What do house foundations do?
A house's foundations transfer the weight of the building — the walls, floors, roof and everything in it — safely down into ground that is stable enough to carry it. They spread that load over a wide enough area, and reach down to firm enough material, that the structure sits still rather than sinking, tilting or cracking over time.
Get the foundations right and you never think about them. Get them wrong — or build on ground that later moves — and the effects show up throughout the house above, in the walls, floors and openings. That's why foundations matter so much to buyers, even though you can't see them.
Common types of house foundation
Different sites and structures call for different foundation types. The most common in residential construction are:
- Strip foundations — a continuous strip of concrete running beneath load-bearing walls, spreading their weight along the ground. This is the traditional, everyday foundation for many houses on reasonable soil.
- Trench-fill foundations — a variation on the strip, where a deeper, narrower trench is filled with concrete almost to the surface. It's quicker to build in some ground and common on modern homes.
- Raft foundations — a single reinforced concrete slab covering the whole footprint of the building, spreading the load evenly. Used where the ground is weaker or where a wide, uniform spread is helpful.
- Pad foundations — isolated square or rectangular pads that support individual columns or piers rather than a continuous wall, often seen in framed structures and some extensions.
- Pile foundations — deep columns driven or bored into the ground to reach firmer material well below the surface. Used where good bearing ground is deep, on poor or made-up soils, or near trees.
How deep are house foundations?
There is no single correct depth. Traditional strip foundations for a house are often somewhere in the region of around a metre deep, but that is a rough starting point, not a rule — the real answer always depends on the ground.
Soil type is the biggest factor: shrinkable clay soils, which swell and shrink with moisture, generally need deeper foundations than stable gravels. Nearby trees and vegetation draw water from the ground and can call for greater depth still. Made-up or previously disturbed ground, slopes and local conditions all feed in too. Because of this, depth is judged site by site rather than assumed.
Signs of a foundation problem
Because foundations sit out of sight, problems usually announce themselves in the structure above. Common signs include:
- new or widening cracks in walls, especially diagonal cracks running from the corners of doors and windows;
- doors and windows that start to stick, jam or no longer close squarely;
- sloping or uneven floors, or a noticeable lean in a wall;
- gaps opening up where walls meet floors, ceilings or an extension.
Not every crack means trouble — many are cosmetic or the result of normal, long-finished settlement. But these patterns can indicate ongoing subsidence (the ground moving away beneath the foundations) or settlement, and they warrant proper investigation rather than guesswork. A property defects report is one way to focus in on a specific concern like this.
What a survey checks — and its limits
It's important to be clear about what a survey can and can't do here. The foundations themselves are below ground and cannot be seen, so a surveyor does not inspect them directly. Instead, the survey assesses the visible, above-ground evidence — the cracking patterns, distortion and movement described above — and forms a professional view on whether the structure appears stable or whether something is going on.
Where a surveyor suspects active movement, the right step is a clear recommendation for further investigation, rather than a definitive verdict on the foundations. That might mean a structural engineer's assessment, monitoring over time, or trial holes dug to expose and examine the actual foundations. A good survey tells you plainly what has been seen, what it might mean, and what needs looking into next.
For a property where movement is a real concern — an older, altered or previously cracked home — a Level 3 Building Survey, from £495, is the appropriate level, because it looks in the most detail at the structure and reports fully on any evidence of movement. If you're weighing the options, our guide to how much a survey costs sets out the levels and starting prices so you can match the survey to the property.
Key takeaways
- Foundations transfer the building's load safely into stable ground below.
- The main foundation types are strip, trench-fill, raft, pad and pile.
- Typical depth varies with soil, trees and ground conditions — there's no single figure.
- Cracking, sticking doors and windows, and sloping floors can signal foundation movement.
- A survey assesses visible, above-ground evidence — not the buried foundations themselves — and recommends specialist investigation where needed.
Frequently asked questions
What are the main types of house foundations?
The most common residential foundation types are strip and trench-fill foundations (a continuous concrete base under load-bearing walls), raft foundations (a single slab spreading the load over the whole footprint), pad foundations (isolated pads under columns or piers) and pile foundations (deep columns that reach firmer ground). The right type depends on the soil, the structure and the ground conditions on the site.
How deep are house foundations?
There's no single depth. Traditional strip foundations for a house often sit somewhere in the region of around a metre deep, but this varies considerably with the soil type, the presence of nearby trees, made-up ground and local conditions. Clay soils and trees generally call for deeper foundations, so depth is always judged case by case rather than to a fixed rule.
What are the signs of foundation problems?
Warning signs include new or widening cracks in walls (particularly diagonal cracks near doors and windows), doors and windows that begin to stick or won't close squarely, sloping or uneven floors, and gaps opening up between walls, floors or extensions. These can indicate subsidence or settlement and warrant proper investigation rather than assumptions.
Does a survey check the foundations?
A survey assesses the foundations indirectly. The foundations themselves are below ground and can't be seen, so a surveyor inspects the above-ground evidence — cracking patterns, distortion, movement — and forms a view on whether the structure appears stable. Where movement is suspected, the surveyor recommends further investigation, such as a structural engineer or trial holes, rather than confirming the foundations directly.
Can you underpin a house?
Underpinning is one recognised way of strengthening or extending foundations that are no longer performing, generally by deepening or widening them so the load reaches firmer ground. It is a significant structural undertaking and isn't always the right or only remedy, so any decision should follow proper diagnosis by a suitably qualified engineer rather than being assumed from the symptoms alone.
This guide is general information, not advice on a specific property. Every building differs — for findings specific to a property you are buying, book a survey.