The Short Answer
On our estimates, a concrete base for a shed or small outbuilding costs about £60 to £80 per m² laid, so £800 to £1,000 for a 3m x 4m base. Small bases cost more per m². Laying takes two to three days, and it needs time to cure before you build on it.
That covers digging out, a compacted sub-base, formwork and the concrete. A garage slab is thicker and costs more, up to £60 to £140 per m². Bad access (everything barrowed through the house) or a sloping garden add to it.
Concrete base prices
| Base | Laid |
|---|---|
| Per m², shed or garden building | £60 to £80 |
| 2m x 3m (6m²) | £500 to £600 |
| 3m x 4m (12m²) | £800 to £1,000 |
| 4m x 5m (20m²) | £1,200 to £1,600 |
| Garage slab, per m² | £60 to £140 |
How a shed base is built
B&Q's guide to laying a concrete shed base sets out the usual method:
- Mark out a base the same size as the shed or slightly bigger, and dig out about 150mm for most sheds.
- Lay 75mm of MOT type 1 sub-base and compact it with a wacker plate. Concrete straight onto soil is likely to move and crack.
- Build timber formwork to the depth of the concrete, and lay a damp-proof membrane inside it, which helps stop condensation in the shed.
- Pour about 75mm of concrete, tamp it level and cover it with plastic so it doesn't dry too fast.
Heavier buildings need more: follow the shed, garden room or garage maker's base specification before anyone pours.
How long before you can build on it?
B&Q says to wait at least 24 hours, 48 if possible, before taking the formwork off and walking on it, and that concrete takes up to 28 days to reach full strength, so it's best to wait that long before building on it.
Planning and alternatives
A hard surface in the back garden of a house is permitted development. In the front garden, one over 5m² has to be porous or drain onto a permeable area. A raised platform over 0.3m isn't permitted development. The building that goes on the base has its own rules: see garden room planning.
Many garden room makers now include ground screws or a plinth system instead of a slab, so ask your supplier what they need before you pour anything. For extension foundations, see groundworks.
Get concrete base quotes
A concrete base is priced by area and thickness. Tell installers what it's for.
Free, no obligation. We're paid by MyJobQuote when a tradesperson buys your job, never by you.
Frequently asked questions
How much does a concrete shed base cost?
On our estimates, about £800 to £1,000 for a 3m x 4m base, or £60 to £80 per m², laid.
How much does a 12x12 concrete slab cost in the UK?
A 12ft x 12ft base is about 13.4m², so roughly £800 to £1,050 for a shed base, or £800 to £1,900 for a thicker garage-grade slab, on our estimates.
Do you need planning permission for a concrete base for a shed?
Not usually in a back garden: hard surfaces there are permitted development. In a front garden, a surface over 5m² must be porous or drain onto a permeable area. The shed has its own outbuilding rules.
How thick should a concrete shed base be?
B&Q's guide uses 75mm of concrete on 75mm of compacted MOT type 1 for most sheds. Garages and heavy buildings need more; follow the building's supplier.
How long does a concrete base take to cure?
Wait at least 24 to 48 hours before walking on it. Concrete takes up to 28 days to reach full strength, and B&Q suggests waiting that long before building on it.
How we priced thisView sources (4, 2 official)
Rules come from the legislation and official guidance, read at source. Prices are our estimates: we check them against trade price data, retailer shelf prices and specialists' price lists, then build them up from their parts so the totals add up. Below are the sources behind the page; the trade price guides we only compare against aren’t listed. Where sources disagreed, we've said so. More on our method.
- OfficialThe Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 1, legislation.gov.uk. Checked 16 Oct 2026.
- OfficialOutbuildings: planning permission, Planning Portal. Checked 15 Oct 2026.
- IndustryHow to build a concrete shed base (2 November 2024), B&Q. Checked 15 Oct 2026.
- IndustryHow much do garden rooms cost? (supplier survey, January 2026), The Garden Room Guide. Checked 15 Oct 2026.
