Basement conversion in London. We design, engineer and build basements under one contract. Basement cost in London, the 2026 figures.
Basement conversions in Fulham, Chelsea and Kensington.
A London basement is not a single building operation. It is a sequence of planning, investigation, temporary support, excavation and permanent structure. Then come waterproofing, drainage, services and fit-out. Each stage must be designed around the property and its neighbours.
The projects that run to plan begin with a disciplined feasibility phase. They also begin with an experienced basement design-and-build team.
1. Establish planning feasibility
Requirements vary between boroughs and proposals. Converting an existing cellar can be different from creating a new excavation. The same goes for a new lightwell or separate unit. Listed status, conservation policy and trees can alter what is acceptable. So can flood risk and previous development.
Use the Planning Portal basement guidance as an overview. Then check the local authority’s current policy and the individual property.
2. Commission the right surveys
- Measured and level survey
- Structural inspection and opening-up where required
- Drainage and sewer information
- Ground and groundwater investigation appropriate to the proposal
- Tree and root assessment where relevant
- Existing services and utility capacity
- Neighbouring foundation information where obtainable
3. Define structural and temporary works responsibility
The engineer must design both the permanent structure and a safe construction sequence. Underpinning, piling, propping, excavation bays and load transfer should all be coordinated. They should fit with access, spoil removal and inspections. The contractor must understand exactly which temporary stages require approval before progressing.
4. Waterproofing and drainage as maintainable systems
Waterproofing should be selected for the ground conditions, use and risk profile. Details at slabs, walls, joints, lightwells and service penetrations matter. They matter as much as the headline system. Drainage channels, pumps and alarms must remain serviceable after finishes are complete. So must access for maintenance.
5. Coordinate MEP and the intended use
A cinema, gym, bedroom suite, pool or wine room each creates different requirements. They cover ventilation, acoustics, drainage, heating, cooling and power. Plant space and ceiling zones must be allowed before the layout is fixed.
6. Address party walls, logistics and neighbours
Basement work often affects adjoining owners. It can require notices and awards under the Party Wall etc. Act. Access, deliveries, skips and spoil removal should also form part of the construction-management plan. So should working hours, monitoring and communication.
7. Build a complete cost plan and programme
Price investigations, design, approvals, temporary works, excavation and structure. Price waterproofing, drainage, MEP, fit-out and specialist rooms too. Add preliminaries, professional fees, contingency and VAT. The programme should include planning, party-wall procedures and procurement, not only the construction period.
Questions to ask the contractor
- Who designed and signs off the temporary works?
- How will movement be monitored?
- Which waterproofing standard and maintainable system are proposed?
- How are pumps, alarms and drainage accessed after completion?
- Who coordinates MEP, structure and interior fit-out?
- How are changes recorded and approved?
- What testing, certification and aftercare are included?
See the South Kensington triple-basement project. It is a completed example of NU Projects’ residential delivery.
Frequently asked questions
Do all basement projects need planning permission?
No, but many excavation and external-alteration proposals do. The answer depends on the work, property and local policy.
Should the interior be designed before the structure?
The intended use should inform the structure and services. But the disciplines must develop together. Neither can be treated as an afterthought.
What contingency is sensible?
It should respond to investigation quality, existing conditions, design maturity and procurement risk. A project-specific quantity surveyor should advise.
Start with feasibility
Contact NU Projects for a confidential discussion about planning, structure, risk, cost and delivery.
This is the work we do. Basement conversion in London, or ask for a cost estimate.
Published 20 October 2021. Written by the NU Projects studio, edited since for house style and accuracy.