House extensions · Portsmouth

How much does House extensions cost in Portsmouth? (2026 price guide)

A single-storey rear extension typically costs £1,800-£2,800 per square metre, meaning a modest 20 square metre extension might run £35,000-£55,000, while a two-storey extension can add £45,000-£90,000 depending on size and specification. Basic groundwork-only projects sometimes start from around £15,000 for very small additions.

Costs vary considerably based on the size and complexity of the design, ground conditions, the standard of fittings and finishes chosen, and whether planning permission or party wall agreements are needed. Extensions requiring structural steel beams or underpinning add significantly to the budget, as does working on properties with restricted access.

Get a detailed breakdown of labour, materials and any contingency allowance, and check that quotes include VAT and waste disposal to avoid surprises later.

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