House Extension Architects in West Hampstead

A measured section, not an area-level contour description, shows whether levels shape a West Hampstead rear extension. Where the subject garden or adjoining land is higher or lower than the internal floor, the survey records those levels before a footprint is agreed. Whether the new floor steps down, the slab is lowered or the garden is terraced then follows from site evidence rather than the West End Green appraisal being treated as a measurement of the property.

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 must be established from the measured site and existing system before floor levels are fixed. Foul and surface-water routes, invert levels and any need for pumping or diversion are site-specific. Locate and classify the drain or sewer. A build-over agreement is not inferred from a shared or private drain, or from proximity alone; check whether the work affects a sewer under the sewerage undertaker's control and whether that undertaker requires an agreement.

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, map the complete application site against the separate West End Green and South Hampstead boundaries, and match any operative Article 4 schedule to the property and proposed work. We then test any dwellinghouse Part 1 route against planning history, class limits and conditions; flats and buildings containing flats cannot use Part 1 because of property type. Only then do we fix the footprint and prepare the applicable planning, Building Regulations and construction information. We do not promise approval or a project programme.