Pros and cons of building a basement

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

That is not always what the market does. When prime London prices stall or fall, 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. The reason people do it anyway is that in much of prime London there is nowhere else to go. The plot is fixed, the roof is already converted, and 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, a cinema, a gym, a utility room, a plant room or a pool, and frees the floors above for bedrooms and living space.

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

The case against

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

It is also the most disruptive work you can do to an occupied house and to 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, and 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, in London basement companies, ten checks before you appoint.

If you are weighing up a basement under a London house, tell us about the property. Nick Jeffries founded NU Projects in 2010 and still answers most of these calls himself.