Side extension cost: what the boundary adds to a narrow build

How side extension cost works

1. Settle whether it is permitted development. A great many side 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 side 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

Can I build right up to the boundary?

Sometimes, and it changes the construction. Building to the line usually means a parapet or a clipped eaves detail rather than an overhang, and the Party Wall Act applies to excavation near a neighbour's foundations even where you are not touching their wall.

What are the permitted development limits?

For a single storey side extension, broadly no wider than half the original house, single storey, within a height limit, and in materials similar to the existing. Designated areas and flats are treated differently.

How do I keep access down the side?

You often cannot keep the full width, so the design should say how bins, bikes and rear access will work afterwards. It is the practical thing people miss until the wall is up.

How much does a side extension cost UK-wide?

There is no single figure, because the boundary decides the build. Building up to a line means scaffolding, guttering and maintenance access have to be resolved, party wall notices are almost certain, and permitted development limits the width and sets conditions on height and materials, so settle those before comparing quotes against one written specification.

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