Single storey timber frame extension

How far out you can build without planning permission

In England, a single storey rear extension is permitted development if it comes out 3 m or less from the original back wall of a terraced or semi-detached house, or 4 m or less on a detached one. It must be no taller than 4 m, and where it sits within 2 m of a boundary the eaves cannot be higher than 3 m. Side extensions are limited to half the width of the original house.

You can double those depths, to 6 m and 8 m, through the larger home extension route. The council consults your neighbours first and the fee is £249. Houses in conservation areas, national parks and areas of outstanding natural beauty do not get the larger allowance. None of this applies to flats or maisonettes, which sit outside permitted development altogether.

Wales follows broadly similar depth limits with its own detail. Scotland has separate householder classes, and the limits there turn on how close the extension comes to the boundary. In every nation, a lawful development certificate from the council puts the answer in writing; in England it costs £274.

The cladding catch

Permitted development comes with a condition that the outside materials are of a similar appearance to the existing house. A brick house with a larch-boarded extension fails that test, however good it looks, and so needs a householder planning application (£548 in England). Most are approved, but allow eight weeks.

If you want to stay inside permitted development, give the frame a brick outer skin, brick slips on a carrier board, or render where the house is rendered. The frame behind is the same either way.

One more limit applies whatever the planning route. A wall within 1 m of the boundary has to resist fire from both sides and can have only very small unprotected areas, so timber boarding and big windows belong on the garden elevation, not the wall that faces next door.

What the walls and roof are made of

A typical external wall, from inside to out: plasterboard, a vapour control layer, 140 mm studs at 600 mm centres packed with insulation, 9 mm structural sheathing board, a breather membrane, a ventilated cavity, then the cladding. To reach the 0.18 W/m²K that building regulations ask of a new extension wall in England, most makers add a continuous insulation layer of 40 to 60 mm across the inside face of the studs.

With boards or render on battens that wall is 250 to 280 mm thick overall. With a brick skin it is about 360 mm, slightly more than a modern masonry cavity wall.

Flat roofs are built warm: joists, a deck, insulation on top and a single-ply or rubber membrane over that. The target is 0.15 W/m²K, which usually means 120 to 150 mm of rigid insulation. Pitched roofs use trusses or cut rafters and need enough fall for the tile or slate you pick.

Where it meets the house

Timber and masonry move differently, so the new frame is never toothed rigidly into old brickwork. The end studs are fixed to the house wall through a damp-proof strip, a sealed vertical joint runs up the outside, and a lead or proprietary flashing with a cavity tray above keeps water out where the new roof meets the old wall.

The opening into the house is formed late, after the shell is dry. It needs a steel or concrete lintel sized by a structural engineer, and building control will want the calculations. Until that day your existing back wall stays shut and the house stays warm.

Glazing, and the 25% rule

Extensions are allowed windows, doors and rooflights up to 25% of the new floor area, plus the area of any openings the extension covers up. A 24 m² room that swallows an old 3 m² patio door can therefore have 9 m² of glass without further sums.

Go beyond that and your designer has to show the whole extension loses no more heat than a compliant one would, usually by improving the walls, floor or roof. It is routine paperwork, but raise it before the frame is drawn, because thicker insulation changes the panel design.