Rear Extensions

Under permitted development a single-storey rear extension may project 3 m from the original rear wall of a terraced or semi-detached house and 4 m from a detached house, with a maximum height of 4 m (3 m at the eaves within 2 m of a boundary). Through the larger home extension prior-approval route, those depths increase to 6 m and 8 m: you notify the council, neighbours are consulted for 21 days, and the council has 42 days to confirm whether prior approval is needed and granted. Extensions on designated land, on flats, or beyond these limits need a planning application. Note that a rear extension on a house with an existing outrigger is measured from the outrigger's rear wall on that part of the house, which is why side-return and wraparound schemes on terraces are often planning applications rather than PD.

Source: Planning Portal, extensions — planning permission; GPDO 2015 Schedule 2 Part 1 Class A.

Opening the rear wall means a steel beam (often two, with a "goalpost" frame where the side wall is also removed) sized by a structural engineer and bearing on padstones or new piers. Foundations in Barnet and Harrow clay are typically 1 m or more deep, and deeper near trees under building control's foundation-depth guidance; where a drain runs under the extension it is either diverted or bridged with a build-over agreement from Thames Water. Roofs are usually a warm flat roof with a single-ply or GRP membrane and one or more rooflights, or a low-pitched roof to match the house.

Sliding doors give bigger panes and a cleaner look; bi-folds open the whole wall but stack in a bundle at one end. Both should be thermally broken aluminium with a threshold detail that keeps water out and lets the floor run level to the terrace. Rooflights over the old rear wall line stop the middle of the house going dark. Where the extension faces south or west, specify solar-control glass or external shading — Building Regulations and common sense both point the same way.

10 to 16 weeks is typical: two weeks of groundworks and foundations, three to four weeks of structure and roof, then glazing, first fix, plastering, second fix, kitchen and decoration. We hoard the site from the house so you can live in it, and we keep the old kitchen running until the new one is ready wherever the layout allows. Skips, scaffold and deliveries are arranged around parking restrictions on the street.