Which extension type fits your home, your plot and your planning context
London extensions divide into four broad families. Side return extensions infill the narrow alley alongside a Victorian or Edwardian terrace's back addition — usually 3–4 metres wide, almost always permitted development, transformative for kitchen and dining space. Rear extensions project from the rear elevation, single-storey, up to 4m on detached and 3m on semi/terraced under permitted development (extendable to 6m and 8m respectively under prior approval). Wraparounds combine side return and rear, creating a full-width open-plan space across the back of the house. Double-storey extensions add a bedroom and en-suite above the ground-floor extension, doubling the value but roughly doubling the cost. We assess all four during the initial design phase — sometimes the obvious answer (a big rear) is not the best one (a wraparound might fit your living patterns better at similar cost). Planning context matters too: conservation areas, Article 4 directions, party wall constraints and overlooking distances all narrow the options. We model the constraints in CAD before committing to a direction.
Permitted development versus full planning: what determines the route
Most single-storey rear and side-return extensions in London proceed under permitted development (PD) — no planning application, just a lawful development certificate confirming compliance. PD limits for detached houses: rear projection up to 4m (8m under prior approval), eaves height 3m, max ridge 4m, side extensions up to half the original house width. Semi-detached and terraced: 3m rear (6m under prior approval), same eaves and height limits, no side extensions on principal elevation. Conservation areas and Article 4 areas (large parts of Hackney, Camden, Islington, Westminster) suspend most PD rights — full planning required. Wraparounds usually need planning because side and rear together exceed PD limits. Double-storey extensions always need planning. We run a free desk-based assessment of your address before committing to a route and have a 100% approval rate on first-submission planning applications over the past three years (we don't submit if we don't think it'll pass).
Structural design: steel, foundations and the things that go wrong if you cheap out
Every extension that removes an external wall (which is almost all of them) needs structural steel to carry the wall above. A typical rear extension uses one 203×133 UB to carry the existing rear wall, plus pad-stones, plus a ring-beam where the new flat roof meets the wall. A wraparound or side return often needs two beams: one across the back, one along the side, with a column at the corner. Foundations on London clay are typically 1.2–1.5m deep mass concrete strip — deeper near trees (the NHBC tree-distance tables determine depth). On reclaimed or made-ground sites we sometimes specify pile and ring-beam at additional cost. Get any of this wrong and you get cracking, subsidence, doors that won't close, water ingress at the junction. Our structural engineer is in-house, calculations are signed and submitted to building control before site mobilisation, and we maintain £10M professional indemnity cover on the design work. We do not work with the cheap-and-cheerful drawing-from-the-internet approach.
Glazing: the single largest visual and cost-driving decision
Glazing is what makes a London extension feel like a London extension. The standard package is a 3-panel bifold or 2-panel slider running the full width of the rear, paired with a roof lantern or 2–3 large rooflights. Cost-driving variables: frame finish (aluminium powder-coat in any RAL colour adds ~£1,200 vs. standard black), slim-frame upgrade (Reynaers SL38 or Schueco ASS70 add £3,500–£6,000 over a budget bifold), thermal performance (1.2 W/m²K minimum to satisfy building regs, 0.8 for premium), and configuration (sliders are sleeker but cost more per linear metre than bifolds). On a typical 4m wide rear extension we'd budget £8,000–£12,000 for bifolds, £14,000–£22,000 for sliders, plus £4,500–£8,500 for a 2.4×1.8m roof lantern. We specify, supply and install through three preferred suppliers (IDSystems, Express Bifolds, Sunflex) — never sub-contracted to the cheapest installer.
Open-plan or broken-plan: how the extension changes the floor below
Most extensions are designed to merge with the existing back room — usually creating an open-plan kitchen-dining-living space spanning the whole rear of the house. This requires removing an internal wall (almost always a structural wall) and installing a second steel beam to carry the floor above. Open-plan delivers light, sociability, and a sense of space, but loses noise separation and visual containment. 'Broken-plan' uses partial walls, screens, level changes or sliding pocket doors to maintain visual flow while preserving acoustic privacy and the option to close off a snug or playroom. We design both, but increasingly recommend broken-plan for families with children — the open-plan-everything trend has peaked, and clients with young kids regularly tell us they wish they had kept a separate living room. The structural cost is similar; the design fee marginally higher (more variables) but worth it.
What you actually pay for: typical cost breakdown on a £100k rear extension
On a £100,000 single-storey rear extension: £12,000 foundations and groundworks, £15,000 superstructure (walls, roof, steel), £13,000 glazing (bifolds + lantern), £8,000 first fix MEP (electrics, plumbing, heating), £7,000 plastering and decoration, £9,000 kitchen fit-out (excluding kitchen units — those are separate), £6,000 flooring, £4,500 second fix and lighting, £3,500 external works (patio, drainage), £10,000 fees (architect, structural, planning/PD, building control, party wall), £6,000 site management and plant, £6,000 margin and contingency. Where it goes wrong with other contractors: provisional sums for groundworks (open-ended), 'PC sums' for kitchens and bathrooms, no allowance for variations, day-rate management. Our quotes are itemised, fixed-scope, with variations only by signed instruction at agreed rates. We publish our day rates and margin policy openly during initial consultation.
Living through a build: what to expect and how to plan for it
The first month is the disruptive month. Demolition of the existing rear wall, foundation digging and pouring, drainage diversion, scaffolding going up — this is the noisy, dusty phase. From week 5 the structure is weather-tight and most work moves indoors; noise drops, dust is contained, life becomes liveable again. Most clients stay in residence with a temporary kitchen set up in another room (we can supply a hire kitchen for £400/month) and the existing rear door sealed off behind a stud-wall partition with insulation and a fire-rated door. Bathrooms and bedrooms are unaffected throughout — we work to keep at least one toilet and one bathroom functional at all times. The final two weeks bring the kitchen install, flooring and decorations, which is messy but contained. From handover, you walk in to a clean, finished space — we don't 'snag-as-we-go', we do a full pre-handover clean and snagging walk with you before key transfer. The 12-month defects period starts on handover with proactive 6-month and 12-month walkthroughs.
