A rear extension in Herne Hill starts with the existing rear building line and how much garden sits behind it. On the larger double-fronted and semi-detached villas towards Dulwich the plots are generous, which can tempt an ambitious footprint, but the useful question is how far the extension can project before it erodes garden amenity, overshadows a neighbour or reads as over-development of the plot. We test single-storey depths in section against the daylight reaching neighbouring windows and the position of boundary walls, then set a projection that is both liveable and defensible to whichever council determines it. That evidence-led depth is what separates a scheme a case officer can support from one that invites objection.
The sloping streets are the feature that makes Herne Hill rear extensions distinct. Many of these villas sit on ground that falls or rises between the house and the end of the garden, so the floor level you extend from and the garden level you extend towards are rarely the same. That changing level is an opportunity — a few steps down into a garden-facing room, or a raised terrace threshold — but it also drives the structural design, the drainage falls and the way daylight enters at the back. We survey the levels precisely at the outset, because a rear extension designed as if the ground were flat will fight the site rather than settle into it.
The choice between single and two storeys changes the assessment entirely. Single-storey rear extensions are the most common here and are judged largely on garden impact and neighbour daylight. A two-storey rear addition is realistic on some of the larger detached and semi-detached villas, but it brings first-floor windows, outlook and a sense of enclosure over a neighbour's garden into play, and is far more sensitive on a semi-detached pair where the two houses share a boundary. We advise early which storey height a given house and plot can carry, so the design is committed to a planning submission at a scale the site can actually defend.
Roof form is where a rear extension is won or lost aesthetically. A flat roof with a discreet parapet, a shallow mono-pitch falling away from the house, or a glazed element to bring light deep into a plan each suit different villas, and the right choice depends on the eaves height, the pattern of the existing rear elevation and how the addition is seen from the garden. On the sloping plots the roof also has to resolve against a changing datum, so a form that reads as subordinate from the house can loom differently from the lower end of the garden. We detail the junction with the original wall so the extension reads as a clear, quiet addition rather than a competing mass.
Daylight and right to light deserve early attention because, even on the more generous villa plots, a rear extension can affect a neighbour. Whichever council applies assesses daylight and sunlight to neighbouring windows using the established BRE tests, and a well-judged section — controlling height near the boundary and stepping the roof down — often keeps a scheme within them. 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 so it can be addressed rather than discovered late in the build. On the close-set terraces near the station this constraint bites hardest and shapes the footprint from the first sketch.
From there the route is a coordinated one. We survey the house and its levels, confirm whether Lambeth or Southwark determines the application, test the extension against the planning and daylight constraints, and develop a design the relevant case officer can support — then produce the planning drawings and, once the principle is settled, the building regulations package for construction covering structure, insulation, ventilation and drainage. On the semi-detached and terraced plots the Party Wall etc. Act 1996 usually applies, so we identify the notifiable neighbours early and coordinate with a party wall surveyor. The result is a rear extension that adds real ground-floor space without unbalancing the house or its garden.
