Basement companies in London, and what to ask each one

Basement conversions are the way most London homeowners add space when there is none left above ground. They are also the hardest residential job to compare on paper, because two quotations of the same length can carry completely different scopes. This guide is about what to ask for, in writing, before you appoint anybody.

Why the rankings you find online are not rankings

There is no public league table of London basement companies, and the ones you find are written by the firms in them or by people paid to write them. The right specialist depends on the property, the ground, the approvals, the access and what the room is going to be used for. A firm that is right for a Victorian terrace in Clapham may be wrong for a listed stucco house in Belgravia.

So do not shortlist on a ranking. Shortlist on evidence, and then read the documents.

The six documents a basement company should hand you

Ask every firm on your list for these six. A specialist will have them ready. A general builder taking on a basement will not.

• A design responsibility schedule. Who owns architecture, permanent structure, temporary works, waterproofing, drainage, mechanical and electrical, and the interior. In writing, with names.
• A temporary works and monitoring approach. How the house is held up while the ground under it is removed, and how movement is measured while that happens.
• A waterproofing design. Not a product name. A system: structure, joints, penetrations, drainage, pumps, alarms, power resilience and access to service all of it in ten years.
• A survey schedule. Measured, structural, drainage, ground, groundwater, trees and utilities. Ask which are done before the price is fixed and which are assumptions.
• A programme with dates. Approvals, party wall, temporary works, excavation, structure, waterproofing, services, finishes, commissioning, handover.
• A cost plan behind the total. Design fees, surveys, structure, waterproofing, drainage, services, finishes, specialist rooms, preliminaries, contingency and VAT, each on its own line.

What the quote usually leaves out

The gap between two basement prices is almost never the work. It is the exclusions. Read them before you read the total.

The items that most often sit outside a low quotation are the structural engineer, the party wall surveyors, building control, the temporary works design, the drainage diversion, the pumps and their standby power, the ventilation, the lighting design, the joinery and the decoration. Any of those can be a five figure line. Published rates for basement work are on our cost calculator, so you can see roughly where a real number sits before you start comparing.

The questions that separate a specialist from a builder

Four questions, and you will learn more from the answers than from any brochure.

Ask how they would support the house during excavation on your street, given the ground and the neighbours. Ask what they would do if water came in at the wall to floor junction two years after handover. Ask who signs off the waterproofing design and what qualification that person holds. Ask for two clients whose basements finished in the last two years, and telephone them.

What NU Projects does on a basement

NU Projects has been building in London since 2010, and basements are a large part of that work. Architecture, interior design, cost planning, basement construction, building services and aftercare sit under one contract, with one project manager, so the interfaces above are ours to manage rather than yours.

You can see completed work on our projects page, including the South Kensington triple basement.

Talk a basement through

If you are considering a basement in London, tell us about the house before the design and the risk responsibilities are fixed, or call the studio on 020 7731 6841.