The first question we ask about a Golders Green basement is whether it is the right way to buy the space at all, and on this stock that is a real question rather than a rhetorical one. The district's interwar semi-detached and detached houses, its double-fronted villas and its Edwardian bay-fronted homes generally sit on plots with something still to give above ground: a margin or driveway at the side, a garage, a rear elevation that can be opened up, a roof that can be converted, a mature garden that can take a modest projection. Metres won above ground are almost always cheaper, quicker and far less risky than metres dug out below it. So the basement briefs that survive scrutiny here are the ones where the plot has genuinely been used up, or where the accommodation wanted does not need daylight in the first place — plant, storage, a gym, a cinema, a pool. We will tell you which of those you are in before you spend design money on either.
Once a basement is the right answer, the survey decides what kind of project it actually is. Where the house already has a cellar or a lower-ground storey — something established by inspection rather than assumed from the age of the building — the work is usually lowering the existing floor and extending the box, and the existing walls only have to be picked up to the new level. Where there is nothing beneath the ground floor, the entire structure has to be formed from scratch under a building that was never designed to stand above one, and every wall the house rests on must be carried down before a single usable metre appears. A new internal stair then has to land somewhere, taking a slice out of the ground floor you already own. Feasibility on a basement is settled by a structural engineer, a ground investigation and a section cut through the house — not by a floor plan.
Nothing about the ground beneath your plot can be taken on trust, and a neighbour's finished basement two streets away is not evidence about yours. Trial pits and boreholes establish what the existing walls are founded on and at what depth, whether made ground, old foundations or obstructions are present, and whether groundwater sits within the depth you propose to dig; monitoring over a period shows whether that level shifts with the seasons. Those findings, and nothing else, drive the retaining strategy, the grade of waterproofing, the temporary works and the movement that has to be predicted in the buildings next door. They also, occasionally, kill a scheme outright — which is a far cheaper thing to learn at investigation stage than after an application has been drawn, submitted and defended. We commission the investigation to inform the design, not to satisfy a validation checklist behind it.
Underpinning is the heart of the work, and on Golders Green's semi-detached and terraced houses — where a flank wall is shared with an occupied home next door — it is the neighbour's problem as much as yours. Walls are picked up in short numbered bays worked in a hit-and-miss sequence, so that only a small length is ever unsupported at any moment: each bay is dug, cast, pinned tight up to the existing footing and left to gain strength before the next one is opened. Where the ground investigation warrants it, reinforced concrete or a piled retaining wall may replace traditional mass-concrete bays, and the new slab is designed both to carry the walls above and to resist water pressure pushing up from below. On a semi-detached or terraced house those bays sit under a wall shared with an occupied home, so the bay sequence, the temporary propping and a movement-monitoring regime are designed by the engineer and belong to the scheme — they are not details left for a contractor to improvise on site.
A basement floor sits below the drainage that already serves the house, which is why water governs the design rather than decorating it. Waterproofing is designed to BS 8102 and normally combines two independent forms of protection: commonly a maintainable cavity-drain membrane lining the walls and floor, drained to a sump with twin pumps, paired with a watertight concrete structure or external tanking. Foul and surface water from the new rooms then has to be lifted up to the sewer with protection against backflow. The pump is not a component you fit and forget — it needs power, a backup supply, a discharge point and servicing for as long as the room exists, and anyone buying a basement should understand that commitment before committing. Insulation, ventilation and condensation control are designed alongside the waterproofing, because a sealed box with poor air movement feels like a cellar even when it is perfectly dry.
What decides most basement applications, though, is the part that is not buried. A habitable room below ground needs daylight, ventilation and a means of escape, and in practice that means a lightwell — with a retaining wall, guarding, steps and a grille or gate — cut into a garden, a front area or a driveway. On these plots it lands in one of two sensitive places. At the front it is seen from the street and, on a semi-detached pair, read against the neighbouring half of a composition drawn as one house. At the rear it sits among the mature gardens the area is known for, and where a conservation designation reaches the address Barnet states that trees in conservation areas are protected by law, so root protection can limit where you may excavate at all. Add the ventilation grilles, the sump discharge, any relocated plant and the hard surfacing over the new slab, and an invisible project acquires a very visible footprint. We design those elements first and let the engineering follow what can actually be consented.
