Under Kensington's garden-square terraces, a basement is how owners add a gym, media room or pool without losing the protected garden above. TrustBuilt Projects designs and builds basement conversions across W8 and SW7 — sequential underpinning, lightwells and twin BS 8102 waterproofing — working within RBKC's Policy CL7 and the 2016 Basements SPD from day one.
Kensington's tall stucco terraces around Cornwall, Stanford and Cottesmore Gardens — and the houses fronting the communal squares off Kensington High Street and Holland Park — have almost no room to grow outward, so a single excavated storey beneath the house and garden is how owners add a gym, cinema, wine room, pool or staff accommodation. The squares themselves are the sensitivity: the Royal Borough protects their greenery and mature trees as fiercely as the listed frontages.
RBKC's Policy CL7 and 2016 Basements SPD set the limits, and on a garden-square house they bite hard: generally a single storey only (double basements are refused in almost all cases), no more than 50% of the garden built over, and at least a metre of permeable soil retained above any garden basement so the square's planting survives and rainwater still drains. The garden 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 in a conservation area, and many homes are flats in mansion blocks where digging simply isn't on the table. For a garden-square house we lead with the structural engineer's design, the basement impact and ground-water assessment and the party-wall awards with neighbours on both sides, protect the trees the policy requires us to keep, then underpin in sequence and tank to BS 8102 so the new storey stays permanently dry.
Basements are the most specialised work we do, and under a prime Kensington garden-square house the figures reflect depth, tree protection, 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 a single storey below ground, restricts a garden basement to 50% of the garden, and requires at least a metre of permeable soil kept above it so the square's planting and drainage survive. We assess your specific W8 or SW7 site against the policy before designing anything.
Yes — tree protection is central in Kensington. The Royal Borough guards the mature trees and greenery of the garden squares, so the structural and arboricultural design has to keep roots and the required permeable soil intact. We build the tree-protection measures and the one-metre soil rule into the scheme from the start.
Generally no. The Royal Borough refuses more than one storey below ground in almost all residential cases — multi-storey basements are only entertained on large, comprehensively planned sites, not garden-square houses. We design a single, well-proportioned storey the council will support.
Almost never. Basement excavation under a purpose-built mansion block or a converted stucco terrace isn't realistic, and lease and freeholder terms rule it out in any case. We're honest about this at survey stage and focus the value where it can actually be won inside the flat.
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