What are the different types of house foundation?

These are not five grades of the same thing, with a cheap one and an expensive one. They are answers to different ground. Choosing on cost is how people end up with a foundation that does not suit the site, and the failure appears years later when nobody remembers the decision.

Safety

Step by step

1. Strip: a band of concrete under a wall.The traditional form. A trench is dug to the required depth, a band of concrete is cast in the bottom, and masonry is built up off it to ground level. Most British houses built between the wars and the 1980s sit on something like this.
2. Trench fill: the same trench, filled.Rather than casting a thin strip and building up, the trench is filled with concrete almost to the surface. It uses far more concrete and far less bricklaying, and it is quicker, which is why it dominates on modern sites where deep foundations are needed anyway.
3. Raft: one slab under the whole building.A single reinforced slab, usually thickened into a beam around the edges and under internal load-bearing walls. It spreads the load over the entire footprint, so it suits weak, soft or variable ground where a strip would concentrate load onto a bad patch.
4. Pads and ground beams: load collected at points.Separate concrete pads take the load at intervals, with reinforced beams spanning between them to carry the walls. Common where the surface ground is unreliable but there is something firm at a workable depth, and standard for framed structures.
5. Piles: reaching for ground that will carry.Columns driven or bored down to firmer strata, or relying on friction along their length, with ground beams spanning between the pile heads. Used where good ground is deep, where trees have influenced a wide area, or on made ground and fill.
6. Notice that the form does not set the depth.A strip foundation on shrinkable clay near trees may need to be far deeper than a raft on gravel. The two questions are separate: what shape spreads this load onto this soil, and how deep does the underside need to be to sit below ground movement.
7. Look at what your house actually is before assuming.Extensions frequently have a different foundation from the house they are attached to, which is one reason a crack often appears exactly at the junction. A different foundation moving differently is not a defect in either — it is why movement joints exist.

Tips

• Trench fill wins on modern sites for a reason that has nothing to do with engineering: it removes a bricklayer working in a deep trench, which is slow and hazardous.
• Wide trench blocks exist specifically so that a single thickness of blockwork can fill the width of a foundation trench instead of building two leaves below ground. If you find very wide blocks below ground level, that is what they are.
• The junction between an old house and a newer extension is the most likely place in the whole building to find a crack, and the least likely place for it to mean subsidence.

Common mistakes

• Assuming a raft is a shortcut on bad ground — A raft is a designed, reinforced structure, not just a thick slab. An unreinforced or under-designed slab on soft ground cracks and the building follows it.
• Matching an extension foundation to what a neighbour did — Ground varies over very short distances, and the neighbour's site investigation, if there was one, was for their plot. Building control will want a design for yours.
• Treating the presence of concrete as proof of adequacy — Plenty of older foundations are concrete, shallow, and entirely fine because the ground has never changed. The depth relative to the moving layer is what matters, not the material.

Questions people ask