Cost Calculator

What decides house extension cost

House extension cost depends far more on the details of the job than on the headline size. The calculator above gives a starting range for a single or double storey extension per square metre; the factors below are what move a real quote up or down from that range. We give a free estimate after a site visit, once we can see the ground, the access and the house the extension joins.

Ground and foundations come first. Clay, trees nearby, drains under the footprint or made-up ground can mean deeper or engineered foundations. Access matters too: if materials have to be barrowed through the house or a narrow side passage, labour goes up.

Structure is next. Opening the back of the house usually needs a steel beam, and wide glazed openings or a large roof span can need more steel. The roof form, whether flat, pitched or with a roof lantern, changes the cost of both structure and finishes.

Finally there is the specification: the brick chosen to match the house, the amount and type of glazing, the kitchen, flooring, heating and electrics. Two extensions with the same footprint can differ a lot once those are chosen.

Costs by type of extension

A single storey extension cost is driven mainly by foundations, the roof and glazing. A double storey extension (two storey extension) has a larger footprint of foundations per room gained, but the upper floor adds bedrooms for comparatively less groundwork, which is why the per square metre rate in the calculator is different. Kitchen extension cost is pushed up by drainage, the steel beam to open into the existing kitchen and the kitchen fit-out itself.

Side return extension cost depends heavily on party wall work and the roof, especially a glazed roof, because the wall usually sits on the boundary. Wrap around extension cost combines a rear and side extension, with a more complicated roof and steelwork at the corner. Porch extension cost is lower overall but higher per square metre, because a small footprint still needs foundations, walls, a roof and a door.

Fees to budget for

Alongside the build, most projects need drawings, a structural engineer's calculations, a Building Regulations application, and sometimes a planning application and party wall agreements. These are separate from the builder's price, so include them in your budget from the start.