The three basement conversion types — and what each costs
Type 1: Cellar conversion (£35,000–£90,000). Refurbishes existing under-house space into habitable use — home office, gym, utility, bedroom. No excavation, no underpinning. Suitable when existing cellar has 1.7m+ headroom. Build duration 8–14 weeks. Type 2: Full basement excavation (£150,000–£350,000). Creates a new basement floor under the existing house by excavating 2.5–3m below the current floor level, underpinning all perimeter walls and casting new structure. Build duration 22–28 weeks. Always requires full planning permission and Basement Impact Assessment. Type 3: Basement extension (£200,000–£500,000+). Combines full excavation under the house with sub-garden extension, adding floor area beyond the original footprint. Build duration 28–36 weeks. Most ambitious type; planning-restricted in central London boroughs.
What drives basement conversion cost
Five factors most affect basement conversion pricing. (1) Excavation type and extent — cellar refurbishment is 4-10x cheaper than full excavation because no ground is moved. Cubic metres of clay removed is the single biggest cost variable on excavated basements. (2) Underpinning extent — full perimeter underpin on a typical London terrace is 25–40 linear metres at £1,200–£2,800/m, so £40,000–£90,000 just for the underpinning. (3) Waterproofing system — Type A tanking £80–150/m²; Type C cavity drain £150–250/m²; combined £250–400/m². For a 50m² basement with 80m² of treated surface area, waterproofing is £15,000–£40,000. (4) Light wells — each external light well adds £15,000–£40,000 including planning, structure and glazing. (5) Borough premium — K&C, Westminster and Camden command 20-30% premium over outer London for the same basement scope, driven by access difficulty, conservation requirements and labour rates.
What's included in a basement conversion cost
A full basement excavation fixed-scope contract typically includes: site setup, scaffolding, hoarding and traffic management; service diversions (gas, water, drainage, electrics); perimeter underpinning to engineer's pin schedule with all mass concrete; bulk excavation and spoil removal; reinforced concrete structural slab; reinforced concrete retaining walls; Type A+C waterproofing system; sump and pump with battery backup; MVHR mechanical ventilation system; first and second fix electrics; first and second fix plumbing including any new drainage; light wells (if planned); plaster, decoration and floor finishes; internal joinery (skirting, doors, architraves); engineer's calcs, building regs Full Plans application and inspections, party wall notices and surveyor co-ordination, Basement Impact Assessment, planning application. Excluded typically: bespoke fit-out items (cinema equipment, pool installation, sauna fittings); landscaping and garden re-instatement beyond standard returfing; furniture.
