10 essential questions to ask before building a basement in London, By Nick Jeffries.

Building a basement in London can unlock valuable living space, boost property value and create some truly spectacular interiors. but it’s not a simple dig and pour. basements are complex, high-stakes projects that require careful planning, the right team, and a solid understanding of what you’re getting into.

before you begin, here are ten essential questions every homeowner should ask.

By Nick Jeffries

1. is my property suitable for a basement?not all homes are viable candidates for basement construction. factors like ground conditions, water table level, existing structure and local planning restrictions all play a role. a feasibility study from a qualified architect or engineer is the first step.

2. do i need planning permission?many basements fall under permitted development, but deeper or more complex digs—especially those with light wells, rear extensions or full footprints—require planning approval. if your home is listed or in a conservation area, permission is a must.

3. how long will it take?a single-storey basement can take 6 to 12 months depending on size and complexity. this includes underpinning, waterproofing, structure and fit-out. timelines can extend significantly for deeper or double basements.

4. what will it cost?for 2025, london basement costs start at around £4,500 per square metre for the shell and structure. full fit-outs with high-end finishes can push costs to £7,000–£10,000 per square metre. factor in design, party wall fees, planning, and contingency.

5. who are my neighbours—and how will they be affected?basements can strain relationships. expect party wall notices, noise, vibration, and temporary inconvenience to adjacent homes. working with a considerate contractor and experienced party wall surveyor helps manage expectations and avoid disputes.

6. what type of waterproofing system will be used?basement waterproofing is critical. we typically use a type c drained cavity system combined with sump pumps and backup alarms. the wrong system, or poor installation, can lead to long-term damp and structural issues.

7. how will natural light be introduced?light wells, glazed walk-on panels, light shafts and open-plan layouts all help bring light into basements. architectural design plays a big role in avoiding dark, uninviting spaces.

8. will the basement include living or sleeping areas?if you plan to use your basement for bedrooms or habitable space, you must meet building regulations around ventilation, fire escape, ceiling height and thermal performance.

9. can i future-proof it?think ahead: will your basement need cinema wiring, underfloor heating, air conditioning or plant rooms? planning these elements early avoids costly changes later.

10. who should manage the build?basement projects are not for general builders. choose a design and build team with proven experience in london basements. the right partner will coordinate structural engineers, architects, waterproofing specialists and contractors under one roof.

at nu projects, we’ve built basements across chelsea, fulham, south kensington and beyond. we know what works, what doesn’t, and how to deliver a flawless finish while protecting your investment.

get in touch if you’re considering a basement project and want to explore what’s possible beneath your feet.