The first question on a West Hampstead basement is not how deep, but whose ground it is. So much of the stock here that reads as a Victorian or Edwardian house from the pavement was carved into flats decades ago, and the owner who most wants a basement is very often the lower-ground or garden-flat leaseholder standing directly on the sub-soil. Standing on it is not owning it. In this kind of lease the ground beneath the building, the main structural walls and the structure between dwellings are commonly retained by the freeholder, so the volume you want to excavate may be an asset you would have to acquire before you could touch it. A basement also alters the load path of every household above you, which gives the freeholder and their surveyor a direct structural interest in what happens under the floor. We ask for the lease, the title plan and the demised-extent drawing at the first meeting, because a below-ground scheme drawn against sub-soil that belongs to somebody else is an expensive way to discover the project never existed.
Where ownership is clear, the next honest answer is that nobody yet knows what is under the house. A neighbour's completed basement, however identical the frontage looks, is evidence about their plot and not about yours. The West End Green appraisal describes that conservation area as sitting roughly between the 55 and 80 metre contours, falling from the higher Finchley Road side toward lower ground — a statement about the area's shape, not a finding about the ground or the water beneath any particular property. What actually governs the design is a site investigation: trial pits establishing what your walls are founded on and at what depth, boreholes recording the strata and any made ground, and monitoring showing whether groundwater sits within the proposed dig and whether that level moves with the seasons. Those results, not a preference and not a precedent, determine the retaining strategy, the waterproofing specification and the movement predicted in adjoining buildings. They also produce, occasionally, the most useful advice we give: that a basement is the wrong way to buy this particular family the space it wants.
Underpinning is not a preliminary to a West Hampstead basement; it is the project. A period terrace was built to carry itself on modest, shallow footings with no expectation of a storey beneath, so the walls have to be picked up and carried down before a single square metre of room is gained. The work proceeds in short numbered bays in a hit-and-miss sequence, each excavated, cast, pinned tight up to the existing footing and left to gain strength before its neighbour is opened, so that only a small length of wall is ever unsupported. Where the ground investigation warrants it, reinforced-concrete underpinning or a piled retaining solution may replace traditional mass concrete, and the new slab is designed to resist water pressure pushing up from below as well as to carry the building pressing down from above. On a terrace those bays sit beneath a wall shared with an occupied home, so the bay sequence, the temporary propping and the movement-monitoring regime are part of the consented scheme, not site decisions taken by a contractor with a deadline.
That is why the Party Wall etc. Act 1996 sits on the critical path of a basement rather than at its edges. Underpinning a shared wall is notifiable in its own right, and excavating within three metres of an adjoining structure to a level below its foundations, or within six metres on the deeper diagonal, requires notice — so on a mid-terrace NW6 house you are commonly serving on owners to both sides, and sometimes to the rear as well. In a converted building the geometry is stranger still: the party structure can run horizontally, above your head, and the adjoining owner may be the household living directly over the excavation. Each adjoining owner may appoint a surveyor; a schedule of condition records their property before work begins, which protects you at least as much as it protects them; and the award fixes method, hours, monitoring and access. Below-ground projects that stall rarely stall at planning. They stall here, because notices were served late.
Keeping the room dry is a lifetime commitment, not a product decision. Waterproofing is designed to BS 8102 and normally combines two independent methods: a maintainable cavity-drain membrane lining walls and floor, drained to a sump with twin pumps, paired with an integral watertight concrete structure or external tanking, with the balance between them set by what the ground investigation actually found. Because the new floor sits below the level of the drainage it has to reach, foul and surface water must be lifted to the sewer with protection against backflow, and where the structure sits over or close to a shared or public drain a build-over agreement with the sewerage undertaker runs on its own timescale alongside planning. The pumps need power, a backup supply, a discharge point and servicing for as long as the room exists. Insulation, ventilation and condensation control are designed with them, because a sealed box with poor air movement will feel damp even when it is watertight, and clients deserve to know that before they commit.
What finally decides the scheme, though, is the half of it that shows. A habitable basement needs daylight, ventilation and a means of escape, and in a terrace that generally means a lightwell — a retaining wall, guarding, steps, a grille or gate — cut into a front area or rear garden, plus a stair down that satisfies the building regulations and steals a slice of the ground floor you already have. Where the address sits within the West End Green Conservation Area, that lightwell, its railings, its paving and any ventilation or plant terminal are assessed against a frontage the neighbourhood plan describes as homogeneous Victorian and Edwardian domestic architecture, and read against the whole run rather than your house alone. So we design the visible elements first and let the engineering follow what can actually be consented, rather than excavating a beautiful box that has nowhere lawful to put its light, its air or its escape. We do not promise approval; we make the case on evidence and tell you early when the evidence is against it.
