What actually drives the cost of a house extension

Groundworks and foundations set the tone

Foundation design depends on soil type, proximity to trees, and any slope across the site. A structural engineer or building control officer will specify what is needed, and that specification drives excavation depth, concrete volume, and how long groundworks take. This stage is rarely visible in the finished building but often accounts for a significant share of the early programme.

Access also plays a part. A tight side passage in a Torquay terrace might mean barrowing concrete and spoil by hand rather than using a pump or machine, which changes labour time even though the extension itself is modest.

Structure, openings and roof form

Steel beams, especially where an extension opens into the existing house, add cost through both the steel itself and the design work behind it. A flat roof extension behaves differently to a pitched one in terms of materials and detailing, and each has knock-on effects for insulation and weatherproofing.

Where an extension needs to tie into an existing roofline, working out flashings, valleys and junctions with the original structure takes skill and time that a straightforward new-build extension would not require.

Specification of finishes

Once the shell is weathertight, the specification you choose for windows, doors, flooring, and internal joinery has a real effect on the final figure. Bifold doors, for example, sit at a different price point to a single set of French doors, and that choice ripples through the frame and lintel design too.

It helps to separate structural essentials from finish choices early on, so you know which parts of the budget are fixed by the building itself and which are flexible depending on what you specify.

What a quote should include and exclude

A clear quote should state what is covered: labour, named materials, waste removal, and site management. It should also be explicit about exclusions, such as kitchen units, decorating, or external landscaping, so nothing is assumed on either side.

• Groundworks, foundations and drainage connections
• Structural elements including steel and roof structure
• First and second fix labour
• External finishes such as render, brick or cladding
• Named allowances for items like windows and doors

Provisional sums and how they work

Where an exact cost cannot be fixed in advance, such as an unknown drain run or a decorative finish you have not yet chosen, a builder may include a provisional sum. This is an estimated allowance, not a guaranteed final figure, and it should be clearly marked as such in the quote so you can track it separately once the real cost is known.

Common questions