London Extension Cost Breakdown: Where the Budget Goes on a Rear Extension

Before a single brick is laid on a London extension, a significant proportion of the preliminary costs are incurred: Scaffold: a two-storey rear London extension requires full-height rear elevation scaffolding. Cost: £2,500-£5,000 for erect, 12-16 week hire, and strike. Skip hire and waste removal: demolition, excavation, and construction waste from a London extension generates multiple skip loads. Cost: £150-£300 per skip, typically 4-8 skips for a medium-sized London extension (£600-£2,400). Site welfare: toilet facilities, temporary power, water supply, and site security. Temporary power connection in London: £200-£500. Site insurance (Contractor's All Risks): the principal contractor's CAR insurance typically covers the project — ensure the contractor's policy covers the full contract value. Temporary works: propping and shoring of the existing structure during demolition of the existing rear wall of the property is required where the extension opens into the existing house. Cost: £1,000-£3,000 depending on structural complexity. Total preliminaries and establishment: £6,000-£12,000 for a medium-sized London two-storey rear extension. This is 8-15% of the total construction cost — often overlooked when budgeting from rates per m².

Below-ground works are a significant and sometimes unpredictable element of a London extension cost: Demolition and excavation: breaking up and removing the existing rear ground floor (often a concrete oversite or original Victorian floor), excavation of the extension footprint, and removal of excavated material. Cost: £2,000-£5,000. Strip foundations (new extension): the most common foundation type for London rear extensions on clay sub-soils away from trees. Trench excavation, concrete fill to engineer's specification, formation of the brick foundation walls. Cost: £150-£300 per linear metre of foundation trench. For a typical 5m x 4m London extension (20 linear metres of perimeter foundation): £3,000-£6,000. Pile foundations (where trees are close): mini pile foundations are used in many London rear extensions where large trees are within 5-10m of the new extension on London Clay. Cost: £5,000-£15,000+ for a mini pile and ground beam system (significantly more than strip foundations). Drainage: new drainage to the extension (ground floor slab drainage, new inspection chambers, connection to existing system). Cost: £2,000-£5,000. Floor slab: reinforced concrete ground floor slab (or beam-and-block alternative). Cost: £80-£150 per m². For a 20m² extension: £1,600-£3,000. Total groundworks and below-ground: £10,000-£25,000 for a medium London two-storey extension (depending heavily on foundation type and depth).

The visible structural shell of the London extension: Masonry superstructure (walls): cavity wall construction (inner leaf of thermal blockwork, outer leaf of facing brick to match the existing property, full-fill cavity insulation, wall ties). Cost: £120-£200 per m² of wall area for supply and fix. For a typical London two-storey rear extension (approximately 50m² of wall area): £6,000-£10,000. Steel beams (RSJs): structural steel beams to support the existing property above the new opening and to create the structural frame where required. A two-storey London rear extension typically requires 2-4 steel beams (RSJs or universal beams). Cost: £1,500-£4,000 per beam installed (supply, fabrication, and installation — including temporary propping). Flat roof (single-storey element): GRP flat roof, warm deck specification. Cost: £90-£150 per m². For a 20m² single-storey element: £1,800-£3,000. Pitched roof (two-storey element): new pitched roof structure to match the existing house roof (rafters, ridge, roof tiles to match existing or complementary specification, underfelt, counterbattens, battens). Cost: £100-£180 per m² of roof plan area. Roof tiles (to match existing): additional £40-£80 per m². Rooflights: flat glass rooflights (Velux or Korniche) are a popular addition to London extension flat roofs, bringing natural light into the rear of the property. Cost: £800-£2,500 per rooflight (supply and installation). Total structure and roof: £25,000-£55,000 for a medium London two-storey rear extension.

First-fix mechanical and electrical (MEP): new electrical circuits from the consumer unit to the extension; new plumbing runs for the kitchen extension (hot and cold supply, waste drainage); any underfloor heating manifold and pipework. Cost: £6,000-£14,000 for a medium London extension first fix (electrics + plumbing). Insulation: floor insulation (PIR rigid board beneath slab or above existing slab in beam-and-block construction); wall cavity insulation (full-fill mineral fibre or PIR); roof/ceiling insulation (PIR board to warm flat roof; mineral fibre between rafters for pitched element). Cost: £2,000-£5,000 for a medium London extension. Plasterboard and plastering: plasterboard to all internal walls and ceilings, skimmed with a finish plaster coat. Cost: £2,000-£6,000 for a medium London extension. Second-fix electrics and plumbing: second-fix electrical (sockets, switches, light fittings, extract fans); second-fix plumbing (kitchen sink, dishwasher connection, washing machine connection). Cost: £3,000-£8,000. Underfloor heating (where specified): wet UFH in the ground floor of the extension is popular — materials and installation (excluding screed): £40-£100 per m². Total MEP, insulation, plasterboard and second fix: £15,000-£35,000 for a medium London two-storey extension.

Finishes (client-variable): the final element of the London extension cost breakdown is the most variable — it depends entirely on the specification chosen by the homeowner. Indicative cost ranges: kitchen supply and installation: £10,000-£60,000+ (budget to bespoke). Floor finishes: £30-£200 per m² supply and fit (LVT, engineered oak, porcelain tile — for a 25m² extension: £750-£5,000). Tiling (kitchen splashback, ceilings, features): £50-£120 per m² supply and fit. Decoration (internal): £2,000-£5,000 for a medium London extension. External windows and glazed doors (bifold, sliding, frameless): bifold doors (3.0m wide, aluminium) supply and installation: £3,000-£8,000. Single french door + sidelight: £1,500-£4,000. Professional fees: architect (design, planning, building control drawings): £8,000-£20,000 for a medium London rear extension project. Structural engineer: £2,000-£6,000. Party wall surveyor (both surveyors' fees): £1,500-£4,000. Principal designer fee (CDM): £1,000-£3,000. Building Control fee: £800-£2,000. VAT: VAT at 20% applies to all construction work on existing dwellings (extensions, alterations, refurbishments). Note: new dwellings are zero-rated. For a £150,000 London extension contract (excl. professional fees): VAT = £30,000. Total professional fees + VAT is often 30-40% on top of the construction contract cost — budget this from the outset. Typical all-in cost for a medium two-storey London rear extension (construction + finishes + fees + VAT): £180,000-£320,000 for a high-quality scheme.