The two dominant ways to build a garden room foundation in the UK are a concrete raft (or slab) and steel foundation anchors. Both work; both have their place. Here's the honest head-to-head.
Concrete raft
- Cost: usually 15-25% cheaper than steel foundation anchors for a small footprint
- Time: 3-5 days including excavation, formwork, pour and cure
- Excavation: 300-500 mm of topsoil removed and disposed of
- Level: needs a flat site or a stepped raft on slopes
- Lifespan: 40-60 years, sometimes shorter on ground that moves
- Removal: effectively permanent — you're leaving concrete in the garden
Ground screws
- Cost: 15-25% more than concrete for the smallest footprints, equivalent or cheaper at larger sizes
- Time: 1 day
- Excavation: none
- Level: any slope, no problem
- Lifespan: 50+ years manufacturer design life
- Removal: reversible — the screws unscrew if the building is ever removed
Which wins?
For the vast majority of garden rooms we build, steel foundation anchors are the better answer. Speed, no excavation, no waiting for cure, reversibility, and unmatched performance on sloping sites make them the sensible choice for a garden building. Concrete is still the right answer on unusual sites — very poor ground bearing, heavy point loads, or where a client specifically wants a slab.