Use cost per square metre carefully
Published 2026 guides put typical extension building costs around £1,800–£3,000 per square metre for many standard projects. That range is broad because a plain shell and a finished kitchen extension with large glazing are not comparable.
Some costs do not grow in proportion to area
A small extension still needs design work, foundations, drainage, roof edges, doors and Building Control. A very small extension can therefore cost more per square metre than a larger simple one.
Structure can change the budget quickly
Long steel beams, temporary propping, difficult access, piled foundations or moving a drain can add significant cost without adding any floor area.
Keep a contingency for discovered work
Existing buildings do not always match drawings. Allowing a sensible reserve is safer than spending the full budget on the initial contract sum.
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