Can I have raft over part + strip over part?

Possible but rarely worthwhile — composite foundations create differential settlement risk at junction. Engineer would typically extend raft over full footprint if any part requires raft, or use deepened strip everywhere. Don't mix without specialist design.

Is raft same as basement floor slab?

Basement floor slab is a raft — RC slab + edge beams + retaining walls + waterproof construction. Typical basement raft 300–400mm thick with 2-layer mesh + RC walls cast off raft up to ground level. Cost £450–£950/m² basement footprint (much more than extension raft due to depth + waterproofing).

When does Building Control reject strip + insist on raft?

Where Soil Investigation shows: bearing capacity <100 kN/m² at strip depth; variable strata risking differential settlement; high water table within 1m of ground level; ground heave/shrink risk above standard; near significant trees outside Influence Zone calculation. Engineer's design either upgrades strip (deepen + reinforce) or moves to raft.

Raft vs Strip Foundation London 2026 | Builderr

Raft vs Strip Foundation — Which Suits a London Extension?

Raft foundation = reinforced concrete slab covering full footprint with edge + intermediate thickening, distributes load across whole area; suits poor/variable ground, high water table, infill sites, basements. Cost £125–£285/m² built. Strip foundation = trench-fill RC under loadbearing walls only, simpler + cheaper, suits firm uniform ground (most London Clay sites); £85–£185/m run. Choose by ground bearing capacity, settlement risk, water table, building loads + budget.

Strip foundation use + spec

Standard London domestic foundation: trench-fill concrete 600mm wide × 1,000–1,500mm deep under each loadbearing wall. Bearing on London Clay at 1m+ depth — capacity typically 100–150 kN/m² adequate for 2–3 storey domestic loads. Pour: lean concrete (C20/25) fills trench to 150mm below DPC; brickwork/blockwork up from foundation. Cost £85–£185/m run for typical 600mm × 1,200mm strip. Application: extensions on uniform firm clay/gravel; majority of central + suburban London where ground tested. Variants: deepened strip (where trees nearby — depth to 2.5m+ pushing into stable clay below heave zone); reinforced strip (with 2× T16 bottom + 2× T16 top bars where soil variable or spans across soft pockets); stepped strip (where ground slopes >1:10). Design: NHBC Standards Chapter 4.4 or Eurocode 7 with structural engineer.

Raft foundation use + spec

Full reinforced concrete slab covering full plan area (typically 200–300mm thick); edge thickening (toe beam 600–900mm deep × 500mm wide) at perimeter under loadbearing walls; intermediate thickening under internal loadbearing walls or columns. Spreads load uniformly across entire footprint — settlement self-correcting. Reinforcement: typically A393 mesh top + bottom + extra T12–T20 bars at edge beams. Suits: poor/variable ground (Made Ground, soft alluvium); high water table (raft + waterproof concrete acts as basement tanking); infill sites (former pond/garage/extension foundation residual); reduced settlement risk (single-piece raft eliminates differential settlement). Cost £125–£285/m². Application: extensions over Made Ground (common in former orchards, allotments, demolished outbuildings); basements (raft + RC walls); near trees where deepened strip impractical.

Cost comparison + decision

Typical 25m² London side-return extension: strip foundation £2,500–£4,500 (trench-fill + reinforcement + concrete + labour); raft foundation £3,000–£7,000 (excavation + DPM + mesh reinforcement + edge beam + concrete + labour). Premium for raft £500–£2,500. Decision factors: (1) Soil Investigation report — if ground variable or weak, raft pays off in reduced settlement risk + may be required by engineer. (2) Adjacent structures — raft spreads load away from neighbouring foundations better than strip. (3) Tree proximity — deepened strip may be 2x depth = 2x cost, raft sometimes equivalent + simpler. (4) Basement — raft is the only option (forms basement floor). (5) Drainage — raft + integrated drainage layout coordinated; strip easier to retrofit drainage later. (6) Programme — raft 1 day excavation + 1 day reinforcement + 1 day pour + 5 day cure (cleaner sequencing); strip 1–2 day excavation + 1 day pour + 5 day cure (similar timeline).