Pros and cons of building a basement

Property is usually a sound investment. You live in it. It tends to rise in value. The price you paid can look small by the time you sell.

That is not always what the market does. Prime London prices may stall or fall. Then owners stop trading up and start improving what they already own. A basement is the most dramatic version of that, and the most demanding.

The case for digging

Building below ground costs more per square foot than building above it. People do it anyway. In much of prime London there is nowhere else to go. The plot is fixed and the roof is already converted. The garden is either protected or too small to lose.

What the space is used for is the other half of the argument. A basement takes the rooms that do not need daylight. These are a cinema, a gym or a utility room. They are also a plant room or a pool. It frees the floors above for bedrooms and living space.

Costs vary widely with ground conditions, depth, access and finish. A single rate is misleading. The NU Build Cost Calculator gives a construction figure by work type and specification. It publishes the basement bands in full.

The case against

A basement is not a straightforward build. Planning and party wall awards have to be resolved before excavation starts. So do temporary works, groundwater, spoil removal and waterproofing. Each of them can move the programme.

It is also the most disruptive work you can do to an occupied house. The same goes for the street it sits on. Most owners move out.

How to choose the contractor

Use a specialist. Ask for completed residential basements of comparable depth and access. Ask what that company designed as opposed to what it subcontracted. Ask to speak to an owner they have finished for. Check the insurance and the certification before you sign anything.

Our longer guide sets out the ten checks worth making. Find it in London basement companies, ten checks before you appoint.

You may be weighing up a basement under a London house. If so, tell us about the property. Nick Jeffries founded NU Projects in 2010. He still answers most of these calls himself.