In short
The same kinds as a brick extension. Trench-fill or strip footings on firm ground, a raft or piles where the ground is poor. Depth is set by the soil, not the weight of the walls, so expect at least 450 mm, 750 mm to 1 m in clay, and more near trees.
On this page
People often hope that a lighter building means a cheaper hole in the ground. With a timber frame extension that is only partly true, and it helps to know which part before you compare quotes.
Lighter walls, same depth
A timber stud wall with board cladding weighs a fraction of a brick and block cavity wall. The load on the footing is lower, so the concrete can sometimes be narrower.
Depth is another matter. Foundations go down to reach ground that will not move, and what makes ground move is frost, and clay that swells and shrinks as it wets and dries. Neither cares what the walls are made of. Building control will ask for the same depth under a timber frame as under blockwork on the same plot.
One caveat. If the frame is to have a brick outer skin, the footing must be wide enough to carry that brickwork as well as the frame behind it. At that point the foundation is, in effect, a masonry one.
The four types
| Type | Where it is used | Typical cost, single storey |
|---|---|---|
| Trench fill | Firm ground. A narrow trench filled almost to the top with concrete | £1,000 to £3,000 |
| Strip | Firm ground. A shallow layer of concrete with blockwork built up from it | £1,000 to £3,000 |
| Raft | Soft, filled or variable ground. A reinforced slab under the whole room | £3,000 to £6,000 |
| Piles with ground beams | Shrinkable clay near trees, deep fill, high water table | Priced by the engineer after a ground investigation; can add several thousand pounds |
The costs are typical national ranges for a room of about 20 to 25 m², including VAT. London and the South East usually cost 20 to 40% more. Read them as a guide, not a quote.
Trench fill is the usual choice for a rear extension. It needs no bricklayer below ground and the trench is open for a day, which matters when it is a metre from your neighbour’s wall.
How deep is deep enough?
Approved Document A, the official guidance on structure in England, gives the starting points. Footings should be at least 450 mm below finished ground so that frost cannot get under them. In clay the minimum is 750 mm, and it rises with how shrinkable the clay is: NHBC’s standards use 750 mm, 900 mm and 1 m for low, medium and high shrinkage.
Trees change everything. Roots draw water out of clay and the ground sinks as it dries. A willow, oak or poplar within its own height of the extension can push the required depth past 2 m, and at about 2.5 m most engineers switch to piles. Tell your designer about every tree and large shrub nearby, including ones that were recently cut down. Clay swells again when a tree goes.
The inspector has the last word. They look at the open trench before any concrete is poured, and if the soil at the bottom is not what the drawings assumed, you dig further.
The kerb and the sole plate
This is the part that is specific to timber frame. The lowest timber in the wall, the sole plate, must stay dry for the life of the building. So the frame never sits straight on the slab at ground level.
Instead, two or three courses of brick or dense block are built up from the foundation to form a kerb. A damp-proof course goes on top, and the treated sole plate is fixed down onto it with straps or anchor bolts. The finished plate should be at least 150 mm above the ground or paving outside. Keep that gap for good: do not let a later patio or flower bed creep up the wall.
Level matters more than it does for masonry. A bricklayer can lose 10 mm in the mortar joints. Factory panels cannot, so frame makers ask for the kerb to be level and square to within a few millimetres and will measure it before they deliver.
Can it sit on ground screws or pads?
For a small, single-storey, timber-clad room, sometimes. Steel ground screws or concrete pads carry a timber ring beam and a suspended, insulated timber floor, with a ventilated void beneath. There is very little digging and no wet concrete.
It is not a way round the rules. A habitable extension still needs building regulations approval, a structural engineer has to design the screws for your soil, and not every building control body will accept them on clay. They cannot carry a brick skin. Ask the question early, before a design depends on it.
Drains and neighbours
Two things beside the soil can move a foundation.
Drains first. If a public sewer runs within 3 m of the new footings you need a build-over agreement from the water company, and the foundation is usually taken down below the pipe or bridged across it. Find the manholes before the design is fixed.
Then the Party Wall etc. Act 1996, which applies in England and Wales. Digging within 3 m of a neighbour’s building, to a level lower than its foundations, needs written notice at least a month ahead. Old houses often have footings only 300 to 500 mm deep, so nearly every new extension foundation qualifies.
What goes on top
Inside the kerb, the floor is normally a concrete slab over insulation, or concrete beams and blocks with insulation and screed above. Both are the same as in a masonry build and both need a damp-proof membrane that laps up to the wall’s damp-proof course.
The slab can be poured while the panels are still being made. That overlap is where a timber frame gets its head start.
If you want a figure for your own plot, the cost calculator lets you switch between firm ground, soft ground and clay near trees. Or send the postcode through the quote form below and let a frame firm look at it.