Basement conversion cost: the four things that decide it before the finish

How basement conversion cost works

1. Waterproofing is the specification, not a treatment. A basement below ground is under water pressure at times, and the answer is a designed waterproofing system rather than a coat of something. That usually means a cavity drain membrane taken to a perimeter channel and a sump, or a structurally applied barrier, designed by somebody qualified to design it and maintainable afterwards. It is the largest single decision in the job.
2. Headroom decides whether it is a conversion or an excavation. If the existing space already has the height for a habitable room, the work is fitting out a dry shell. If it does not, the floor has to come down, which means underpinning the walls in sequence, temporary works and a structural engineer, and that is a different order of cost and time. This is the question to answer first.
3. Where the water goes when it is below the drain. A basement WC, shower or utility sits below the invert of the existing foul drain, so the waste has to be pumped. That means a sealed pumped station, power that must not fail and a maintenance regime, and it has to be designed rather than added at second fix.
4. Light, and what a room without it is worth. A basement with no natural light is a store with plasterboard. Light wells, a lowered external area with a retaining wall and drainage, or rooflights under a rear terrace are how basements become rooms, and each of them is groundworks with its own cost and its own planning question.

Insurance and paperwork

Common questions

Do basement extensions need planning permission?

Converting an existing cellar or basement into a habitable room often does not, on its own. Excavating to create or enlarge one, or forming a light well or external stair, frequently does, and in some boroughs basement development is specifically controlled. Ask locally before you design.

Is a converted basement worth what it costs?

It is the most expensive space a house can gain per square metre and the one with the largest range. Where the alternative is moving, it can be worth it; where an extension or a loft is possible, those are usually cheaper for the same floor.

Can I do the waterproofing myself?

This is the one part where the answer is genuinely no. A failed waterproofing system in an occupied room is destructive and expensive to correct, and the design, the installation and the guarantee are all worth buying from somebody who does it for a living.

Registered contractors by region

• Registered roofers in the East Midlands postcode area
• Registered roofers in the East of England postcode area
• Registered roofers in the London postcode area
• Registered roofers in the North East postcode area
• Registered roofers in the North West postcode area
• Registered roofers in the Scotland postcode area
• Registered roofers in the South East postcode area
• Registered roofers in the South West postcode area
• Registered roofers in the Wales postcode area
• Registered roofers in the West Midlands postcode area
• Registered roofers in the Yorkshire and The Humber postcode area