The first Finchley basement question is whether below-ground space is the right way to buy the square metres at all. On a typical N3 or N12 semi there is often still something to give above ground — a side garage, a rear projection that has not used its full depth, a loft that can take a dormer. Metres won above ground are almost always cheaper, quicker and less risky than metres dug out. The basement briefs that survive scrutiny here are those where the plot has genuinely been used up, or where the accommodation wanted does not need daylight — plant, storage, a gym or cinema.
Once a basement is the right answer, the survey decides the job type. Where a cellar exists — established by inspection, not assumed from the age of a 1930s house — the work is usually lowering the floor and extending the box. Where there is nothing below the ground floor, every wall the house rests on must be carried down before usable space appears, and a new stair takes a slice out of the ground floor you already own. Feasibility is settled by a structural engineer, ground investigation and a section through the house.
Ground conditions under Finchley’s interwar streets cannot be taken on trust from a finished basement two roads away. Trial pits and boreholes establish foundation depths, made ground, groundwater and obstructions. Those findings drive the retaining strategy, waterproofing grade, temporary works and the movement that must be predicted next door. They also, occasionally, kill a scheme early — far cheaper at investigation stage than after drawings are complete.
Underpinning is the structural heart of the work, and on a Finchley semi it is the neighbour’s problem as much as yours. Walls are picked up in short numbered bays in a hit-and-miss sequence so only a short length is unsupported at once. On a terraced or semi-detached house those bays sit under a wall shared with an occupied home, so the bay sequence, propping and movement monitoring are designed by the engineer and belong to the scheme — not left for a contractor to improvise on site.
A basement floor sits below the drainage that already serves the house, so water governs the design. Waterproofing is designed to BS 8102 and normally combines two independent forms of protection — commonly a maintainable cavity-drain membrane drained to a sump with twin pumps, paired with a watertight structure or external tanking. Waste is pumped up to the sewer with backflow protection. Pumps need power, backup, discharge and lifelong servicing.
What often decides the application is the part that is not buried. Habitable rooms below ground need daylight, ventilation and escape — usually a lightwell with retaining wall, guarding, steps and a grille. On a mock-Tudor semi the lightwell is read against a paired composition drawn as one house; on a short Edwardian garden it competes with remaining amenity and any trees. We design those external works first and let the engineering follow what Barnet can actually consent.
