Most Kensington basements are formed by underpinning: the existing walls are supported in short, alternating sections — a hit-and-miss sequence — while new reinforced concrete is cast beneath them to carry the load down to a lower level. Where a whole new storey is dug, temporary propping holds the structure as spoil is removed, often by hand or conveyor, through a tightly constrained site. This sequence is engineered before anything is excavated, because the order of works, the width of each bay and the depth of each pour all govern whether the building above stays stable throughout the dig.
The Royal Borough applies one of London's most developed basement policies, and feasibility has to be tested against it from the outset. In general terms it restricts new basements to a single storey below the lowest floor, limits how much of an unbuilt garden may be excavated — commonly around half — and requires a depth of soil above any garden basement so planting can survive. A Basement Impact Assessment and a construction traffic and method statement are typically expected. These positions must be confirmed for the specific address, because the borough scrutinises basement proposals closely and no outcome is guaranteed.
The Basement Impact Assessment is where ground and water are examined rather than assumed. Much of Kensington sits on London Clay, but perched water, made ground and the line of culverted watercourses — the historic Counter's Creek runs down the borough's western edge — mean groundwater flow and flood risk are assessed on their merits. The report considers how the excavation affects neighbouring structures, drainage and the wider water environment, and it is coordinated with the structural design. On a tightly packed Kensington plot it is frequently the document on which the planning case turns, so it is commissioned early.
Waterproofing a Kensington basement is a design discipline, not a product choice. Because the finished space can sit below the water table for parts of the year, we work to a strategy that normally combines more than one method — an external tanking or barrier system with an internal drained cavity that channels any water to a sump and pump — so a single failure does not flood the room. The detailing at wall-to-floor junctions, service penetrations and the join with retained historic fabric is where basements succeed or leak, and it is set out fully in the building regulations and specialist waterproofing drawings.
Excavating within three or six metres of a neighbour's foundations engages the Party Wall etc. Act 1996, and in Kensington's terraces and mews — where houses share walls and sit hard against one another — every basement does. Notices must be served on each affected owner before excavation begins, and where a neighbour dissents, appointed surveyors produce an award recording the neighbouring property's condition and setting conditions for the works. The process runs to weeks, so it is planned into the programme early. Accurate drawings and a clear excavation sequence underpin a fair award and reduce the scope for dispute.
Getting light, air and a safe means of escape into a below-ground room shapes the whole design. Front and rear lightwells, sub-pavement vaults and egress stairs have to work structurally and satisfy building regulations, while in Kensington's conservation areas the appearance of a front area, its railings and any rooflights are closely controlled. We resolve these together — the structure, the waterproofing line, the escape route and the heritage detail — rather than leaving them to collide on site. A basement designed this way reads as part of the house, not an afterthought bolted beneath it.
