How much does a basement conversion cost?

On a tight North London plot, the space under the house can be the last untapped square metreage you own. Basement projects split into two very different jobs with very different budgets: converting a cellar you already have, and digging out new space.

Converting an existing cellar

If your house already has a cellar with reasonable head height, converting it is the affordable route: tanking or cavity-drain waterproofing, an insulated floor, ventilation, electrics and finishes. In our area this typically runs from £1,500 to £2,500 per square metre, so a 20 square metre cellar often lands between £30,000 and £50,000.

A full dig-out

Creating new basement space by lowering the floor or excavating under the house involves underpinning, structural design and serious groundworks. Costs are considerably higher, commonly £3,000 to £5,000+ per square metre, which is why full digs make most sense on high-value streets where space cannot be added any other way.

The one thing you cannot compromise on

• Waterproofing designed to BS 8102, usually cavity drainage with a sump and pump for habitable rooms.
• A structural engineer for anything touching foundations or floor levels.
• Ventilation and escape routes so the room is signed off as habitable.
• Drainage that can lift foul water if the basement sits below the sewer.

A basement lives or dies by its waterproofing. Done right it stays dry for good; done cheap it fails in the first wet winter. That is why we design the waterproofing before we price anything else.

Frequently asked questions