Basement conversions and extensions
Lower ground digs and lightwell conversions, engineered for water and managed for neighbours.
Arrange a site visit →A basement is the most engineering-heavy project in residential construction: an excavation under or beside a live house, a waterproof structure designed for groundwater that is always present, and a party wall process that involves every neighbour your foundations come near. It is exactly the kind of project a design and build company should be doing — one team owning the engineering, the dig and the finish.
We build lower ground floor conversions, lightwell extensions and full basement digs under gardens and houses in Kensington, Chelsea, Belgravia, Marylebone and Hampstead. Prime central boroughs regulate basements tightly, and the difference between a consented, buildable scheme and an expensive refusal is in the design stage.
What the boroughs allow
RBKC's basement policy is the strictest in London: a single storey under the house only, covering no more than 50% of the garden, with the retained garden left open to the elements — and no basement at all under many listed buildings. Westminster requires demonstration of no harm to neighbouring properties and a full construction management plan, and Camden's policy resists anything beyond a single-storey dig under part of the house. We design inside the relevant policy from the first sketch rather than designing first and finding out at committee. Every scheme is drawn to satisfy the borough's own validation requirements, not a generic template.
Water, structure and the dig
Basement structures are engineered for water for the life of the building: reinforced concrete or lined masonry with a full waterproofing system — typically a drained cavity membrane with managed discharge, so any water that finds its way in is collected and pumped, not fought. Lightwells are designed for daylight and escape, engineered retaining walls do the structural work, and the dig sequence is planned around underpinning the existing house. Structural design is signed off by our engineers and verified by Building Control.
The neighbours, managed properly
A basement touches the party wall more than any other project — underpinning, excavation within measured distances, and temporary works all require party wall agreements with each sharing owner, plus dilapidation surveys before and after. We serve notices, coordinate the surveyors, agree the retention and monitoring regime, and sequence the dig so agreements are in place before the first bucket. In central London the logistics are managed too: parking suspensions, delivery windows and dust control are part of the plan we submit, not an afterthought.
Related
Answers, before you ask.
Can I dig a basement under my garden?
Often, but the borough rules differ: RBKC allows basement under no more than 50% of the garden with the rest left open, while other boroughs assess each case. We design to your borough's policy from the start and tell you plainly what it will and will not accept.
How deep can a London basement go?
Policy in the strict boroughs caps most residential basements at a single storey below ground level. Deeper schemes are refused in RBKC and heavily restricted in Westminster and Camden. One storey, well designed with generous lightwells, delivers the space in almost every case.
How is a basement waterproofed?
We build to a managed-water standard: a full waterproofing membrane system with a drained cavity and sump pump, so the structure tolerates groundwater permanently rather than trying to seal it out absolutely. The system is designed by our engineers and maintained for the life of the build with access points kept serviceable.
Do my neighbours have to agree to the basement?
Underpinning and excavating near the boundary are notifiable under the Party Wall Act; neighbours cannot veto the work, but their surveyors agree the method and safeguards in an award. We run the process so the dig starts with every award in place.
How long does a basement conversion take?
A single storey basement dig typically runs several months on site, dominated by the excavation, underpinning and concrete stages; the fit-out follows like any other room programme. We give a fixed programme before works begin.
Planning a project?
Send us the drawings and we will price it properly — one contractor, fixed programme, itemised quote.
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