What extension can I build on a semi-detached house in London?

Side extension on the open side

The side wall is the semi's advantage. Paragraph A.1(j) allows an extension beyond a side wall if it is one storey, no more than 4 m high and no wider than half the width of the original dwellinghouse. On a semi that is 7 m wide, the side extension can be up to 3.5 m wide, which is often wider than the side passage itself. Because the new wall usually ends up within 2 m of the side boundary, paragraph A.1(i) also limits the eaves to 3 m there.

Converting the side garage into a room, or replacing it with a new side extension, is one of the most common semi projects. Our single-storey side extension drawings guide sets out the full drawing set for that job. A two-storey side extension is never permitted development, so it goes in as a householder planning application, and some councils' design guides ask for it to be set back from the front and down from the main ridge so the pair still reads as a pair.

Two-storey rear extension and the 7 m rule

A two-storey extension at the back of a semi can be permitted development, but the limits are tight. Paragraph A.1(h) rules it out if it would extend beyond the rear wall of the original dwellinghouse by more than 3 metres, or be within 7 metres of any boundary of the curtilage opposite the rear wall. Government guidance confirms that this applies whether the extension is new or the addition of a storey onto an existing single storey extension.

The 7 m is measured to the back fence, so a semi with a short garden may fail it even at 3 m deep. Upper-floor side windows must also be obscure-glazed. Where a two-storey back extension fails any of these tests, the plans go to the council as a householder application, and the officer looks closely at the light and outlook from the attached neighbour's nearest windows.

If a single-storey extension is already there, adding a storey on top of it explains how these limits apply.

Loft and wraparound options on a hipped-roof semi

Most London semis have a hipped roof, and the usual loft move is a hip-to-gable with a rear dormer. Government guidance allows up to 50 cubic metres for semi-detached and detached houses of extra roof space, against 40 for a terrace, which is often enough for a third bedroom and a shower room. Our loft conversion drawings guide explains the Class B tests.

A wraparound joins a rear and a side extension into one L-shaped room. On a semi it is common, but the corner piece is judged under both the rear and side limits at once, so it nearly always needs a householder application. The wraparound extension guide covers that case.

The attached neighbour and the party wall

On a semi you have one neighbour you share a wall with and one you do not. Building a new wall on the line of junction, cutting into the shared wall for a steel beam, or raising it for a two-storey extension are all work under the Party Wall etc. Act 1996. Section 3 says a party structure notice must be served at least two months before the date on which the proposed work will begin.

That notice is separate from planning and from building regulations. The same attached neighbour may be consulted on a prior approval application and then receive a party wall notice for the same extension, so it helps to talk to them early with the drawings in hand. Building regulations approval is needed for every extension, whichever planning route it takes; our building regulations drawings cover the structure, insulation and drainage detail the builder works from.