The short answer
A basement conversion in the UK typically costs between £1,000 and £4,500 per square metre, but the total depends heavily on what work is involved. Refitting an existing dry cellar is the lower end, often £15,000–£40,000. Converting a cellar that needs tanking or a cavity drainage membrane and some headroom gained usually runs £40,000–£80,000. Lowering the floor with underpinning to create full ceiling height is more involved, commonly £70,000–£150,000. Digging an entirely new basement under or beside a house is the most expensive route at £150,000–£400,000+. The main drivers are how much you dig, the waterproofing system, structural underpinning, access for muck-away, and whether you add a lightwell or separate entrance.
Basement costs vary more than almost any other home project because the starting point differs so much. The figures below set out typical ranges for the main types of conversion, from a straightforward refit to a full new dig.
Typical UK costs
- Refit a dry cellar£15,000–£40,000
- Convert with waterproofing£40,000–£80,000
- Lower floor + underpin£70,000–£150,000
- Dig a new basement£150,000–£400,000+
- Per square metre~£1,000–£4,500/m²
What you are paying for
A basement conversion is priced around the scale of the structural and waterproofing work, not just the floor area. A cellar that is already dry and has decent headroom mostly needs lining, insulation, a damp-proofing system, electrics, heating and finishes, so it sits at the lower end. The cost climbs sharply once you have to dig: lowering the floor to gain headroom means underpinning the existing footings in sequence, removing spoil, and forming a new structural slab. Creating a basement where there was none means full excavation, temporary support, a tanked concrete box and often a separate access, which is why new digs cost several times more than a refit.
| Type of conversion | Typical UK cost | What it involves |
|---|---|---|
| Refit an existing dry cellar | £15,000–£40,000 | Lining, insulation, electrics, finishes |
| Convert with waterproofing | £40,000–£80,000 | Tanking or CDM, some headroom gained |
| Lower floor + underpin | £70,000–£150,000 | Underpinning, new slab, full headroom |
| Dig a new basement | £150,000–£400,000+ | Full excavation, structural box, access |
Indicative UK figures for guidance. Sources: Checkatrade and Homebuilding & Renovating basement cost guides.
The hidden line items
The headline build figure is rarely the whole cost. Most basement projects carry professional and statutory fees on top: a structural engineer to design the underpinning and slab, a party wall surveyor if you share walls with neighbours, building control and any planning fees, and often a specialist waterproofing designer. Excavation also brings disposal costs, because spoil has to be carried out by hand or conveyor and taken away by skip or grab lorry, which is expensive in tight terraced streets. Then there is the fit-out: a basement used as a habitable room needs heating, ventilation, lighting that suits a windowless space, and a compliant means of escape, all of which add to the per-square-metre rate.
Why two quotes can differ so much
Basement quotes vary widely because firms scope the work differently. One quote may assume an existing slab is sound while another allows for breaking it out; one may price a simpler tanking system while another specifies a full cavity drainage membrane with a sump and pump. Headroom is a common variable, since gaining an extra few hundred millimetres can mean the difference between a light refit and a full underpinning job. To compare fairly, you need each quote to state the waterproofing system, whether underpinning is included, who carries the structural design, and what is excluded, such as decoration, kitchens or bathrooms.
Region matters too. London and the South East carry a clear premium, partly because so many basement digs there are in terraced and listed streets with difficult access and strict party wall situations. A like-for-like conversion can cost noticeably more in inner London than in much of the rest of the country, which is one reason national averages are only a starting point.
Frequently asked questions
Is a basement conversion cheaper than an extension?
Per square metre, a basement is usually one of the most expensive ways to add space because of the excavation, underpinning and waterproofing. It tends to make sense where there is no room to extend outwards or upwards, or where the existing cellar already gives you a head start.
Does a basement conversion add value?
A well-finished, dry, properly waterproofed basement can add usable floor area and value, especially in areas where land is scarce. Whether it pays back the cost depends heavily on local prices, so it is worth checking comparable values before committing to a full dig.
How long does a basement conversion take?
A simple cellar refit might take a few weeks, while a floor-lowering and underpinning job often runs three to six months. A full new dig with a separate entrance can take six months to a year, depending on access and ground conditions.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.