Extension cost calculator UK: what any calculator has to assume about your plot

How extension cost calculator uk works

1. Settle whether it is permitted development. A great many house extension are permitted development, which means no planning application, but the limits on depth, height, materials and how much of the plot is already built on are specific and a designer should read them against your own house before anybody prices anything.
2. Get drawings and a structural design. Existing and proposed drawings, then a structural engineer's calculations for whatever is being opened up or carried. On a house extension this is a small share of the total and it is what turns an idea into something three builders can price identically.
3. Serve the party wall notices in time. Work on or near a shared wall, or excavation within three or six metres of a neighbour's foundations, engages the Party Wall etc. Act and the notice periods are set in months rather than weeks. Serving late is the commonest reason a start date slips.
4. Price it against a specification, not a sketch. Ask each builder to price the same written specification, with provisional sums shown as provisional sums. Two quotes that differ by a third almost always differ in what is included: the kitchen, the flooring, the decoration and the drainage diversion are the usual absentees.
5. Agree stages, retention and building control. A staged payment schedule against completed work, a small retention until sign-off, and a clear statement of who is notifying building control. The completion certificate is what a future buyer's solicitor will ask for.

Insurance and paperwork

Common questions

Why separate the groundwork from the house extension cost per m2?

Because it is the line that varies most and the line you can do least about. Trees, made ground, a shallow water table or a sewer crossing the site all change it, and none of them appears in a floor area.

What percentage should I allow for fees?

Enough for drawings, structural calculations, the planning or certificate application, building control and, where notices are served, a party wall surveyor. Ask each of those professionals for their own fee rather than applying a rule of thumb.

Should I hold a contingency?

Yes, and hold it separately from the builder's price. Ground conditions and existing services are where surprises live, and a contingency that is already spent on a nicer kitchen is not a contingency.

What can an extension price calculator not see?

The two things that most often blow a budget: foundation depth and drainage. A calculator multiplies floor area by a rate per square metre and adds a percentage for fees, and neither of those lines scales with floor area, so treat its answer as a first sketch rather than a price.

Registered contractors by region

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