Complete the project information below. After the calculation, enter your email address and your indicative estimate will be sent directly to you.

Free calculator · updated September 2026
Construction cost calculator

Calculate your build cost

Select your main project below. For an extension or loft conversion, you can also include refurbishment of the existing house, with a separate floor area and finish.

Your postcode sets the cost area automatically.

10 m² 50 m² 200 m²

Enter only the additional floor area, across all new floors. Keep existing rooms in the refurbishment area below.

Add the existing floor area being refurbished. Do not include the extension or loft area a second time.

Count new or replacement kitchens and bathrooms once across the whole project. Their fit-out allowances are added separately to the area costs. Leave at 0 if none are being fitted.

Important

This is an early budgeting guide, not a quotation. The result excludes VAT, professional fees, statutory charges, surveys, structural engineering, Party Wall matters, specialist reports, abnormal site conditions and major structural alterations to the existing house. Area allowances include general building work and internal finishes; kitchen and bathroom fit-outs are added only through the quantities above. Refurbishment assumes renewal of internal finishes and services. The estimate range reflects pricing uncertainty; allow separately for a project contingency.

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