Extension Cost Breakdown: Where Does the Money Go?

A single-storey rear extension in London can be broken into the following major cost categories. The percentages are indicative — they vary by project type, finish level, and whether the work is design-and-build or managed separately.

**Groundworks and foundations: 10–15% of total build cost** Excavation, concrete strip or pad foundations, insulation under slab, ground floor concrete slab. This category is heavily influenced by soil conditions, proximity to trees, and depth required by the structural engineer.

**Superstructure (walls and roof): 20–30% of total build cost** Masonry walls (blocks and bricks), flat or pitched roof structure, waterproofing membrane (for flat roofs), roof tiles or felt and battens (pitched), structural steel including RSJ beams and padstones, structural openings in the existing rear wall.

**Windows, doors, and glazing: 8–15% of total build cost** Aluminium bifold or sliding doors, fixed glazing panels, rooflights, and any new windows. A project with extensive glazing (full-width bifolds, large rooflights) will see this category reach the higher end of the range.

**First-fix plumbing and electrics: 5–10% of total build cost** First-fix electrical cable runs, socket and switch back-boxes, consumer unit upgrade, UFH manifold and pipe loops (if UFH specified), radiator pipework runs, soil/waste pipe runs.

**Insulation and plastering: 8–12% of total build cost** PIR or mineral wool insulation (walls, floor, roof), plasterboard and skim plaster finish, or full wet plaster system. Premium finish levels (thin-coat skim, seamless cornice-less junction) add cost.

**Second-fix plumbing and electrics: 5–8% of total build cost** Sanitary ware, radiators or UFH commissioning, sockets, switches, lighting, consumer unit connections.

**Floor finishes: 5–10% of total build cost** Large-format porcelain tiles (fitted), engineered wood, or other client-specified finish. Note — under RCB standard conditions, second-fix materials (including floor tiles) are client-supplied; only the installation labour is in the contractor's cost.

**Decoration: 3–6% of total build cost** Mist coat, two coats finish paint, skirtings, architraves, door furniture.

**External works: 3–6% of total build cost** External drainage connections, patio or landscaping adjacent to the extension, soakaway or rainwater drainage from new roof, external render or brickwork pointing.

The build cost is not the only cost in an extension project. Professional fees and statutory charges can add 10–20% to the overall project cost — a figure that clients on tight budgets sometimes overlook.

• **Architect fees**:
• •Design, planning, and technical drawings: £3,000–£8,000 for a standard single-storey rear extension
• •Full project management (architect of record through to completion): £5,000–£15,000+
• •Reduced services (design and planning drawings only, no site visits): £2,000–£4,000

**Structural engineer fees**: £900–£1,800 for a standard rear extension including foundations, beam design, and Building Regs calculations.

**Planning application fee**: £258 (Householder Application, England, 2025).

**Building Regulations application fee**: Varies by local authority and project value — typically £300–£700 for a standard rear extension (Full Plans route).

**Party wall surveyor fees**: £800–£2,400+ depending on whether an Agreed Surveyor is used or separate surveyors are appointed by each party.

**Structural surveys / asbestos survey**: £300–£800 for an asbestos survey (required for properties built before 2000 before demolition or alteration works); £400–£900 for a structural survey of the existing building.

• **Total professional fees (indicative, standard rear extension)**:
• •Minimal: ~£5,000 (PD, architect design only, no party wall issues)
• •Typical: £8,000–£15,000 (planning, architect, engineer, party wall)
• •Complex or high-specification: £15,000–£25,000+

When a project runs over budget, clients often ask where costs can be reduced. Understanding which cost categories are structural (difficult or inadvisable to reduce) and which are finish-level (more flexible) helps make smarter decisions.

• **Categories where reducing cost is inadvisable**:
• •Foundations — never cut corners on foundation depth or concrete specification; the consequence is structural failure
• •Structural steel — beam sizes are determined by the engineer; using smaller beams to save cost is dangerous and will be rejected by Building Control
• •Waterproofing (flat roofs) — cheap flat roof systems leak within 5–10 years; the remediation cost far exceeds the saving
• •Fire safety — doors, smoke detectors, and fire protection cannot be omitted or substituted below the required standard

• **Categories where value engineering is reasonable**:
• •Glazing — standard aluminium windows and doors rather than slim-frame premium products; fixed glazing rather than opening where ventilation is available from other sources
• •Floor and wall finishes — client-supply of tiles or engineered wood saves margin; standard tile sizes are cheaper than large-format
• •Decoration — two coats on new plaster only, with the client completing additional decorating themselves
• •External works — simplifying patio design or landscaping; delaying external works until Phase 2
• •Rooflights — standard flat rooflights rather than premium slim-frame units

**The procurement effect**: The way the project is procured (design-and-build versus separately contracted) can significantly affect total cost — not because one is always cheaper, but because different procurement routes carry different risk allocations and design responsibilities. On a tight budget, a design-and-build approach with a single fixed price is often more controllable than an architect-led project with separate specialist contractors.