A house extension in Dartmouth Park begins with a levels survey, not a floor plan. The neighbourhood's grain is hillside — terraces and villas on streets that slope between Highgate, Kentish Town and Hampstead Heath — and on a plot with a cross-fall the ground behind the house is rarely at the level a plan drawing implies. The rear garden may rise behind the house as a bank, fall away toward the bottom boundary, or fall sideways across the width of the plot so that one flank boundary sits noticeably higher than the other. Each of those conditions produces a different extension. The single most common failure we are asked to unwind is a footprint fixed before the levels were understood, because once the depth and the floor level are wrong against the ground, the retaining, the drainage and the neighbour relationship all have to be forced afterwards. We measure the fall across the whole plot, front to back and side to side, before committing to a size.
The decision that follows is whether the extension steps down to meet the garden or holds the existing floor level and retains against the ground. Holding the internal level keeps the house legible as one floor plane and gives a flush threshold to a terrace, but where the garden falls away it means the new floor sits above the ground and the extension gains apparent height at the rear — height that a downslope neighbour will feel. Stepping down brings the room closer to the garden and reduces the mass seen from below, but it introduces a change of level inside, a stair or a ramp, and a lower floor slab that has to be drained and tanked where it sits against wet ground. Where the garden rises instead, the extension is effectively cut into the bank and the retaining question moves to the rear wall. There is no default answer; the fall dictates it.
Where the ground has to be held back, retaining becomes the structural core of the extension, and it does not necessarily stop at your own boundary. Where you cut into a rising garden, or where the neighbouring plot sits higher than your new floor, the extension's rear or flank wall becomes a retaining structure holding somebody else's ground, and it has to be designed for lateral earth pressure, surcharge from anything standing on that ground and long-term drainage behind the wall. That is a different engineering problem from a wall that simply carries a roof. It changes the foundation depth, the wall construction, the waterproofing specification and the sequence of the build, and it usually means the structural engineer is engaged at concept stage rather than after planning. It also changes the party wall position, because excavating below the level of a neighbour's foundation on sloping ground brings the three- and six-metre excavation tests into play at depths a flat-site scheme of the same floor area would never reach.
Water is the next thing the slope decides. On a falling plot, surface water arrives at the back of the house from the garden above rather than leaving it, and an extension placed across that path will dam it unless the design gives it somewhere to go. Land drainage behind a retained wall, a positive fall away from the new threshold and a considered outfall are not details to be added at building-regulations stage; they are part of the footprint decision. The existing drainage in an older terrace also tends to be shallow, gravity-fed and laid to suit the original ground, so lowering a floor to meet a falling garden can put the new level below the invert of the drain it is meant to connect to. We establish where the foul and surface-water drains actually run, and at what depth, before the layout is fixed.
Neighbour impact reads differently on a hillside, and this is where flat-site intuition misleads people most. An extension of a given height is experienced by a downslope neighbour as taller and more enclosing than the same extension on level ground, because it stands above their outlook rather than beside it; an upslope neighbour, conversely, may look down onto its roof, which makes the roof form and the position of any rooflights an amenity question rather than a purely aesthetic one. Camden assesses daylight and sunlight to neighbouring windows using the established BRE tests, and a section drawn through the real ground — not an assumed flat datum — is what shows whether a scheme passes. Right to light is a separate civil matter that planning consent does not resolve, so where an extension rises toward a neighbour's window we flag it early rather than let it surface late.
One caution is worth stating plainly: an extension approved on a flatter Kentish Town plot, or on a differently oriented site toward Highgate, is not a precedent for your house. That consent may have turned on a ground condition you do not share, and a depth or height that was unremarkable there can be materially different here once the fall is taken into account. So the route through is deliberately ordered. We survey the plot with levels, establish the drainage and boundary conditions, and only then test extension depths and floor levels in section against the neighbours on both sides of the fall. Where the property sits within the conservation area, the design is framed against Camden's Dartmouth Park Conservation Area guidance, with the rear massing and garden setting addressed directly. From there we prepare the householder application and, once the principle is settled, the building-regulations package covering structure, waterproofing, drainage, insulation, ventilation and any internal change of level, with retaining design coordinated with a structural engineer and party wall notices programmed alongside. We do not promise approval. We build the case on evidence, and we make sure the scheme submitted is one that can actually be built on the ground it stands on.
