#NU BLOG

10 Questions to Ask Before Building a Basement in London

Jul 25, 2025
2 min read

Updated: Aug 1

A basement creates valuable space, but it also places major structural and waterproofing responsibility beneath an existing home and beside neighbouring properties. The appointment should therefore be based on evidence, technical control and clarity—not a headline excavation rate.

After more than 16 years working on London homes, these are the ten questions NU Projects recommends asking before choosing a basement contractor.

1. Have you completed comparable basements?

Ask for projects matching the property type, excavation method, depth, access and intended fit-out. A simple cellar refurbishment is not equivalent to a new basement beneath a period townhouse.

2. Who owns the design responsibility?

Identify who coordinates planning, permanent structure, temporary works, waterproofing, drainage, MEP and interiors. Responsibility must be named and documented.

3. What surveys and investigations are required?

The answer may include measured, structural, drainage, ground, groundwater, tree and utility information. Ask how uncertainty will be reduced before construction.

4. What is the planning strategy?

The team should understand the local authority's basement policy, property constraints and likely supporting documents. National basement planning guidance is only a starting point.

5. How will the building and neighbours be protected?

Ask for the excavation sequence, propping, underpinning or piling method, movement-monitoring plan and hold points for inspection. Understand how party-wall requirements and neighbour communication will be managed.

6. What waterproofing and drainage system is proposed?

The answer should cover design rationale, junctions, penetrations, maintainability, pumps, alarms, power resilience and access for servicing—not simply a product name.

7. How are ventilation and building services coordinated?

Below-ground rooms need carefully designed air quality, heating, cooling, drainage, lighting and controls. A pool, gym, cinema, wine room or bedroom suite creates different environmental demands.

8. What does the price include and exclude?

Compare investigations, design, approvals, temporary works, excavation, structure, waterproofing, MEP, fit-out, specialist rooms, preliminaries, professional fees, contingency and VAT. Large unexplained allowances are a warning sign.

9. How will changes and progress be reported?

Variations should be described, costed and approved in writing before work proceeds. Progress reporting should connect design decisions, procurement, programme, risk and commercial status.

10. What happens at commissioning and aftercare?

Ask how pumps, alarms, drainage, ventilation, heating and controls will be tested, documented and maintained. Confirm warranties, handover information, defects procedures and the response if a system needs attention.

The evidence checklist

Comparable completed basement projects
Named engineer and design responsibilities
Property-specific surveys and planning route
Written temporary-works and monitoring approach
Coordinated waterproofing, drainage and MEP design
Transparent cost plan and programme
Documented testing, handover and aftercare

Review the NU Projects portfolio and ask for the technical story behind the finished photographs.

See the South Kensington triple-basement project for a completed example of NU Projects’ residential delivery.

Frequently asked questions

Should I appoint the lowest basement quote?

Only after confirming that every bidder has priced the same design, responsibilities, temporary works, waterproofing, services, fit-out, allowances and exclusions.