How to use the Foundations Cost Calculator
- Enter total foundation length (l/m). For extensions, sum all trench runs.
- Select type (strip or trench fill) and typical depth/width for your soil.
- Optionally add spoil cart-away, overbreak and reinforcement allowances.
- Review totals (inc VAT) and export or copy the result.
Use this for budgeting. For tender pricing or engineered designs (rafts, pads, beams), request a QS estimate with BoQ, prelims, overheads and profit, and risk.
Typical foundation costs (planning guide)
- Strip foundations: ~£140–£220 per l/m including VAT (typical £180) assuming ~450–600 mm wide × ~900–1000 mm deep, C25 concrete and standard spoil removal.
- Trench-fill foundations: ~£170–£280 per l/m including VAT (typical £220) assuming ~600 mm wide × ~1.2 m deep and higher concrete volume.
Depth, width and soil conditions drive variance. Near trees in clay or on poor ground, expect deeper trenches and higher concrete/muck-away costs.
Depth, soil and inspection basics
- Depth: commonly 900–1200 mm, but Building Control will confirm on site after the first dig.
- Soils/trees: shrinkable clays, high water table or adjacent trees can require deeper trenches or alternative designs.
- Concrete: C25/C28 common; trench fill reduces blockwork below DPC but uses more concrete.
Worked example: 20 l/m trench-fill
20 l/m × £200 per l/m (midpoint) = £4,000 including VAT. Add allowances if Building Control requests deeper excavation or if cart-away distances/fees are high.
Common uplifts and allowances
- Overbreak/soft spots: extra excavation and make-up.
- Spoil cart-away: distance, grab hire vs skip and landfill charges vary.
- Reinforcement: trench mesh or strip rebar to engineer’s detail.
- Pumping: add if concrete wagon cannot discharge directly.
Programme and inspections
- Typical small extension: 2–4 working days for dig, inspect and pour (weather dependent).
- Book Building Control inspection of trench depth/ground before pouring concrete.
- Coordinate services and trial holes early to avoid delays.
Scope assumptions
Excavate trenches, dispose of arisings, place concrete to agreed depth/width, basic trench mesh where specified, allow for site setup and tidy. Excludes piling, rafts, pads/ground beams, dewatering, contamination disposal, tree root barriers and structural redesign.