For owners of prime London houses planning a basement
Basement Conversion Cost in London: What Drives It
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 4 min read
The short answer
A basement conversion cost in London depends first on whether you convert an existing cellar or dig a new basement. The Home Owners Alliance puts a 40 m² basement with excavation and underpinning at around £100,000 to £160,000; a prime basement with a pool, cinema or wine room goes far beyond that.
- Converting an existing cellar usually needs no planning permission; digging a new basement usually does.
- Underpinning, propping, waterproofing and spoil removal are most of the cost of a new dig.
- Several central London boroughs have their own basement policies and ask for a construction method statement.
- Every basement next to a neighbour needs party wall notice; add VAT, fees and a contingency.
A basement conversion cost in London depends first on one question: are you converting an existing cellar, or digging a new basement? The Home Owners Alliance puts a 40 m² basement with excavation and underpinning at around £100,000 to £160,000.
A prime basement with a pool, a cinema, a gym or a wine room goes far beyond that figure. This guide explains what drives the cost, what the borough will ask for and how long it takes.
Planning a lower floor? Our basement conversion in London service designs, gets the consents for and builds luxury basements as one team. Discuss your project and we will look at the house and the ground with you.
Cellar conversion or a new dig
The two routes cost very different amounts, because one reuses a hole and the other makes one.
| Route | What the work involves | Planning |
|---|---|---|
| Cellar conversion | Lowering the floor if needed, waterproofing, insulation, light, ventilation and finishes | Usually not needed for an existing basement |
| Extended cellar | Digging out under more of the house, with underpinning | Often needed |
| New basement | Excavation under the house and often the garden, underpinning, a new structure | Likely to be needed |
| Double-depth or iceberg | Two or more storeys below ground, with heavy temporary works | Restricted in some boroughs |
The Planning Portal says converting an existing basement into living space is unlikely to need planning permission, while excavating a new one is likely to. A lightwell at the front, visible from the street, faces stricter rules than one at the back.
Structure, underpinning and waterproofing
Most of the cost of a new basement is invisible once it is finished. It sits in the ground, the walls and the water strategy.
- Ground investigation. Boreholes and a groundwater check before anything is designed.
- Underpinning. The walls of your house, and often the party walls, are deepened in short sections.
- Temporary works. Propping holds the house and the neighbours' walls while the ground is open.
- Spoil removal. Every cubic metre dug is carried out, often by conveyor through the house.
- Waterproofing. A tanked or cavity-drained system, with pumps and a maintenance plan.
- Light and air. Lightwells, walk-on rooflights and mechanical ventilation.
The deeper and wider the dig, the more of each. A pool adds a structural tank and a plant room on top.
Borough basement policies and party walls
In central London, the borough's basement policy shapes the design before the engineer does. Kensington and Chelsea, for example, adopted a dedicated basement policy in 2015 and a basements planning document in 2016.
- Limits. Some boroughs limit how many storeys a basement may have and how much of the garden it may cover.
- Method statement. A construction method statement, signed off by an engineer, comes with the application.
- Traffic. A construction traffic management plan sets out how spoil and deliveries reach the street.
- Listed and conservation. A listed house needs listed building consent for work to its fabric.
Almost every London basement needs party wall notice. Excavating near a neighbour's foundations needs at least one month's notice, and work to a shared wall two months. Party wall surveyors and any security for the neighbours belong in the budget.
What drives the cost of a basement conversion
Once the route is chosen, these decide where your basement lands.
- Depth and footprint. One storey under the house costs far less than two storeys under the garden.
- Ground and water. Clay, gravel and a high water table change the temporary works and the waterproofing.
- Access. A mews house or a terrace with no side access means slower digging and more labour.
- What goes in it. A pool, cinema, gym or wine room brings plant, acoustic and environmental systems.
- Specification. Stone, joinery, lighting and audio-visual systems at the level of the rest of the house.
- The house above. Making good floors, the stair and the services where the basement joins the house.
Our build cost calculator gives a first indication. Add VAT at 20%, engineering and design fees, party wall costs and a contingency of 5% to 10% of the construction cost.
Programme and living nearby
A new basement is the longest single job most houses will ever see. Most of the time goes into what happens before the dig.
- Investigation and design. Ground investigation, structural design and the layout of the rooms.
- Planning and method statement. The application, with the construction method statement and traffic plan.
- Party wall awards. Notices, surveyors and agreed awards with every affected neighbour.
- Excavation and structure. Underpinning in sections, propping, the slab and the walls.
- Fit-out. Waterproofing, services, plant and the finishes.
On a new dig, most owners move out for the excavation and structural stages. Neighbours stay, so noise, dust and working hours are planned with them from the start. Our guide to house renovation cost in London covers the work on the rest of the house.
This was a full high-end renovation of my Victorian rental apartment in Westminster, just off Sloane Square and Belgravia. The scope included full fit-out: electrics, plumbing, a new bespoke kitchen, and a luxury bathroom refurbishment. Craftex took care of everything and made the process easy. They weren’t the cheapest, but you can tell they really know what they’re doing when it comes to prime London property renovations. The team was professional, organised, and genuinely seemed to care about getting everything right. I was especially impressed with the quality of the finish – the attention to detail really shows in every corner. I’m very happy with the results and would definitely recommend Craftex to anyone looking for reliable, high-quality renovation work in Central London.
How we price a basement
We do not publish a price per square metre, because the ground, the depth and the neighbours differ on every house. We start with the ground investigation, draw the basement and price it as it is drawn.
The figure is fixed before work starts, and any change is agreed in writing and priced first. Read our guides to planning and building regulations drawings and listed building consent for the consents. Have a house in mind? Discuss your project and we will reply the same working day.
