How much does a 3m by 4m extension cost?

Twelve square metres is at the point where fixed costs still dominate. Foundations, the structural opening into the house, the party wall award, building control and professional fees cost much the same on a 12m² extension as on a 20m² one, so the cost per square metre is higher on smaller builds. Extending to 16 or 18m² often adds far less than proportionally.

The specification swings the number more than the size. A solid roof with two rooflights, standard French doors and a plastered finish sits at the lower end. A structural glass roof lantern, aluminium bifolds across the full width and polished concrete flooring can add £25,000 to the same footprint. Decide the glazing early, because it is the single biggest variable.

For a Fulham terrace there is a further consideration: whether the 3m depth triggers a planning application. A 3m rear extension on a terraced house is within permitted development limits in principle, but not in a conservation area and not on a flat — and several parts of Fulham are conservation areas. Getting this checked before design is free and saves a great deal.

As a rule of thumb for budgeting, take the shell figure and add 40 to 60 percent if the extension contains a kitchen. Clients frequently price the building work and forget that the room needs to be finished and fitted out, which is when a £45,000 extension becomes a £75,000 project.

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