Basements in South West & West London
Cellar conversions and basement dig-outs: a whole new floor beneath the house
A whole new floor without giving up an inch of garden. An old cellar becomes a room you use every day, or a new floor is dug out beneath the house, and one team takes it from the survey to the last coat of paint.
Inside the Basement
Scroll, and the house goes down a floor: the old coal cellar converted into a cinema, and the back half dug out new into a gym with a light well. Underpinned wall by wall, waterproofed and fitted out. Then the first evening downstairs.
What It Adds
A floor added where the house already stands: the most room a London house can gain without going up or out.
*Illustrative guide figures while launch pricing is finalised — your exact numbers follow the free survey.
What We Do
A basement is the one way to add a whole floor to a London house without building up or out. Where a house has an old coal cellar, it can be converted: the floor taken down, the walls underpinned, then waterproofed, insulated and fitted out as a proper room. Where it has none, a new basement can be dug out beneath the house, the walls underpinned a section at a time, with a light well to bring the daylight down. The structure, the waterproofing, the services and the finished room are planned and built as one programme by one team, so the details that keep a basement dry and sound are never left between two contractors.
Key Benefits
- A whole extra floor without losing any of the garden
- Rooms the house never had: a cinema, a gym, a playroom or a guest suite
- Dry, warm and fresh: waterproofed, insulated and ventilated from the start
- Where the house allows, a light well bringing the daylight down
What's Included
- Structural survey & feasibility
- Structural engineer's design
- Party wall notices coordinated
- Underpinning & structural support
- Excavation & spoil removal
- Steel & concrete structure
- Type C cavity-drain waterproofing
- Sump & pump system
- New staircase
- Insulation, screed & linings
- Ventilation, electrics & lighting
- Building regulations & structural sign-off
Our Process
- 01
Survey & Feasibility
We look at the house, the ground and the walls you share: the footings, the drains, where the water sits, and whether a light well is possible. Then a clear plan of what the basement can be.
- 02
Design & Approvals
The structural engineer's design, the waterproofing design, planning where your borough needs it, building control, and the party wall notices to your neighbours.
- 03
Underpinning & Excavation
The walls underpinned in a set order, a section at a time, and the earth dug out in layers under temporary props, the house above carried at every stage.
- 04
Slab & Structure
The new concrete slab, steels on padstones carrying the walls above, the light well and the new stair.
- 05
Waterproofing
A cavity drain membrane on the walls and floor, draining to a channel and a sump with its pumps, so any water that comes through the structure is taken away.
- 06
Fit-Out & Handover
Insulation, screed, linings, ventilation, electrics and lighting, and the finished room, with building control sign-off at handover.
Why Choose OKPW
Structure to Finish, One Team
The underpinning, the waterproofing and the finished room built by the same team, so nothing that keeps a basement dry falls between two contractors.
Built in Sequence
Each wall underpinned a section at a time in a set order and the earth dug out under props, so the house above is carried at every stage.
Transparent Pricing
Detailed, itemised quotations with no hidden costs. You know exactly what you're paying for.
Before you start
Do I need planning permission?
Converting an existing cellar often doesn't need it. Digging out a new basement usually does, and so does adding a light well; many London boroughs have their own basement policies. We check your house and your borough on the survey.
Do I need a party wall agreement?
Almost always. Underpinning the walls you share with your neighbours is work the Party Wall Act covers, so your neighbours must be given notice. We explain which notices your job needs on the survey.
How much does a basement cost?
Our guide prices run from £60,000 for converting an existing cellar to £600,000 for a full dig-out under the whole house, plus VAT; a partial dig-out under part of the house is £150,000 to £300,000. The cost calculator shows the price for your own house.
How do you keep a basement dry?
With a cavity drain membrane on the walls and the floor. Any water that comes through the structure runs down behind it to a drainage channel and a sump, and pumps take it away. It is the system our prices are based on.
What's the difference between a cellar conversion and a dig-out?
A conversion makes a proper room of a cellar the house already has, usually lowering its floor and underpinning its walls. A dig-out makes a new basement where there was none, under part of the house or all of it.
Make Your Home Worth More
A free, no-obligation survey by the people who do the work — covering Central, South and West London and Surrey, out to Guildford and Dorking.
