How much does a house extension cost?

This is paid to your builder, not your designer, and depends heavily on size, specification and site. As a very rough guide, extension build costs are usually quoted as a price per square metre, with kitchens, bathrooms, glazing and groundworks pushing the figure up.

• Size and storeys, two-storey extensions are often better value per m² than single-storey, because the roof and foundations are shared.
• Specification, kitchens, bathrooms, large glazing (bi-folds, roof lights) and high-end finishes.
• Groundworks and access, drainage, foundations, slopes and tight access.
• Structural complexity, big openings and steels.

This is where a designer comes in, and good design here often saves money on the build by avoiding mistakes and helping you get accurate builder quotes. Typically this covers the measured survey and design, the planning application (council fees are separate), and the building-regulations drawings and structural calculations.

• Local authority planning and building-control fees (paid to the council).
• Structural engineer calculations (we coordinate these).
• Party Wall surveyor fees if you share a boundary.
• VAT, and a sensible contingency (often 10–15%).

Get the design right before you build, obtain like-for-like quotes from two or three builders using proper drawings, and be realistic about specification. A clear, well-detailed design is the single best tool for an accurate, competitive price.