A basement is the one project in South Hampstead where the thing you propose to alter is the part of the building nobody thinks of as theirs. The stock here runs heavily to red-brick mansion blocks and large houses long since divided into flats, and in that stock a lease normally demises the inside of a home — the internal faces, the finishes, the volume between them — while the fabric the building stands on remains with the freeholder. Foundations, below-ground party walls, the slab and the sub-soil under it are ordinarily building-wide assets rather than anybody's private ground. So the owner of a garden or lower-ground flat, standing on what feels entirely like their own floor, may hold no right whatever to dig beneath it. That is not a planning point, and no planning permission cures it. We therefore open a basement enquiry with the lease and the title plan, establish where the demise actually stops in the vertical direction, and only then decide whether a below-ground scheme is a design problem, a negotiation, or something that cannot honestly be built.
Where the building is a mansion block, we say the difficult thing early. Excavating beneath a block means excavating into the foundations that carry every home above it, and no single leaseholder holds the right to do that; nor, in most cases, will a freeholder entertain it, because the risk lands on the whole building while the benefit lands in one demise. Selling a design for that is not something we will do. What is genuinely available to a lowest-storey flat is narrower and more useful than it sounds: waterproofing and tanking a chronically damp room, winning daylight and ventilation within the volume that already exists, reworking the plan inside the demise, and — where the freehold is held collectively and every leaseholder is at the table — a properly negotiated variation that brings the sub-soil into the demise before an engineer is ever instructed. Each of those is leasehold-led work needing consent and real technical evidence. A house in single ownership meets none of these obstacles, which is why the address decides the project and the area never does.
Basement policy is the second gate, and it is a demanding one. A below-ground proposal is judged on what it can demonstrate about structural stability, about ground and groundwater conditions, about drainage and about the effect on neighbouring buildings, and that evidence — structural and, where the ground demands it, hydrogeological — is usually drawn together into a Basement Impact Assessment. Whether the authority wants it in that single form, and what else must sit beside it, is established from its requirements for the address before a consultant is instructed. What we will not do is quote you a permitted depth, a share of the garden or a storey count from memory. Those tests belong to the policy as it currently stands, they are the part of the regime most likely to have moved since whatever precedent someone shows you, and they have to be confirmed for the specific address and the specific authority before a footprint is worth drawing. Permitted development offers no way round any of it: a flat carries no such rights at all, and on conservation-area land what a house retains is commonly curtailed. We build the assessment as the spine of the application rather than as a report bolted on at the end.
Then comes the part nobody expects. A basement is invisible — right up until it needs daylight and a way out. Habitable rooms below ground must take light, ventilation and escape from somewhere, and on a terraced or semi-detached plan that somewhere is very often the front: a lightwell cut into the forecourt, guarding around it, a gate, an altered boundary wall, new paving, sometimes a vent at low level. Camden describes its Article 4 controls in this area as reaching front elevations and street-facing sides, and separately front boundary treatments — the walls, the gates and the fences themselves. The consequence is neat and slightly perverse: of all the project types here, the below-ground one is the single project whose only visible element lands squarely on the controlled face. The lightwell is therefore not a detail to resolve after consent. It is the planning application. We design it as a conservation element from the first sketch — proportions, guarding pattern, gate, paving and the depth of the void — because it is the piece a case officer will actually look at.
Excavation is also the work most likely to bring neighbours, and the households above you, into the project as parties rather than spectators. The Party Wall etc. Act 1996 is engaged by building on the line of junction and by excavating within three or six metres of a neighbour's structure, and a basement does both almost by definition: it goes deeper than the foundations next door and it works against the shared wall. Notices run to their own periods, an adjoining owner may appoint a surveyor, and a schedule of condition together with a movement-monitoring regime is a normal outcome rather than an unlucky one. In a subdivided South Hampstead building the list of interested parties is longer than the street suggests, because the freehold owner, and every household living directly above the dig, has a genuine interest in the answer. None of this is discharged by a planning consent, and unresolved neighbour procedure — not planning — is what most often stalls a below-ground scheme after permission has been granted.
Water and drainage are where a basement is won or lost technically. Waterproofing is designed rather than bought: the normal approach for a habitable below-ground room is dual protection, combining protection built into the structure itself with a drained cavity that collects any water reaching the inside and carries it to a sump. The sump needs a pump, the pump needs a power supply, a high-level alarm and a maintenance regime, and in a building of several homes somebody has to own that liability for decades. Ground and groundwater conditions are a site question, never an area assumption; they are established by investigation for your plot before a specification is written. Drainage compounds it, because a new floor commonly finishes below the level of the drain it must discharge into, so the discharge is pumped — and in a converted house the below-ground runs you excavate through serve the flats above, which have to stay in service throughout. Building regulations then cover structure, fire safety, insulation, ventilation, drainage, stairs and waterproofing, and escape from a below-ground habitable room constrains the plan from the outset. We do not promise approval; we build the case the evidence supports.
