Cellar conversions, lowered floors and full basement excavations — waterproofed to BS 8102, structurally engineered, and built by a team that also installs the drainage, heating and power that a habitable basement depends on.
"Basement conversion" covers work that ranges from a fortnight to the better part of a year. Establishing which one you're actually contemplating changes the budget by an order of magnitude.
Everything else in a basement is conventional construction. Keeping it dry, permanently, is the part that separates a basement that works from one that has to be opened up again in five years.
The governing standard is BS 8102, and it defines three approaches: Type A (barrier — a tanking membrane), Type B (structurally integral — waterproof concrete) and Type C (drained protection — a cavity drain membrane feeding a sump and pump). For habitable space in London ground conditions, Type C, often combined with Type B, is the usual specification, because it manages water rather than trying to resist it absolutely.
A Type C system is only as good as its maintenance. That means twin pumps with a battery backup, accessible rodding points designed in from the start, and a service regime afterwards. We design for the inspection access at the outset — retrofitting it is close to impossible.
Alico is a registered installer with Delta Membranes, and we specify and install the waterproofing rather than subcontracting it away from the team responsible for the structure it sits against.
Lowering a floor or excavating a new basement means taking the load of the existing walls down to a new level, in short sequenced bays, with each bay cast and cured before the next is opened. It is slow, methodical work and the sequence is designed by a structural engineer, not decided on site.
Because that excavation is almost always within three metres of a neighbouring property and below the level of their foundations, the Party Wall etc. Act applies in nearly every case. Expect to serve notice, expect at least one neighbour to dissent, and expect surveyors to be appointed and an award agreed — including a schedule of condition of their property — before you break ground. One month's notice minimum for excavation, and in practice the process takes considerably longer.
Several London boroughs also apply specific basement planning policies restricting depth, extent under gardens, and construction hours. The Royal Borough of Kensington and Chelsea in particular has among the most restrictive basement policy in the country. Establish what your borough permits before commissioning a design.
A straightforward cellar conversion with adequate head height can be 8–12 weeks. A lowered floor with underpinning is typically 20–30 weeks. A full excavation under a house is usually 9–18 months including the consents. The party wall and planning process in front of the build is often six months on its own.
A correctly designed and installed Type C system with twin pumps, battery backup and accessible maintenance points is the industry standard for habitable basements and comes with an insurance-backed guarantee on the membrane. What no one can honestly guarantee is a system that is never serviced — the pumps and drainage channels need periodic inspection, and we set that out at handover.
It depends entirely on your borough and whether you're extending beyond the existing footprint. Converting an existing cellar with no external alteration often doesn't. Excavating under a garden, forming a lightwell, or altering the front elevation almost always does — and several London boroughs, RBKC especially, apply specific and restrictive basement policies. Check locally before you design.
For any excavation near their foundations, the Party Wall etc. Act 1996 requires notice. In practice you should expect surveyors to be appointed, a schedule of condition of the neighbouring property to be recorded before work starts, and an award setting out working hours, monitoring and access. Handled early it's administrative. Handled late it stops the job.
In prime central London, where floor area is the constraint and the plot can't be extended outwards, frequently yes. Further out, an extension or a loft will almost always give you more usable space per pound. We'll tell you honestly which one your property suits.
What it costs, what consent you need, and what to check before taking a quote.
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