London Building Costs 2026: A Realistic Guide to Construction Prices

Before quoting ranges, it is worth understanding why costs vary — because two projects of the same square footage can cost very different amounts.

**Specification level:** The single largest driver of cost variation is the specification of materials and finishes. A kitchen extension with budget-end tiles, standard sanitary ware, and a basic kitchen (client-supplied) will cost significantly less than the same-size extension with premium stone flooring, bespoke joinery, and high-end glazing.

**Structural complexity:** A simple box extension on a straightforward foundation is cheaper than one involving unusual ground conditions, complex steelwork, large structural openings, or works close to existing services (sewers, trees, underground structures).

**Access and site conditions:** London sites often have constrained access — narrow passages, no-dig zones near protected trees, tight working space, scaffolding complexity, restricted skip placement. These constraints add cost.

**Programme speed:** A project with a tightly compressed programme (premium paid for weekend or out-of-hours working, or for additional labour to accelerate) costs more than one with a normal programme.

**Market conditions:** Construction costs fluctuate with material prices, labour availability, and demand. The cost ranges in this guide reflect market conditions in late 2025 / 2026 — they will vary from those applying in earlier or later periods.

**VAT:** All figures in this guide are exclusive of VAT (20% standard rate applies to residential extensions and refurbishments). New builds are zero-rated for VAT on construction.

• **Rear extensions — single-storey:**
• •Basic specification (concrete block, standard finish): £1,400–£1,800/m² construction cost (excluding kitchen)
• •Standard specification: £1,800–£2,400/m²
• •Good specification (premium glazing, stone or engineered wood floor, detailed finishes): £2,400–£3,500/m²

• Typical single-storey rear extension in London (25m²):
• •Basic: £35,000–£45,000
• •Standard: £45,000–£60,000
• •Good specification: £60,000–£87,000

• **Extensions — double-storey:**
• Double-storey extensions cost less per m² than single-storey (foundations and roof costs spread over two floors):
• •Standard specification: £1,600–£2,200/m² (per floor, so a 2-floor extension totalling 50m² costs £1,600–£2,200 per floor area)
• •Good specification: £2,200–£3,000/m²

• **Loft conversions:**
• •Velux (no structural change, rooflights only): £25,000–£40,000
• •Rear dormer, single bedroom + bathroom: £45,000–£70,000
• •L-shaped dormer (typical terraced terrace, 2 rooms): £55,000–£85,000
• •Hip-to-gable + rear dormer (semi-detached): £55,000–£90,000
• •Mansard (flat-topped dormer, typically central London): £80,000–£130,000+

• **Full house refurbishments:**
• •Light refurbishment (decorating, new kitchen, new bathrooms, no structural work): £400–£700/m² of GIA
• •Standard refurbishment (rewire, replaster, new bathrooms and kitchen, some structural): £700–£1,200/m²
• •Comprehensive strip-back refurbishment (full rewire, full replaster, new M&E, structural work): £1,200–£2,000/m²

• Typical 3-bed Victorian terrace (90m²) full refurbishment:
• •Standard: £63,000–£108,000
• •Comprehensive: £108,000–£180,000

• **Garage conversions:**
• •Straightforward integral or attached garage to habitable room: £18,000–£35,000
• •More complex or with significant services upgrade: £35,000–£50,000

• **Basements:**
• •New basement excavation (tanked, insulated, including retaining walls): £3,000–£5,000/m² of new basement floor area
• •Existing underpinned basement conversion: £2,000–£4,000/m²

• **New build:**
• •Standard specification: £2,500–£3,500/m² GIA
• •Good specification: £3,500–£4,500/m²
• •High specification or complex: £4,500–£7,000+/m²

• **Typically included in a contractor's construction cost:**
• •Labour and plant
• •Structural works (foundations, steelwork, structural openings)
• •External envelope (walls, roof, windows, external doors)
• •Internal finishing (plastering, screed)
• •First and second-fix mechanical and electrical (plumbing, heating, electrical wiring and final connections)
• •Tiling
• •Decoration
• •Basic floor finishes (where specified)

• **Typically excluded from the construction cost:**
• •**Kitchen units and appliances:** At rcbGroup, second-fix materials are client-supplied as standard — clients source their own kitchen. Kitchen units and appliances are excluded from the construction cost; rcbGroup provides the installation.
• •**Bathrooms and sanitaryware:** Often client-supplied at rcbGroup (WC suites, shower enclosures, basins, baths). Plumbing and tiling are included in the construction cost; fixtures may not be.
• •**Floor finishes (carpets):** rcbGroup does not install carpet. Engineered wood, LVT, or stone floor finishes may be included or excluded depending on the contract.
• •**Furniture and soft furnishings**
• •**Professional fees:** Architect, structural engineer, party wall surveyor, planning consultant
• •**Planning and Building Regulations fees**
• •**VAT**
• •**Contingency:** A 10–15% contingency above the contract sum should be budgeted separately

• **The true total project cost:**
• For a £60,000 construction contract on a rear extension:
• •Construction: £60,000
• •Client-supplied kitchen: £8,000–£25,000 (depending on specification)
• •Professional fees (architect, SE): £5,000–£8,000
• •Planning and Building Regulations: £1,500–£2,500
• •Party wall: £1,500–£3,000
• •Contingency (12%): £7,200
• •VAT (20% on construction): £12,000
• •**Total all-in: approximately £95,000–£120,000**