L-Shaped Extensions: Design, Roofs and Planning

When the L shape is the right answer

The shape suits houses where the existing kitchen sits in a narrow back addition with a passage beside it, which is common on period terraces and many semis. Filling the passage and extending across the rear lets the whole back of the house become one space rather than a string of small rooms.

It is less useful where the garden is short, where the side is already occupied by a garage, or where only a modest amount of extra space is needed. In those cases a simple rear or side return extension often gives most of the benefit for less complexity.

The roof junction is the critical detail

Where the side wing meets the rear wing there is an internal corner in the roof. With pitched roofs it becomes a valley; with flat roofs it becomes a change of level or a shared deck. Either way it has to collect a lot of water and send it somewhere, so gutter sizes, falls and the position of downpipes need designing rather than improvising on site.

Many L-shaped extensions use a flat roof on the wing and a pitched or glazed roof over the side, or one continuous flat roof with lanterns. The flat roof coverings guide compares the membranes that are commonly used.

Structure: opening up the old walls

The point of the shape is one open room, which means removing the back wall of the house and the side wall of the back addition. That usually needs steel beams, sometimes a goalpost frame, and padstones or posts carried down to foundations. A structural engineer designs it and building control inspects it.

The corner where the two old walls met is often where the heaviest load lands, and it is worth resolving early because it affects ceiling lines and where rooflights can go.

Planning permission and neighbours

Because a wrap-around extends both to the side and the rear, it normally falls outside permitted development and needs a householder planning application. The side wing often runs along a shared boundary, so a party wall notice is likely, and neighbours' light and outlook will be considered by the council.

The planning step-by-step guide on this site explains the application route and typical timescales.

Getting the layout and light right

The finished room is large, so zoning and daylight need as much attention as the structure.

Put the kitchen where the drains and existing services make it easiest, often in the side wing · Use rooflights or a glazed strip over the side wing, where walls give little daylight · Keep the dining area near the garden doors, where people naturally gather · Plan heating for a big open space; underfloor heating is common · Think about acoustics, since large hard-surfaced rooms can be echoey

What to budget for

No separate L-shaped price is published on this site. The extension cost per square metre guide and the house extension cost guide give the published ranges, and a wrap-around typically sits towards the more complex end because of the steelwork, roof junction and glazing.

Frequently asked questions

Does an L-shaped extension need planning permission?

Usually yes, because extending to both the side and rear normally falls outside permitted development.

What is the difference between an L-shaped and a wrap-around extension?

The terms are used for the same thing: an extension that wraps around the side and rear of the house in an L shape.

What is the hardest part of an L-shaped extension?

The roof junction where the two wings meet, and the structural steel needed to open up the old walls into one room.

How do you stop the middle of an L-shaped extension being dark?

Rooflights or a glazed roof over the side wing, and keeping large glazed doors on the garden side.