Basements are the most expensive square metres you can add to a London house. They are also, on a constrained urban plot with no garden to spare and a roof already converted, sometimes the only square metres available.
The cost gap between the cheapest and most expensive basement project is enormous, and it comes down almost entirely to one question: are you converting something that already exists, or digging something new?
The three types, and why the price differs so much
Cellar conversion. You already have a cellar with reasonable head height. The work is tanking, drainage, lighting, ventilation and finishes. No structural excavation. This is by far the cheapest route and the one worth checking for first.
Lowering an existing cellar. You have a cellar but the head height is 1.8 metres and you need 2.4. The floor is dug down and the existing walls underpinned to suit. Significantly more expensive than a straight conversion because underpinning is slow, sequential work.
Full excavation. No usable cellar exists. The ground beneath the house is excavated, the existing structure underpinned throughout, and a new waterproof box constructed. This is the expensive one, and it is what most people mean when they say basement conversion.
What each costs per square metre in London 2026
Cellar conversion, existing head height: £1,500 to £2,500 per square metre. A 25 square metre cellar comes in around £40,000 to £62,000.
Cellar lowered and underpinned: £3,000 to £4,500 per square metre. The same 25 square metres runs £75,000 to £112,000.
Full excavation under an existing house: £4,000 to £6,000 per square metre. A 40 square metre new basement runs £160,000 to £240,000.
Excavation extending under the garden: £5,000 to £7,500 per square metre, because you add temporary works, waterproofing over a larger area and often a light well or garden reinstatement.
In central boroughs such as Notting Hill and Kensington, where access is difficult and every load has to come out through the front of the house, add 20 to 30 percent to these figures.
Before anything else: find out whether you already have a cellar and what its head height is. The difference between converting an existing cellar and excavating a new one is roughly three times the cost per square metre. It is the single most important variable in the whole project.
Where the money actually goes
Roughly, on a full excavation project:
Excavation and spoil removal, around 20 percent. Digging is not the expensive part. Getting the spoil out of a terraced house in London is. Conveyor systems, grab lorries and restricted lorry access all cost.
Underpinning and structural works, around 30 percent. The existing house has to be supported while the ground beneath it is removed. This is done in carefully sequenced sections and it cannot be rushed.
Waterproofing, around 15 percent. Usually a combination of external tanking and an internal cavity drain membrane with a sump and pump. Never skimp here. Remediating a leaking basement after the fact costs several times what doing it properly costs first time.
Drainage and services, around 12 percent. A basement sits below the sewer, so waste has to be pumped. That means a pumped drainage system with backup.
Light and ventilation, around 8 percent. Light wells, sun pipes, mechanical ventilation with heat recovery. Building regulations require adequate ventilation and a habitable basement needs proper daylight to feel like a room rather than a cellar.
Internal finishes, around 15 percent. Varies with specification like any other space.
The costs on top of construction
Basement projects carry heavier professional and statutory costs than any other domestic work.
Structural engineer, £4,000 to £12,000. Not optional and not a place to economise. The engineer designs the underpinning sequence that keeps your house standing.
Party wall surveyors, £2,000 to £6,000. Every adjoining owner must be served notice under the Party Wall Act, and excavation near neighbouring foundations always triggers it. Mid-terrace means two sets of neighbours.
Ground investigation, £1,500 to £4,000. Boreholes and a soil report. London clay behaves differently from river gravels, and you need to know which you have before designing anything.
Basement impact assessment, £2,000 to £6,000. Several London boroughs require one, covering hydrology, ground stability and the effect on neighbouring structures.
Planning and building control fees on top.
Structural warranty and insurance, which lenders and future buyers will ask about.
Realistically, professional and statutory costs add 15 to 20 percent to a basement project rather than the 10 to 15 percent typical of an extension.
Does it pay back
Sometimes. It depends heavily on where the house is.
In central London where property values are high per square metre, a well-executed basement can add value at or above what it costs. In outer boroughs where values are lower, the sums are much harder and it is common for a basement to cost more than it adds.
The honest test is this: compare the total project cost per square metre against local property value per square metre. If a basement costs £5,000 per square metre to build and local property sells at £8,000 per square metre, the economics work. If local property sells at £5,500 per square metre, you are building for lifestyle rather than return, which is a perfectly valid reason but should be a conscious one.
Also worth weighing: a basement bedroom is generally valued below an above-ground bedroom by both surveyors and buyers. Basements used as living space, media rooms, gyms or utility tend to be received better than basement sleeping accommodation.
Planning and the practical realities
Basement excavation requires full planning permission. It is not permitted development. Several boroughs have specific basement policies limiting depth, footprint under the garden, and requiring assessments before an application will even be validated.
Timelines are long. Design and planning takes 4 to 8 months. Construction runs 8 to 14 months for a full excavation. Total project time of 12 to 20 months from first meeting is realistic, and longer in boroughs with restrictive basement policies.
You will almost certainly need to move out for a substantial part of the build, which is a real cost people forget to budget.
When a basement is the right answer
When the plot is genuinely constrained, the roof is already converted, the garden cannot be given up, and local property values are high enough that the arithmetic works. Under those conditions a basement is a sensible way to add substantial space to a house you want to stay in.
When there is garden to spare and planning would allow a wraparound extension or a double storey extension instead, those will almost always deliver more space for less money with far less risk.
To compare options on your own property, our cost calculator covers extensions and loft conversions and will give you a range to weigh against these basement figures.