What Does a Basement Conversion Cost?

Cost by Conversion Type

• Existing cellar conversion (light touch): £1,000–1,500/m² — waterproofing, damp-proofing, basic fit-out
• Cellar conversion (full): £1,500–2,000/m² — underpinning, lowering floor, full fit-out
• New basement dig: £2,500–3,000+/m² — full excavation under existing property

What’s Included

• Structural underpinning or bench foundations
• Waterproofing (cavity drain membrane or tanking)
• New concrete floor slab
• Insulation (walls and floor)
• Electrics, plumbing, and HVAC
• Staircase access
• Plastering and decoration
• Light well or window installation

Additional Costs

• Planning permission: £258 (if required — many conversions are PD)
• Building Regulations: £400–800
• Structural engineer: £1,000–3,000
• Party wall agreements: £700–1,500 per neighbour
• Sump pump system: £1,500–3,000

Is It Worth It?

A well-executed basement conversion can add 20–30% to a property’s value in high-value areas. In London and the South East, the value added often exceeds the cost. In lower-value areas, the return may be marginal.