Concrete slab under a garden room
Priced on our reference 4 m × 3 m insulated office: the base stage costs £428 in materials, of a £9,423 whole-build materials total.
£428base-stage materials
£9,423whole build, ex VAT
£1,033/m²per internal m²
What is in the base stage
| Item | Spec | Cost |
|---|---|---|
| MOT Type 1 sub-base | 2.5 t – 100 mm compacted under the slab | £113 |
| Formwork timber + pegs | Slab perimeter 14 m, e.g. 25×150 boards | £34 |
| Ready-mix concrete | Concrete slab, 150 mm | £270 |
| Damp-proof membrane (DPM) / ground barrier | 1200-gauge polythene, footprint | £12 |
When to choose it
For: Solid, thermally massive, doubles as the finished floor with the right build-up.
Against: Dig + spoil + cure time (days), hardest to reverse, needs good access for concrete.
Compared with the alternatives (same 4×3 build)
| Base | Base stage | Whole build |
|---|---|---|
| Ground screws | £588 | £10,230 |
| Concrete slab | £428 | £9,423 |
| Concrete pad foundations + bearers | £788 | £10,430 |
Twenty Paces is a planning aid, not professional structural or planning advice. Costs are materials-only at indicative merchant rates (excluding labour and delivery); span tables, U-values and regulations thresholds are indicative. Verify your design with a structural engineer, your local planning authority and Building Control before you build.