03 / 15 Construction & conversions
A basement conversion adds living space without taking any of your garden or changing how your house looks from the street. It can turn a damp, low cellar into a family room, a home office, a gym or a guest bedroom, or create a completely new level under the house.
Whether a basement stays dry and comfortable depends on three things: the waterproofing, the structure and the ventilation. We plan all three from the start, carry out the structural and waterproofing work, and then fit out the room with the electrics, heating, plumbing and finishes, as one project with one quote.
Types
Many older houses already have a cellar, often used only for storage because it is damp, dark or low. A cellar conversion waterproofs, insulates, lights and ventilates the existing space so it can be used as a habitable room. It is usually the simplest and least disruptive type of basement project.
If an existing cellar is too low for a comfortable room, the floor can be dug down to increase the ceiling height. Because this goes below the existing foundations, the walls usually need to be strengthened first by our underpinning contractors, following a structural engineer's design.
Where there is no cellar, a new basement can be dug out beneath the house, or under the garden next to it. This is the most complex type of project, involving underpinning, temporary supports, a new concrete structure and full waterproofing, and it usually needs planning permission.
A quiet space away from the rest of the house, well suited to a cinema room because there is little natural light to control.
A separate workspace with its own lighting, sockets and data points.
A solid floor and good ventilation make a basement practical for exercise or for children.
Possible where there is a safe means of escape and enough natural light, usually through a lightwell.
A practical use of space close to the ground-floor kitchen.
Where waste pipes are below the sewer, a pumped system or a Saniflo installation carries waste up to the drains.
Waterproofing
Waterproofing is the most important part of any basement. The British Standard for below-ground waterproofing, BS 8102, describes three types of protection, and most basements use two of them together for extra security:
Where a pump is used, it should have a backup, such as a second pump or battery power, and be serviced regularly. We choose the system for your basement after checking the ground, the water table and the existing structure, and we carry out the drainage connections, checked first with a CCTV drain survey where needed.
Is it right for you
At the survey we check:
What you need to know
Converting an existing cellar into a habitable room, with no change to the outside of the house, usually does not need planning permission. You are more likely to need it if:
Several London boroughs have their own planning policies on basements, so we check your council's rules at the survey and prepare the drawings for any application.
What you need to know
Turning a cellar or new basement into a habitable room needs building regulations approval. Building control inspects the work at key stages and issues a completion certificate at the end. The main areas are:
What you need to know
Most basement projects in terraced and semi-detached houses fall under the Party Wall etc. Act 1996, because they involve digging close to, and often deeper than, the neighbouring foundations, or underpinning a shared wall. You must give your neighbours written notice before this work starts. If they agree, work can go ahead; if not, a party wall surveyor prepares an award. We tell you at the survey which notices are needed and build the time into the programme.
Cost
Every basement is priced after a free survey, and you receive a written, itemised quote. These are the main factors:
Converting an existing cellar costs least; lowering the floor costs more; a new basement excavation costs most.
How much of the house needs underpinning, and the steelwork and concrete required.
High water tables and difficult soils need more robust waterproofing and pumping.
The amount of soil to dig out and take away, and how easy access is.
Lightwells, walk-on glass and escape windows.
Pumped drainage for WCs and showers, and sump pumps.
Heating, electrics, flooring, a shower room, kitchenette or built-in storage.
Structural design, planning, building control and party wall surveyors.
No guesswork: you receive a written, itemised quote after your free survey, with payments staged against clear points in the work.
Timescale
A cellar conversion that needs only waterproofing and fit-out is the quickest type. Lowering a floor takes longer because of the underpinning, and a new basement excavation takes the longest. Before building starts, allow time for structural design, planning permission where needed, building regulations approval and party wall notices. Your quote includes a programme showing each stage.
10 — One team
A basement brings together structural, waterproofing and finishing trades. We coordinate all of them:
Lighting, sockets, data points and smoke alarms through our domestic electrical installation service.
Warm floors through our underfloor heating installation service, which keeps the walls free of radiators.
Shower rooms, WCs and utility rooms through our bathroom plumbing service.
Tiling, plastering and painting, so the room is ready to use on handover.
Process
Every stage is planned before we start, with one project manager from survey to sign-off.
we inspect the cellar or ground floor, check head height, drains and access, and talk through how you want to use the space.
structural design, a waterproofing specification and an itemised quote with a programme.
planning permission where needed, building regulations approval and party wall notices.
underpinning, excavation and new concrete, with temporary supports where needed.
membranes, drainage channels, sumps and pumps, or tanking, as specified.
insulation, wiring, heating, plumbing, plastering, flooring and decorating.
building control completion, electrical certificates, and guidance on looking after the pump and waterproofing.
Areas
We carry out basement conversions across London, with a focus on east and south-east London. Some Victorian and Edwardian houses in East Ham and Ilford were built with cellars that can be converted, while houses without a cellar can sometimes have a basement dug beneath them, subject to the ground and planning. Find out more on our pages for basement conversions in London, basement conversions in East Ham, basement conversions in Ilford, basement conversions in Barking, basement conversions in Dagenham and basement conversions in Orpington.
FAQs
Can't see your question? Call or WhatsApp us — we're happy to help before you book.
It depends mainly on whether you are converting an existing cellar, lowering the floor or digging a new basement, and on the ground conditions and waterproofing needed. Cellar conversions cost the least and new excavations the most. We give you a fixed, itemised quote after a free survey.
Usually not for converting an existing cellar with no change to the outside of the house. Digging a new basement, lowering the floor substantially, or adding lightwells or external steps usually needs planning permission, and listed buildings need consent. We check your council's rules at the survey.
A cellar conversion is the quickest. Lowering the floor takes longer because of underpinning, and a new basement takes the longest. Design and approvals come before the build, and your quote includes a programme for each stage.
Using the methods in British Standard BS 8102: tanking, which stops water getting in; waterproof concrete; or a cavity drain membrane that channels any water to a sump and pump. Most basements combine two of these for extra protection.
In most terraced and semi-detached houses, yes. Digging near or below a neighbour's foundations, or underpinning a shared wall, needs written notice under the Party Wall etc. Act 1996.
Yes. If the basement is below the level of the sewer, a pumped system or macerator carries the waste up to the drains. We design the drainage as part of the conversion.
One team, every trade
Next step
Tell us about your cellar or your house and how you would like to use the space. We will survey it, explain what is involved and send you a written, itemised quote.
Send us photos of your cellar on WhatsApp to get started, or use our contact form.
