How Much Does a Basement Cost in London? 2025 Guide

Cost is usually the first question homeowners ask when considering a basement project. The honest answer is that basement construction costs vary enormously depending on the size, depth, ground conditions and specification of the project.

A new basement construction in London typically costs between £2,000 and £3,500 per square metre for the structural shell, waterproofing and basic fit-out. A finished basement with good quality finishes might cost £3,000 to £4,500 per square metre. For a typical 50m² basement, that means a total project cost of £100,000 to £225,000.

Excavation and Soil Removal In London, access is often restricted — narrow side passages, no rear access, terraced houses — which means soil must be removed by hand or with small machinery. Typical excavation costs: £15,000 to £40,000 depending on size and access.

Structural Work and Underpinning Creating a new basement requires underpinning the existing foundations. Reinforced concrete walls, floors and ceilings add further structural costs. Typical structural costs: £30,000 to £80,000.

Waterproofing A properly specified waterproofing system is non-negotiable. We use Delta cavity drain membrane systems with dual sump pumps and battery backup. Typical waterproofing costs: £8,000 to £20,000 for a 50m² basement.

Mechanical and Electrical Basements require ventilation, lighting, electrical circuits and potentially heating. Typical M&E costs: £5,000 to £15,000.

Fit-Out and Finishes Basic finish (plastered walls, screed floor, painted): £10,000 to £20,000. High-specification fit-out: £30,000 to £60,000 and above.

Professional Fees Structural engineer, architect and party wall surveyor fees. Typical professional fees: £5,000 to £15,000.

• Cellar conversion (existing space): £30,000 to £80,000
• New basement construction (single storey): £100,000 to £200,000 for a 50m² basement
• Large or complex basements: £200,000 and above

In most parts of London, yes. A well-constructed basement typically adds 10 to 15 per cent to the value of a property. In prime central London, the return can be even higher.

The only way to get an accurate price for your specific project is to have a specialist contractor visit the site and assess the conditions. At Making Space Below Ground, we offer free site visits and no-obligation quotes.

Call us on 07405 046475 or 020 8087 4829, or fill in our contact form to arrange a free site visit.