The decision that shapes a West Hampstead rear extension is usually made in section rather than on plan. Camden's West End Green conservation area appraisal — recorded by Camden as adopted, with the management strategy, on 28 February 2011 and used to assess applications in that conservation area — places the conservation area roughly between the 55 and 80 metre contours, sloping from the higher Finchley Road side toward lower ground. That is a statement about the area, not a measurement of your garden, but it tells you what to look for. Where the ground behind the house does not sit obediently at internal floor level — and the levels should be surveyed for the exact address rather than assumed either way — the footprint stops being the interesting question. Whether the new floor steps down, whether the slab is lowered, where the threshold lands and how the roof is read from ground that sits higher all follow from a level survey. We take spot levels and draw the existing section early, before any footprint is agreed, because on a stepped plot the section is what the design, the cost and the planning case all hang from.
The rear building line is the second decision, and it cannot be settled by copying the neighbours. Where the ground falls, a projection that would be unremarkable between two houses standing at the same level can read as a much larger mass when it is seen from a garden below, or expose a whole run of roof when it is seen from ground that sits higher. Daylight and sunlight to neighbouring windows are assessed against the established BRE tests, and those tests respond to the height of the new wall relative to the neighbouring window — which a change in level moves. So we model the extension against the actual neighbour relationship on both flanks: the boundary wall, the level at which the adjoining house sits, and where its ground-floor windows and garden doors are. Right to light is a separate civil matter that a planning consent does not resolve, so where an extension approaches a neighbour's window we flag it early rather than letting it surface late. The aim is a footprint that is evidenced, not optimistic.
Inside, the levels decide how the house actually works. A garden sitting below the kitchen floor gives you three broad moves. You can step down into the new room and accept a change of level within the ground-floor plan. You can lower the slab across the extension so the whole rear of the house arrives at garden level. Or you can hold the existing floor level and terrace the garden up to meet it. Each carries consequences the drawings have to resolve. A lowered slab buys level access and generous ceiling height, but it means excavating below existing foundations, retaining the ground on the higher side, and designing waterproofing and drainage for a floor that now sits below external ground on at least one edge. Building regulations cover structure, fire safety, insulation, ventilation, drainage, stairs and waterproofing, and a lowered floor engages most of that list at once. A split level is cheaper to build and often reads well, but it consumes usable width and makes a genuinely level threshold to the garden harder to achieve.
Drainage is where a stepped plot quietly gets expensive. Foul and surface water normally run by gravity, and a floor that has been dropped to meet the garden can find itself close to — or below — the invert of the drain it has to reach. That is not fatal, but it has to be established before the floor level is fixed rather than after the slab is poured: the answer may be a pumped system, a re-routed run, or a finished floor set a little higher than the client wanted. A larger roof also has to discharge somewhere, and on ground that falls the surface water tends to arrive at the lowest corner of the plot, which is usually the corner the extension is standing on. Where the new structure sits over or close to a shared or public drain, a build-over agreement with the water authority runs alongside the planning application on its own timescale. We survey the drainage and confirm invert levels as feasibility work, not as a building-regulations afterthought.
Character still matters at the back of the house. The neighbourhood plan describes the West End Green Conservation Area as having a 'London village' character and as a homogeneous piece of Victorian and Edwardian domestic architecture, and Camden's material describes red-brick houses and mansion blocks in the side streets. Where an address falls inside that designation, a rear addition is expected to be subordinate to the host building: kept below the eaves, restrained in material, and detailed so the original elevation stays legible. The complication that belongs specifically to sloping ground is that the roof of a rear extension is not always the invisible surface it is elsewhere. Where neighbouring gardens or the ground behind sit higher, that flat roof becomes a fifth elevation, and its parapet, upstands, rooflights and any plant are all seen. We design that surface deliberately — a considered finish, rooflights set out to a rhythm rather than scattered across it — instead of treating it as the top of a box.
The route we run is deliberately front-loaded. We take a measured survey with levels, draw the existing section, confirm with Camden whether the address sits in West End Green, close to the separate South Hampstead Conservation Area, or outside any designation, and check whether Article 4 controls apply to the property — because permitted development rights may be restricted or removed, previous extensions change what is left, and a flat or maisonette carries no householder rights at all. Only then do we fix the footprint, prepare the planning drawings and, once the principle is settled, the building regulations and construction package. The trap we see most often in NW6 is an extension priced as a simple box when the site levels make it a more technical piece of work: retaining, waterproofing, drainage, foundation depth and party wall notices are all attached to the same change in level. It is far cheaper to price that at the start than to discover it at tender. We do not promise approval; we make the case properly and evidence it.
