Beneath many Hammersmith homes sits space the house has never used. A basement conversion turns it into a home cinema, a gym, a guest suite or a second living space, without changing how the house looks from the street.
Sovran converts existing cellars and excavates new basements in Hammersmith, Brook Green, Ravenscourt Park and Brackenbury Village as one team: planning, structural design, waterproofing and fit-out under one contract. Our studio is in Isleworth, west London.
Bottom line: a new basement excavation costs £2,800 to £5,500 per m² and takes 20 to 28 weeks on site; converting an existing cellar takes 16 to 22 weeks. In Hammersmith and Fulham, the planning route is the first thing to settle.
New excavation and cellar conversion follow different structural and planning paths. Knowing which applies to your house is the first decision.
A new level dug beneath the house or part of the garden, for houses with no cellar. Underpinning and temporary works make it the most demanding route, and it takes 20 to 28 weeks on site. In Hammersmith and Fulham it is limited to one storey.
A house with an existing cellar or crawl space gains a full room by lowering the floor, underpinning and fitting out. The result meets the same waterproofing and Building Regulations standard as a new excavation and takes 16 to 22 weeks on site.
Hammersmith and Fulham Council has a dedicated basement policy. It limits new basements to a single storey and to no more than half the depth of the garden, to protect neighbouring homes and green space. An existing cellar may need no permission, depending on the works. Since April 2018 a borough-wide Article 4 Direction has removed permitted development rights for basement development, so a new excavation always needs full planning permission.
Basement rules in Hammersmith and Fulham at a glance (council sources checked November 2026)
| Rule | What it means |
|---|---|
| Storeys | New basements are limited to one storey |
| Garden | No more than half the depth of the garden |
| Permitted development | Removed borough-wide since April 2018, so a new excavation needs full planning permission |
| Supporting report | Basement Impact Assessment for new excavation |
| Flood risk | Assessed where the property is in a flood zone |
| Neighbours | Party Wall Act agreements |
| Sewers | Thames Water build-over agreement within 3m of a public sewer |
The council expects a new excavation to come with a Basement Impact Assessment. It sets out how the ground, groundwater, drainage and neighbouring houses are protected, and it is prepared with the structural design rather than added at the end.
Most Hammersmith houses are terraced or semi-detached, so party wall agreements with neighbours are part of the process. Within 3m of a public sewer, a Thames Water build-over agreement is also required. Sovran manages both.
Some streets close to the river lie in a flood risk zone. Where that applies, the application is supported by a flood risk assessment, and the drainage and waterproofing strategy is designed into the structure from the start.
Many Hammersmith streets sit in conservation areas, and Listed Building Consent is required to add a basement beneath a listed building.
Our planning team includes former local authority planning officers, and 97% of our planning applications are approved. We set the planning route before design starts.
Ground conditions change from street to street, and the water table is higher closer to the river. A ground investigation sets the groundwater level the design has to resist. The waterproofing strategy follows BS 8102, is designed with the structure and is agreed before the price is fixed, so the space stays dry for the life of the house.
A new excavation typically costs £2,800 to £5,500 per m², and more for a swimming pool or cinema build. The price is fixed once the structural design and waterproofing strategy are resolved, not before.
| Type | Time on site | Cost guide |
|---|---|---|
| New excavation basement | 20 to 28 weeks | £2,800 to £5,500 per m² |
| Existing cellar conversion | 16 to 22 weeks | Priced after survey |
Head height, lightwell position and the stair down are decided at the first drawing, because they decide whether a basement feels like a room or a cellar. The same studio designs and builds it, under one contract.
Surveyed and understood before anything is drawn.
Shaped around light, headroom and the connection to the floor above.
Planning, Building Regulations and party wall agreements managed for you.
One team on site, one point of contact.
A whole new room, finished as imagined.
A new excavation needs full planning permission in Hammersmith and Fulham. Converting an existing cellar may not, depending on the works, but Building Regulations approval is still required and the rules are tighter in conservation areas and for listed buildings. Sovran confirms the planning route for your address before design starts, and our planning team includes former local authority planning officers.
The council's basement policy limits new basements to a single storey and to no more than half the depth of the garden. The aim is to protect neighbouring properties, drainage and green space. We design to the policy from the first drawing, so head height, lightwells and layout work within what the council will approve.
New excavation is normally supported by a Basement Impact Assessment, which covers structure, groundwater, drainage and the effect on neighbouring properties. Party wall agreements with neighbours are also part of the process, and a Thames Water build-over agreement is needed within 3m of a public sewer. Sovran manages these processes as part of the project.
A new excavation typically costs £2,800 to £5,500 per m², and more for pool or cinema builds. An existing cellar conversion is priced after survey because the condition of the floor and walls decides the work. Sovran fixes the price once the structural design and waterproofing strategy are resolved. The cost calculator gives an early estimate before the survey.
On site, a new excavation takes 20 to 28 weeks and an existing cellar conversion 16 to 22 weeks. Design, planning and party wall agreements come first. We agree the programme at the start and manage it as one team, so you know what happens each week and who to call.
By designing the waterproofing together with the structure, to BS 8102, and agreeing the strategy before the price is fixed. Near the river, drainage and flood risk are assessed as part of the design. The strategy is built into the walls, floor and drainage rather than added at the end, so the space is dry from the first day.
It can add significant living space, and the value depends on the finish, the natural light and the ceiling height. A well-placed lightwell and staircase make the difference between a room and a cellar, which is why they are decided at the first drawing.
Explore our Basement Conversions service, the waterproofing guide, our Design and Build approach and the cost calculator.
Tell us about your house and the space beneath it. We will come back with a clear scope and the next steps.
Every great space starts somewhere. Pick a time that works for you and we'll show you what's possible.