Terraced house extensions

Three shapes that suit a terrace

Rear extension. Straight out from the back of the house, across part or all of the plot width. On a flat-fronted terrace with no outrigger this is the natural choice: often 3 metres deep and 5 to 6 metres wide, around 15 to 18 m².

Side return. On terraces with a back addition, the narrow yard beside it. Small floor area, big change to how the ground floor works. See side return extensions.

Wraparound. The side return and a rear extension in one L-shape, giving a kitchen, dining and living space across the full width. The most expensive of the three, and almost always a planning application.

What you can build without planning

Terraced houses have the tightest permitted development limits. A single-storey rear extension can project 3 metres beyond the original rear wall and stand up to 4 metres high. Within 2 metres of a boundary, the eaves can be no higher than 3 metres, which matters on a narrow plot because the whole extension is within 2 metres of a boundary. Materials must look similar to the house.

The larger home extension scheme lets you go to 6 metres after a prior approval application (£249), in which the council asks your neighbours for comments. It does not apply in conservation areas, and flats and maisonettes have no permitted development rights at all. Many London terraces have been split into flats, so check the title before anything else.

Two neighbours, two party walls

A full-width rear extension on a mid-terrace means digging foundations within 3 metres of the houses on both sides, often against both garden walls. Each neighbour gets a notice under the Party Wall etc. Act 1996, and each may agree, or appoint a surveyor. Budget £700 to £1,500 per neighbour with one agreed surveyor; you pay for theirs too if they choose their own.

A schedule of condition (a record of the neighbour’s rooms before work starts, typically £200 to £600) protects you as much as them.

Drains under the garden

Many London terraces share one sewer that runs across the back gardens, connecting every house in the row. If the extension goes over or within 3 metres of a public sewer, you need the water company’s agreement before building. In most of London that is Thames Water. The check is quick; a diversion is not, so find out early.

Costs for a London terrace

As London guides including VAT, before fees: a 15 m² rear extension £38,000 to £69,500, an 18 m² rear extension £45,500 to £83,000, and a 28 m² wraparound £70,500 to £129,500. The spread covers finish and how hard the street is. Use the cost calculator for your own size.