The short answer
A concrete base — excavated, with sub-base, formwork and slab — typically costs around £150–£250 per square metre in the UK, depending on thickness, reinforcement and access. As a worked example, a single-storey extension base of around 20m² (4m × 5m) often totals roughly £4,680–£9,500, commonly around £6,000–£7,500 outside London and the South East, once excavation and spoil removal, trench-fill or footings, hardcore sub-base, the oversite slab and a basic drainage allowance are added together. Per-m² is a useful guide for the slab itself, but foundations are usually priced per linear metre of trench, so most groundworks quotes mix the two.
'Per square metre' is handy shorthand for a concrete base, but groundworks rarely prices on area alone. Here is what the m² figure covers, a worked extension example, and why your quote will also show per-metre items.
Typical UK figures
- Concrete base~£150–£250 / m²
- 20m² extension base~£4,680–£9,500
- Typical mid (ex-London)~£6,000–£7,500
- Excavation & spoil (20m²)~£800–£2,200
- Basic drainage allowance~£400–£900
What the per-m² figure covers
A per-square-metre base price usually bundles the excavation, hardcore sub-base, any reinforcement and the concrete slab for a defined area. The £150–£250/m² range moves with slab thickness, whether mesh or rebar is needed, and how easily concrete and plant can reach the pour. It does not automatically include the foundations to the walls, the drainage connections or carting spoil off site — those are often separate lines, which is why a base 'price per m²' and the full groundworks figure are not the same thing.
| Item (20m² extension) | Typical figure | Notes |
|---|---|---|
| Excavation & spoil removal | ~£800–£2,200 | depends on volume & tip distance |
| Trench-fill / footings | ~£1,700–£3,200 | varies with depth |
| Hardcore sub-base | ~£380–£700 | compacted MOT type |
| Oversite slab | ~£1,100–£1,900 | thickness & mesh |
| Basic drainage | ~£400–£900 | connections allowance |
Indicative build-up for a 20m² single-storey extension base, excluding the London/SE premium. Source: ConcreteMath extension base cost data.
Why quotes mix per-m² and per-metre
The slab is a flat area, so it makes sense per square metre. The foundations are trenches around a perimeter, so they make sense per linear metre. Drainage is per run, and muck-away is per tonne or per load. A clear groundworks quote therefore reads as a build-up of these units rather than one headline rate — which is a good sign, because it shows the contractor has measured the job rather than guessed a round number.
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Frequently asked questions
How much do groundworks cost per square metre in the UK?
A concrete base — excavated, with sub-base and slab — typically costs around £150–£250 per square metre, depending on thickness, reinforcement and access. Foundations themselves are usually priced per linear metre rather than per m².
How much does a 20m² extension base cost?
A single-storey extension base of around 20m² often totals roughly £4,680–£9,500, commonly around £6,000–£7,500 outside London and the South East, once excavation, footings, sub-base, slab and basic drainage are added.
Why isn't groundworks just priced per square metre?
Because different parts price on different units — the slab per m², the foundations per linear metre of trench, drainage per run and muck-away per tonne. A measured quote shows these as a build-up rather than one headline rate.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.