A Hammersmith basement begins with what already exists below the ground floor. Many of the area's Victorian terraces have a shallow cellar under the front rooms, often with brick barrel vaults running out beneath the pavement, and the first question is whether the scheme lowers and waterproofs that existing space or excavates a genuinely new basement under the full footprint. Lowering an existing cellar by a modest amount to win head height is a very different structural and cost proposition from digging down and out beneath the whole house. The vaults themselves are frequently in shared or highway ownership, which adds a consent question before they can be incorporated. We survey the existing lower ground, the wall foundations, the ground-floor joist level and the vaults before advising which of those two routes the specific house and its neighbours can realistically carry.
The defining technical issue here is water. Hammersmith's proximity to the Thames means the riverside terraces and mansion flats sit in a context where ground water and flood risk shape a lower-ground scheme, and even away from the immediate riverside a basement puts habitable space below the external ground and, in places, below the water table. That drives the waterproofing strategy, which under the British Standard for below-ground structures is normally a combination of approaches rather than a single line of defence — a drained cavity system with a sump and pump backed by an integral watertight concrete structure — so that no one element is relied on alone to keep the room dry. It also drives the flood response: where a lower-ground floor sits in a flood-risk context, the design has to consider how water leaving the ground is dealt with and how the finished floor and any electrics are protected, and any resistance measures are set out for the specific address rather than assumed from the wider area.
Excavation and underpinning are what turn the design into a structural engineering exercise. Lowering a floor or forming a new basement beneath the existing walls usually means underpinning them in short, sequenced bays so the house is supported at every stage, then casting a new reinforced structure that carries the loads above. On the two- and three-storey terraces those walls are shared party walls, so the neighbour's house is physically dependent on the same fabric being worked on, and any movement has to be kept within tight limits that are often monitored during the dig. A structural engineer designs the underpinning sequence, the retaining walls, the ground-floor transfer structure and the temporary works, and that design is what the neighbour's party wall surveyor scrutinises before an award is agreed. Ground conditions established by a site investigation, not assumed, set how deep and how wide each bay can safely be.
Daylight is what makes a lower-ground floor a room rather than a store. Because a basement has little or no natural light of its own, the design leans on light-wells — a front light-well against the pavement vault line, a rear light-well or lowered garden area, and rooflights set into the garden above a basement that extends beyond the house. Each of those is also the element planning cares about, since it is the part of the scheme that appears above ground, and on a conservation street the railing, the retaining wall and the way the light-well meets the pavement are all scrutinised. We size and place the light-wells to bring usable daylight and, where they double as an escape, a compliant means of egress into the lower ground, while keeping any street-facing element subordinate to the period frontage.
Fire escape and the internal stair shape the plan more than owners expect. A habitable basement needs a protected escape route, and building regulations look for either a window or external door giving egress or a protected stair to a final exit, together with interlinked alarms and, on a deeper or more enclosed scheme, often a sprinkler or mist system. The new internal stair connecting the lower ground to the floor above has to meet the going, rise and headroom of Approved Document K, and on a narrow terrace it competes for the same tight plan as the existing hall and rear rooms. We fix the stair and the escape strategy at feasibility, because they decide how much of the excavated area is actually usable, and because retro-fitting a compliant escape into a finished basement is expensive and disruptive. Ventilation and mechanical extract also have to be planned in, since a lower-ground room cannot rely on opening windows the way an above-ground room can.
Bringing it together is a coordinated, sequenced job. We survey the house and its lower ground, set the excavation depth and footprint against the neighbour relationship, and work up the waterproofing, structure and light-wells as one design before producing the planning drawings and, once the principle is settled, the building regulations and technical package a contractor and building control need. On the terraced and riverside plots typical of Hammersmith the Party Wall etc. Act almost always applies and the structural design is central, so we identify the notifiable neighbours and coordinate the engineer and party wall surveyor early — the sequence in which a basement is dug, and the order in which consents fall into place, is as much a design decision as the room itself, and getting it wrong is what stalls projects on site.
