Some of the best space in a London home is already under it. A basement conversion turns an unused cellar — or solid ground beneath the house — into a full storey of living space, without losing a square metre of garden and without the planning fight that often comes with building upwards or outwards.
What we build below ground
Construserv delivers basement projects end to end: underpinning and structural works, waterproofing, drainage and pumped systems, ventilation, heating, lighting and the full interior fit-out. We have built indoor swimming pools, spa and sauna suites, home cinemas, gyms, home offices, guest suites, wine rooms and utility space — and we have done it in occupied family homes in London and the Home Counties, where keeping the house liveable during the works matters as much as the finish.
Lowering an existing cellar, or digging new
Most projects fall into one of three types. Converting an existing cellar is the simplest: the structure is already there, so the work is waterproofing, drainage, head height and finishes. Lowering an existing cellar adds usable ceiling height through sequenced underpinning, and is the most common route in Victorian and Edwardian terraces. Excavating a new basement under the footprint of the house — or under the garden — gives the most space and takes the most engineering. We will tell you honestly which of the three your property suits before you spend money on drawings.
Water is the whole job
Everything below ground lives or dies on waterproofing. We design to BS 8102 and build in redundancy rather than relying on a single barrier: external tanking where the excavation allows it, a cavity drain membrane system inside, perimeter drainage to a sump with twin pumps and battery backup, and accessible inspection points so the system can be maintained for the life of the building. A basement that is dry on handover but cannot be serviced in ten years is not finished work.
Permissions, neighbours and the Party Wall Act
Many basement conversions under the existing footprint fall under permitted development, but a light well, a separate entrance, a listed building or a conservation area will usually need a planning application, and several London boroughs have their own basement policies. Building Regulations approval is always required, structural calculations must be signed off by an engineer, and where you are underpinning or excavating close to a shared wall the Party Wall etc. Act 1996 applies — which means serving notice on your neighbours and appointing surveyors. We handle the co-ordination, set the programme around the approvals rather than against them, and keep the neighbours informed, because a neighbour who understands the programme is the difference between a smooth job and a stopped one.
How long it takes
A straightforward cellar conversion is typically eight to twelve weeks on site. Lowering and underpinning an existing cellar usually runs four to six months. A full new excavation with a pool or spa is a nine to eighteen month programme. Design, engineering, planning and party wall notices sit in front of all of those and commonly add three to six months — start that early and the build runs to time.
Talk to us before the drawings
The cheapest hour you will spend on a basement is the first one. We will walk the property, tell you what is realistic, flag the party wall and drainage issues early, and give you a cost range you can plan around rather than a number designed to win the job.