When to use a raft instead of strip foundations
Poor ground (made ground, soft clay, fill on former gardens): a raft spreads load over the whole footprint rather than concentrated under walls. Common on parts of east London (Newham, Tower Hamlets) where made ground from Victorian fill predominates, and on parts of south London (Lewisham, Greenwich) where shrinkable clay underlies. Shallow founding requirement: where service routes, tree roots or rock prevent excavating strip foundations to 1.0–1.2m, a raft at 300–500mm depth distributes load. Basement floors: the basement slab functions as a raft, designed to span between perimeter underpinning. Extensions on weak ground next to listed buildings: a raft avoids the differential settlement risks of strip foundations next to original shallow foundations.
Cost breakdown by raft type
Standard reinforced concrete raft (200–250mm thick with A393 mesh): excavation £25–£45 per m², blinding £8–£15 per m², DPM and reinforcement £35–£55 per m², concrete £80–£120 per m², thickenings at perimeter £30–£60 per m². Total £180–£280 per m². Insulated raft (with 100–150mm perimeter EPS and 100–150mm under-slab EPS plus UFH): add £100–£180 per m² for the insulation and UFH. Total £280–£450 per m². Engineered raft on poor ground (300–400mm thick, heavy reinforcement, reinforced edge thickenings, possibly with ground improvement or vibro-replacement): structural engineer design £2,500–£6,000, plus base raft cost £450–£700 per m². Used on basement construction and on extensions over deep made ground.
Insulated raft systems for Passivhaus extensions
Brands include Isoquick (UK), KORE (Ireland) and Supergrund (Germany). Pre-cut EPS panels deliver to site, are assembled like a jigsaw to form the perimeter and underside insulation, then UFH and reinforcement laid over and concrete poured in one pour. Achieves U=0.10–0.14 floor U-value as standard, eliminates the thermal bridge at slab edge, integrates underfloor heating, foundation and ground floor in one operation. Cost premium £100–£180 per m² over standard raft but saves 1–2 weeks programme and achieves Passivhaus-level fabric performance.
