In most parts of London a house extension starts with a question about depth. In South Hampstead it starts with a question about tenure, because a large share of what people call a 'house' here is a house that was subdivided a long time ago. Before any design work is useful we establish which of three positions you are actually in: you own the whole building as a single-family house; you own a ground-floor or garden flat within a converted house and want to extend into the garden behind it; or you own a flat in a mansion block whose rear elevation is a shared elevation. These three routes have almost nothing in common with each other. They differ in who has to consent, in what the drawings need to prove, and in whether an extension is legally available at all. Getting this wrong is the single most expensive mistake we see, because it is usually discovered after a design has been paid for.
If you own the whole house, the route is the most familiar one: a Camden householder application for a rear or wrap extension, designed against the neighbour relationship and the character of the conservation area where the address falls within it. The useful piece of local knowledge is where the scrutiny actually sits. Because Camden's Article 4 controls in South Hampstead are described as applying to fronts and street-facing sides of single-family houses, the rear of the building is the quieter face and the front and any visible side are where permitted development cannot be assumed. That does not make a rear extension automatic — the projection still has to be defensible against the neighbour's daylight, the boundary and the amount of garden left behind — but it does tell you where the design effort and the evidence need to be concentrated.
The ground-floor flat in a converted house is the route that most often surprises people. Planning permission and property rights are separate systems, and here you need both. The garden may be demised to your flat, it may be shared with the flats above, or it may be retained by the freeholder with your lease granting only a right of use — and the lease plan, not the fence line or thirty years of habit, decides which. Extending across land you do not hold, or building over a garden that other leaseholders have rights across, is not something a planning permission cures. So the sequence we work to is: read the lease and title, establish what is demised, open the conversation with the freeholder or managing agent about consent and a Licence to Alter, and only then fix a footprint. Where the lease does not permit what the design needs, a lease variation may be required and it runs on its own timetable.
The mansion block is the honest conversation. A red-brick mansion block's rear elevation is a shared elevation: it belongs to the building rather than to any one flat, and no single leaseholder can extend it. We say this plainly because the alternative is to sell a design that cannot be built. What is usually available instead is a reconfiguration inside the demise — reworking the plan to recover space, opening structure between rooms, relocating a kitchen or a wet area, or taking a lateral route where two flats can be joined. Each of those is leasehold-led work in its own right, and in South Hampstead a managing agent will expect acoustic floor build-ups, fire separation and stopping, structural calculations for any opening, drainage falls, riser routes, waterproofing to wet areas, ventilation and common-part protection before it will approve anything. That evidence is the project, and a thin pack gets refused.
Where an extension is genuinely available, the design question that decides its planning outcome is whether it stays at the rear or turns the corner. A single-storey rear addition sits on the quiet face and is argued on garden amenity, neighbour daylight and its subordination to the host building. The moment it wraps onto a flank that can be read from the street, the conservation and Article 4 test switches on and the deliverable changes: existing and proposed elevations, photographs of the visible side, a material schedule and a written explanation of why the addition sits comfortably against the area's ordered red-brick character. The same logic applies to the small things that get added late — an external vent, a flue, a condenser unit or a solar panel. Camden consultation material for Hampstead, South Hampstead and Belsize refers to photovoltaic and solar thermal equipment under Article 4 front or side controls, so equipment that migrates onto a visible face can turn a quiet scheme into a contested one.
The technical work then follows the building rather than a template. In a converted house the soil stack, the rainwater goods and often the kitchen wastes for the flats above run down the rear elevation you are proposing to build against, so a rear extension is a drainage-diversion project as much as a structural one, and the routes have to be resolved with the upper leaseholders' service continuity in mind. Building regulations cover structure, fire safety, insulation, ventilation, drainage, stairs and waterproofing, and in a subdivided building the fire separation between your flat and the ones above is a live constraint on any new opening or ceiling. Daylight and sunlight to neighbouring windows is assessed using the established BRE tests, and right to light is a separate civil matter that a planning consent does not resolve. We survey, establish the tenure position, test the scheme against these constraints, then produce the planning and building regulations drawings. We do not promise approval; we make the case on evidence.
