Single storey extension cost, and why it is decided underground first

How single storey extension cost works

1. Settle whether it is permitted development. A great many single storey 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 single storey 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

How deep do the foundations have to be?

Deeper than most owners expect, and deeper again near trees on shrinkable clay or over a drain. The depth is a design decision made from ground conditions rather than a standard, which is why a quote given without a trial hole is an estimate.

What if there is a drain or a manhole in the way?

Building over or near a public sewer needs the water company's agreement and often a diversion or a new access chamber. It is a real cost and a real delay, and it should be identified before you sign rather than discovered when the digger arrives.

Is a single storey extension permitted development?

Often, within depth, height and coverage limits that differ for detached and other houses, with a prior approval route for larger rear extensions. Designated areas and removed rights change the answer, so it is worth confirming rather than assuming.

How much does a single storey extension cost?

Its price is decided underground before it is decided anywhere else. Foundation depth against nearby trees and drains, whether a manhole has to be moved, and how the new floor meets the old one separate a straightforward job from an expensive one, and none of them shows in a drawing of the finished room, so ask each builder to price the same written specification.

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