeks, and a Lawful Development Certificate is £274. Building control is separate again at typically £500 to £1,200 in London.

How should you use the figure?

As a budget envelope before design, not as a target after it. Build cost should drive the design rather than follow it: a scheme drawn without a budget is a scheme that gets value-engineered after planning consent, which usually means a second application. Fix the range first, design to it, then price the defined drawing package with two or three contractors.

Ranges are indicative London market rates observed across domestic projects and are not a quotation. Statutory figures are the England rates in force in 2026 under the Town and Country Planning (Fees for Applications) (England) Regulations. Building control charges are set by each local authority or registered building control approver.