Marylebone's residential grain — Georgian estate terraces, stucco-fronted townhouses and mansion blocks packed along streets like Harley Street and the Portman estate — means almost every basement is dug hard against a neighbour. Most houses already have a lower-ground floor, so the work is usually lowering and extending an existing storey rather than digging from nothing. Beneath the pavements sits London Clay with made ground and gravels near the buried Tyburn, the stream that gave Marylebone its name. That combination of shared foundations, restricted access and variable ground is what makes excavation here a structural exercise first and an architectural one second.
Gaining head height under an existing lower-ground floor almost always means underpinning the external and party walls. Traditional mass-concrete underpinning is carried out in short, numbered bays — a hit-and-miss sequence — so no more than a small length of wall is ever undermined at once, and each bay is pinned up to the existing footing before the next is opened. Where a full new box is formed, reinforced-concrete underpinning or a piled retaining wall may be more appropriate. The sequence, temporary propping and monitoring regime are set by the structural engineer and must be agreed before excavation begins.
Below-ground rooms have to stay dry for the life of the building, so waterproofing is designed to BS 8102 and usually combines two independent methods. A cavity-drain membrane (Type C) lining the walls and floor, drained to a sump and twin pumps, is common because it manages any water that reaches the structure and can be maintained. It is frequently paired with an integral watertight concrete box (Type B) or external tanking (Type A). Near the Tyburn's old course the water table can sit high, so drainage capacity and a maintenance strategy matter as much as the concrete itself.
Westminster applies one of London's more restrictive basement policies. In broad terms the council resists basements deeper than a single storey below the lowest floor, limits how much of a garden can be excavated, and expects a margin of soil to be retained for planting and sustainable drainage. A structural method statement and, where the ground or scale warrants it, a Basement Impact Assessment are normally required, together with a construction management plan governing lorries, hours and spoil removal. These constraints shape the achievable size early, so we test the policy position before layouts are fixed.
Excavation next to a neighbour engages the Party Wall etc. Act 1996. Underpinning a shared wall is notifiable work, and digging within three metres of an adjoining structure to a level below its foundations — or within six metres on a deeper diagonal — requires notice under section 6. In Marylebone's terraces you are often notifying owners on both sides, and sometimes above in a mansion block. Each adjoining owner may appoint a surveyor, a schedule of condition is recorded, and an award is agreed before work starts. Serving early keeps the party wall timetable from delaying the build.
Above ground the visible moves are lightwells, railings and rooflights. A front lightwell brings daylight and ventilation to new habitable rooms and can provide a means of escape, but it alters the front area, railings and pavement vaults that Westminster and the estates protect. Rear lightwells affect neighbouring outlook and drainage, and any external plant — ventilation, cooling or a sump discharge — needs a discreet position. We coordinate the architect's drawings with the structural engineer, waterproofing designer, party wall surveyor and, where needed, the Basement Impact Assessment team, so the scheme that gains consent is the one that can actually be built.
