The short answer
After a house is demolished, its foundations can be left in the ground, broken out and removed, or in some cases reused — the right choice depends on what the site is for next. If nothing is being built immediately, foundations are sometimes left in place. If you want a clear, level building plot, the slabs and footings are usually grubbed out and removed, which is heavy work and often a separate cost from the demolition itself. Reusing old foundations for a new building is possible but not automatic: a structural engineer must confirm they suit the new design and ground conditions, and building control will want to be satisfied. New designs rarely match the old footprint and loadings, so foundations are frequently replaced. The decision affects both cost and how the next project starts.
The foundations are the part of a demolition people think about least, yet what happens to them shapes the cost and the next build. Here are the options.
Foundations after demolition
- Option 1Leave in place
- Option 2Break out and remove
- Option 3Reuse (if suitable)
- Grubbing outOften a separate cost
- Reuse checkEngineer + building control
The three options
What you do with the foundations depends on the site's next use. The table summarises the options and when each applies.
| Option | When it applies | Cost effect |
|---|---|---|
| Leave in place | No immediate build; ground stable | Lowest — no extra work |
| Break out and remove | Clear, level plot wanted | Adds grubbing-out cost |
| Reuse for new build | New design matches and suits | Survey + checks needed |
Indicative summary for guidance only. Sources: NHBC and building control foundation guidance; Checkatrade demolition cost guide, 2026.
Leaving them in or digging them out
- Leaving in place: if no build is planned soon, or the area is being landscaped over, foundations may simply be left. This is the lowest-cost route as it avoids grubbing out and the associated waste.
- Breaking out (grubbing out): for a clear, level building plot, the ground-floor slab, footings and any below-ground structure are excavated and removed. This is heavy work generating significant waste, and it is frequently quoted separately from the demolition of the structure above.
- Part removal: sometimes only the parts that obstruct the new foundations are removed, with the rest left, depending on the new design.
Reusing old foundations
Reusing existing foundations for a new building can save money and avoid excavation, but it is never automatic. A new house or extension usually has a different footprint, layout and loadings than the building that stood there, so the old foundations may be in the wrong place or not strong enough. A structural engineer must assess whether they suit the new design and the ground conditions, and building control will need to be satisfied that the foundations are adequate. In practice, foundations are often replaced rather than reused, because the cost and risk of proving and adapting old ones can outweigh the saving. Where the old footprint and the new design genuinely line up and the foundations are sound, reuse is possible — but it is a decision for the engineer, not an assumption.
Deciding what to do with the foundations
The sensible order is to fix the site's next use first, then choose the foundation strategy that serves it at the lowest cost and risk. Treating foundations as an afterthought is what leads to either paying to dig out concrete you could have left, or discovering buried footings that obstruct a new build.
- Settle the next use early: a clear building plot, a landscaped garden, or an immediate rebuild each point to a different foundation decision, so decide this before the demolition is quoted.
- Make grubbing out an explicit line: if you want a level plot, confirm foundation removal is in the quote rather than assumed, since it is a common and costly exclusion.
- Get an engineer's view before reusing: never assume old foundations will serve a new design; have them assessed against the new loadings and ground conditions, with building control involved.
- Consider a site investigation: for a rebuild, the ground conditions drive the new foundation design, so investigating them informs whether reuse is even worth exploring.
- Plan the waste: grubbing out generates heavy rubble, much of which can be crushed and reused on site, reducing haulage and tipping.
Decide the destination of the site and the foundations together, and the demolition quote can be scoped accurately from the start. Leave the question open, and the foundations tend to become the surprise that turns a tidy demolition into an unexpectedly large excavation bill.
Foundations and a new build on the same plot
Where the demolition is the first stage of a rebuild, the question of what happens to the old foundations is bound up with the design of the new ones, and getting the two to work together is what keeps the groundworks efficient. The old foundations are rarely a simple asset you can take or leave.
- New designs change the loads: a new house or extension usually has a different footprint, layout and weight distribution, so the old foundations may be in the wrong place or not strong enough for the new structure.
- Ground conditions drive the new design: a site investigation establishes the soil type and bearing capacity, which determines the type and depth of the new foundations — strip, trench-fill, raft or piled — independently of what was there before.
- Old foundations can obstruct new ones: if left in place, old footings can get in the way of the new foundation line, so they often have to be removed even if you had hoped to leave them.
- Reuse needs proof: where the old footprint and the new design genuinely align, reusing foundations is possible, but a structural engineer must demonstrate they are adequate and building control must be satisfied.
- Coordination saves money: planning the demolition, foundation removal and new groundworks together lets the site be cleared to exactly the condition the new foundations need, avoiding duplicated excavation.
In practice, foundations on a rebuild are more often replaced than reused, because the cost and effort of proving and adapting old ones can outweigh the saving, and because the new design rarely matches the old. That is not a failure of planning but a normal outcome of building something different on the plot. The key is to treat the old foundations as part of the groundworks decision for the new build, informed by the site investigation and the engineer's design, rather than as a separate question settled at the demolition stage. Handled together, the removal of the old and the laying of the new flow as one coordinated groundworks operation, which is both cheaper and more reliable than dealing with each in isolation.
Frequently asked questions
Are foundations always removed when a house is demolished?
No. Many demolitions cover the structure above ground only, leaving the foundations in place. They are broken out (grubbed out) when you want a clear, level building plot, which is usually a separate cost. Confirm what your quote includes.
Can I build a new house on the old foundations?
Sometimes, but not automatically. A structural engineer must confirm the old foundations suit the new design, footprint and ground conditions, and building control must be satisfied. New designs often differ enough that foundations are replaced rather than reused.
What does 'grubbing out' the foundations cost?
It is heavy excavation work that generates significant waste, so it is often quoted separately and can add a meaningful amount to the demolition. The exact figure depends on the size and depth of the foundations and how the waste is handled. Get it itemised.
Sources & further reading
- Checkatrade — demolition cost guide
- Planning Portal — foundations and building regulations
- MyJobQuote — house demolition cost guide
Figures on this page are typical UK ranges drawn from published sources and depend on your specific building. They are guidance, not a quotation.