In Kensington, going down is often the only way to add real space. TrustBuilt Projects designs and builds basement conversions across W8 and SW7 — underpinning, lightwells and twin BS 8102 waterproofing — working within the Royal Borough's strict basement policy (CL7) from day one.
Kensington's grand stucco terraces and garden-square houses have little room to extend outward, so a basement — a new excavated storey beneath the house and garden — is how owners add a media room, gym, pool or extra bedrooms without altering the protected elevation. The catch is that the Royal Borough of Kensington and Chelsea runs one of the toughest basement regimes in the country, so the planning route matters as much as the build.
RBKC's Policy CL7 and 2016 Basements SPD set firm limits: generally only a single storey below ground (double basements are refused in most cases), no more than 50% of the garden may be built over, and at least one metre of permeable soil must be kept above any garden basement so planting survives and water can drain. The original garden level can't be lowered, and a Subterranean Construction Method Statement, Construction Traffic Management Plan and the borough's Code of Construction Practice all apply.
Much of W8 and SW7 is listed or within a conservation area, which tightens things further — basements under or affecting a listed building need listed building consent and are heavily restricted. We lead with the structural engineer's design, the basement impact and ground-water assessment and the party wall awards, then underpin sequentially and tank to BS 8102 so the new space stays permanently dry.
Basements are the most specialised work we do, and in prime Kensington the figures reflect depth, listed status, lightwells and waterproofing. As a guide:
| Project type | Typical cost | Timeline |
|---|---|---|
| Single-room basement (under existing footprint) | £180,000 – £350,000 | 20–32 weeks |
| Full-footprint basement + lightwell | £350,000 – £650,000 | 32–52 weeks |
| Basement under house & garden | £600,000 – £1,000,000+ | 12–18 months |
| Listed / complex deep basement | POA | 12–24 months |
Every project is quoted in writing after a free site visit — these ranges are a guide only.
In most cases yes, but within strict limits. RBKC's Policy CL7 generally allows only a single storey below ground, restricts garden basements to 50% of the garden, and requires at least one metre of permeable soil above any garden basement. We assess your specific site against the policy before committing to a design.
Generally no. The Royal Borough refuses more than one basement storey in almost all residential cases — multi-storey basements are only entertained on large comprehensively planned sites, not typical houses. We design to a single, well-proportioned storey that the council will support.
It's much harder. Basements under or affecting listed buildings are heavily restricted and need listed building consent alongside planning. We advise honestly at survey stage on what's realistic, and prepare the heritage and structural case where a scheme is feasible.
We design twin waterproofing to BS 8102 — Type A tanking plus a Type C cavity-drain membrane draining to a sump with dual pumps and battery backup — so there are two independent lines of defence. The ground-water and basement impact assessment informs the drainage strategy for your street.
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