Constructum Code builds basement conversions across London as main contractor, working to your architect’s and structural engineer’s drawings. A basement most often forms part of a wider project — a whole-house refurbishment, or an extension delivered in the same programme — and we also take one on in its own right where the scope warrants a fully managed project.
Below ground the work comes down to three things: keeping the house supported while the ground beneath it is removed, keeping the finished space dry against water it sits below, and moving waste out of a room that is lower than the sewer. Everything above that is finishing.
Interested in Basement Conversions?
The three kinds of basement project
Converting an existing cellar
Where a cellar already has workable head height, the job is to make it habitable: waterproofing, insulation, drainage, lighting, ventilation and a compliant stair. No excavation, no underpinning, and the shortest programme of the three.
Most Victorian and Edwardian terraces have a cellar under part of the ground floor, though few have the height to convert without digging.
Lowering an existing cellar
The common London project. The cellar is dug out to gain height, and because the walls then sit above the new floor level, the existing foundations are underpinned down to it. Underpinning is carried out in short sequential bays, each excavated, formed and allowed to gain strength before the next is opened, so the house is never unsupported across a long span.
This is where the structural engineer’s design sets the programme, and where the party wall process usually decides when work can start.
Excavating a new basement
Digging beneath a house with no cellar, or extending out under the garden with a lightwell for daylight. The largest of the three, the longest programme, and the one most likely to need full planning permission.
Where a house extension or a whole-house refurbishment is also under consideration, it is generally worth delivering both within one programme — the structural work and the party wall process overlap, and new drainage and ventilation cost far less to install before the finishes go in than after.
How we work
Survey first. Existing foundations and their depth, ground conditions and water table, drainage runs, the position of neighbouring structures, and how spoil and materials will reach the street. On a terrace with no side access that last point shapes both the programme and the price.
Approvals. Planning is secured by your architect or planning consultant before the build begins. From there we coordinate the building control route, stage inspections and the notifications the works require, and work with the structural engineer and the party wall surveyor, whose award sets conditions we then build to.
Costs agreed upfront. You receive a detailed, itemised estimate before work begins. Where something changes below ground — which under older foundations it occasionally does — it is priced, explained and agreed with you before the work proceeds.
One team, one programme. Temporary works and underpinning in sequential bays, then the dig, the slab, waterproofing, drainage and services, followed by insulation, plastering and second fix. On most residential projects this is delivered by the same site team and project manager from start to finish. 95% of our projects are completed on or before the agreed schedule.
Regular communication. Progress updates throughout the build, and direct contact with the project manager running your site.
Minimal disruption. Spoil removal is the noisiest and dustiest part of a basement, and the part neighbours notice, so it is planned and protected rather than improvised. Noise, dust, waste and deliveries are managed around the fact that the house remains occupied and the neighbours remain next door.
Guaranteed on completion. A 10-year warranty covering the structural work, and a 12-month guarantee on first fix — the pipework, cabling and carpentry installed before plastering.
Fully insured and accredited. We carry £10m employers’ liability, £5m public liability and £1m professional indemnity cover. Constructum Code Ltd is a TrustMark registered firm, and registered with Certass and FairTrades.
What a basement conversion costs
Basement conversions in London with Constructum Code start from £79,000 plus VAT.
That figure reflects a conversion where the existing cellar structure and head height are largely workable — waterproofing, drainage, services and finishes, without substantial excavation. Lowering a floor or digging a new basement sits well above it.
The main variables are:
- Depth of dig. Every additional course of underpinning brings more structural work, more spoil and more time, and the relationship is not linear.
- Underpinning length. How much of the perimeter is taken down, and whether it runs along party walls on one side or both.
- Spoil removal. On a terrace with no side access, every cubic metre leaves through the house or over it. One of the largest single costs, and set by the site rather than the design.
- Waterproofing system. A drained cavity with pumps and maintenance access costs more than a tanked cellar, and on habitable space it is usually the right answer.
- Lightwells and external works. Excavating beyond the footprint brings in planning, retaining structure and a longer approvals period.
- Finish level. Joinery, sanitaryware, flooring and lighting, as on any project.
We price after the survey, so the estimate reflects the ground and the access on your site.
Common questions
Do I need planning permission?
A basement beneath the existing footprint, with no external alteration, often falls under permitted development. Lightwells, external stairs and any excavation beyond the footprint usually need full permission. Several London boroughs also apply their own basement policies limiting depth, footprint and working hours, and some have removed permitted development rights through Article 4 directions. Your architect or planning consultant will confirm the position for your address before the design is developed.
Will I need a party wall agreement?
Almost certainly. Underpinning a party wall requires notice, and so does excavating within three metres of a neighbouring structure to a depth below its foundations, or within six metres where the excavation cuts a line drawn at 45 degrees from the base of those foundations. On a terrace that covers both neighbours. Awards take time and they set conditions you then build to, which is why the party wall process starts early rather than when the diggers are booked.
How is a basement kept dry?
Waterproofing is governed by BS 8102, and habitable space is Grade 3 — dry, with no damp patches. Systems are Type A, a barrier applied to the structure; Type B, structurally integral waterproof concrete; or Type C, a drained cavity behind an internal membrane, which is the usual retrofit choice because it manages water rather than resisting it. The design is produced by a waterproofing design specialist alongside your structural engineer, and we build to that specification.
How does drainage work below the sewer?
By pump rather than gravity. A sump collects the waste and a pump lifts it to the sewer, and on habitable space that means a second pump and a battery backup, because one pump is a single point of failure in the lowest room in the house. The pumps and ventilation fall within our building services, and the maintenance schedule forms part of what you receive at handover — these systems fail when nobody can reach them, which is why access points are planned into the layout rather than closed in.
What about escape and ventilation?
A habitable basement needs a protected escape route to the outside under Approved Document B, and mechanical ventilation where openable windows cannot do the job. Both shape the layout, so they are designed in from the start rather than resolved at the end.
