The short answer
Usually the same concrete strip footings as a brick extension. A timber frame is lighter, which can allow a narrower footing, but depth is set by the ground and not by the weight above it. Expect at least 750 mm in clay, often 1 m, and more near trees. The part that is different is accuracy, because the frame is cut to the drawing before the base exists.
Depth comes from the ground
A footing has to reach soil that will not move. Two things move soil near the surface: frost, and clay that swells when wet and shrinks when dry. Approved Document A gives 450 mm as the least depth to stay clear of frost, and 750 mm as the least in clay. Building control officers in clay areas commonly ask for 1 m. Where trees stand near the extension, the roots dry the clay further down and the depth is worked out from the species, its mature height and its distance. It can exceed 2 m.
None of that changes because the walls are timber. A light building on shallow footings in shrinking clay cracks just as a heavy one does.
What the lower weight can change is width. Approved Document A sizes strip footings by the load per metre of wall and the type of soil. A single-storey timber frame wall carries less than a brick and block one, so the table may allow a 450 mm wide strip where masonry would need 600 mm. On a small extension that saves a few barrowloads of concrete. If your frame has a brick outer skin, the skin sits on the footing too and the saving goes.
Strip, trench fill, raft or piles
Strip footings are a layer of concrete at the bottom of a trench, with blockwork built up to ground level. They are the standard answer in firm ground.
Trench fill means filling the trench with concrete almost to the top. It uses more concrete and less labour, and nobody has to lay blocks in a narrow trench. Many groundworkers prefer it for small jobs.
A raft is a reinforced slab under the whole extension, used where the ground is soft or variable. It needs an engineer’s design.
Piles carry the load down to firm ground through soil that cannot be trusted, such as deep fill or clay near large trees. Screw piles, wound in by a small machine, suit light timber buildings well and can be installed in a day with little spoil. They also need an engineer’s design, and not every building control body is equally familiar with them, so raise it early.
National price guides put strip foundations for a typical single-storey extension at £1,000 to £3,000 and a raft at £3,000 to £6,000. Piles are priced by the engineer once a ground investigation is done, and can add several thousand pounds more. These are guide figures, not quotes.
Level and square: the part that is different
A bricklayer corrects a slightly uneven footing in the first courses without anyone noticing. Timber frame extension kits cannot do that. The panels were cut in a factory to the dimensions on the drawing, and they expect the base to match.
Every supplier sets a tolerance for the base. Ask for it in writing and give it to your groundworker before work starts. Then check three things yourself before the delivery date is fixed:
- the length of each side, against the drawing;
- both diagonals, which should be equal if the base is square;
- the level at each corner and the middle of each side, using a laser level or a long straight edge.
Small errors in level are packed out under the sole plate. Errors in size or squareness are harder: a base that is too small means the frame overhangs, and one out of square means panels do not meet at the corners.
The sole plate and the damp-proof course
Timber must not sit in damp. The sole plate, the treated timber the wall panels stand on, goes on a damp-proof course at least 150 mm above the finished ground outside. In most extensions that height comes from a low plinth of two or three courses of brick or dense block. The plinth also lines the new floor up with the floor of the house.
The plate is fixed down to resist wind lifting the building, with straps or bolts at spacings given in the frame calculations. Your groundworker needs that drawing before building the plinth, because some fixings are cast or built in.
Drains and neighbours
Dig a trial hole before anything is ordered. It shows the soil, the depth of the house’s own footings and, often, a drain nobody knew about.
A footing within 3 m of a public sewer needs the water company’s consent, and that consent often comes with a condition that the footing goes deeper than the pipe. It is a separate application from building regulations and takes weeks, not days. Our post on the build over agreement covers it.
If you will dig within 3 m of a neighbour’s building and go deeper than its foundations, the Party Wall etc. Act 1996 requires written notice to that neighbour at least a month before you start. On terraced and semi-detached houses this applies to most rear extensions.
Where the new footing meets the house
The house has had decades to settle. The extension has not. However good the new foundations are, the two will move slightly differently, and a rigid joint between them will crack. Frame suppliers detail a movement joint where the new wall meets the old, and the ties holding the frame to the house are slotted so they can slide a little. Do not let anyone fill that joint with mortar.