London Construction Costs Per Square Metre 2025: Extension, Loft & Refurbishment Rates

Extension costs per m2 in London vary significantly based on whether the extension is single-storey or two-storey, structural complexity, and the level of finish. The figures below are for the BUILD cost only — they exclude VAT, professional fees (architect, structural engineer), planning fees, party wall costs, and client-supplied materials.

**Single-storey rear extension (standard specification):** - Budget specification (functional but basic): £1,800-£2,400 per m2 - Mid specification (good quality standard materials, reasonable finishes): £2,400-£3,200 per m2 - High specification (quality glazing, underfloor heating, good kitchen or WC): £3,200-£4,500+ per m2

**Two-storey extension:** - Two-storey extensions are generally more cost-efficient per m2 than single-storey (the roof and foundations are shared between two floors) - Budget specification: £1,600-£2,200 per m2 - Mid specification: £2,200-£2,900 per m2 - High specification: £2,900-£4,000+ per m2

**Side return extension:** - Often the most expensive per m2 due to narrow site access, party wall constraints, and structural complexity - Typically £2,500-£4,000 per m2

**Basement or cellar conversion:** - Highest cost per m2 of any project type: £2,500-£5,000+ per m2 depending on depth, waterproofing system, and existing structure - Basement new-build (digging out below an existing building): £3,500-£6,000+ per m2

**Wraparound extension (combining side return and rear extension):** - £2,200-£3,800 per m2 depending on width, structural complexity, and glazing specification

Loft conversion costs per m2 are calculated on the usable floor area of the new loft room(s), not the footprint of the house. Because loft conversions involve significant structural and roofing work for relatively contained floor areas, they typically appear expensive per m2 compared with ground floor extensions.

**Velux (roof light) loft conversion:** - £1,200-£1,800 per m2 of new floor area - The most cost-effective type — no change to the roof line

**Rear dormer loft conversion:** - £1,800-£2,800 per m2 of new floor area - Most common in London; good balance of cost and new space created

**L-shaped dormer:** - £1,800-£2,600 per m2 of new floor area - Slightly more efficient per m2 than a single dormer due to greater floor area from the same structural complexity

**Mansard loft conversion:** - £2,500-£3,500+ per m2 of new floor area - Most expensive per m2 due to the full roof replacement involved

**Hip-to-gable conversion:** - £2,000-£3,200 per m2 of new floor area - Common in semi-detached houses; requires replacing a hipped roof with a gable to gain additional floor area

**En-suite in loft conversion:** - A bathroom in a loft conversion adds £8,000-£18,000 to the overall cost, depending on specification - Per m2 of bathroom floor area: typically £3,000-£5,000 per m2 (bathrooms are expensive per m2 due to intensive M&E, waterproofing, and fitting work)

Refurbishment costs per m2 are calculated on the gross internal floor area being refurbished. Full house refurbishments vary enormously based on the state of the existing building and the level of work being done.

**Light refurbishment (decoration, flooring, fixtures):** - £200-£400 per m2 - Cosmetic works only: painting, new flooring, replacing kitchen or bathroom without structural change

**Medium refurbishment (includes M&E upgrade, plastering):** - £400-£800 per m2 - Includes new electrics or partial rewire, new plumbing or boiler, replastering, new floors, new kitchen and bathroom

**Full refurbishment (gut-out, structural changes, full M&E replacement):** - £800-£1,600 per m2 - Full strip back, new first fix electrics and plumbing, new bathroom and kitchen, structural alterations, full redecoration

**High-specification full refurbishment (luxury finishes, structural works, bespoke joinery):** - £1,600-£3,000+ per m2 - Used for high-end London properties with premium specification

**Flat refurbishment (London leasehold flats):** - Light: £250-£500 per m2 - Medium: £500-£900 per m2 - Full: £900-£1,800 per m2 - Flat refurbishments often benefit from simpler access (no external works, no roof) but may have communal areas restrictions and lease conditions affecting specification

Understanding why costs per m2 vary helps you assess whether a quote is realistic for your project:

**Specification and finishes**: The largest driver of cost variation. A kitchen extension with budget units and standard flooring might cost £2,400/m2; the same footprint with a bespoke kitchen, high-end glazing, and heated stone floors might cost £4,500/m2. Specify clearly before comparing quotes.

**Structural complexity**: Complex steelwork, difficult foundations, underpinning, or basement tanking dramatically increase cost per m2 relative to simple construction.

**Ground conditions**: Made ground, high water table, tree root zones, or proximity to services require more expensive foundation solutions, increasing cost per m2.

**Access constraints**: Narrow access to rear gardens, no side access, scaffold requirements on busy streets, or restricted working hours in central London all add cost.

**Party wall and conservation area constraints**: Additional professional fees (party wall surveyors, specialist architects) add to the overall project cost and therefore the effective cost per m2.

**London location**: Inner London costs are generally 15-25% higher than outer London for equivalent work, reflecting higher labour and logistics costs. Central London (Westminster, Kensington, Chelsea) can be 25-40% above the London average.

**Market conditions**: London construction costs rose significantly in 2021-2023 due to materials and labour shortages. In 2025, costs have stabilised but remain elevated versus pre-2020 levels.

Cost per m2 should be used as a RANGE INDICATOR, not a precise budget figure. Here is how to use it effectively:

1. **Define the gross internal floor area**: For an extension, measure the proposed footprint multiplied by the number of floors. For a loft, measure the actual new floor area created.

2. **Apply the range**: Multiply the floor area by the lower and upper bounds of the appropriate cost per m2 range to get a budget range.

3. **Add professional fees and planning costs**: Architect fees (8-12% of construction cost), structural engineer (£800-£2,000), party wall (£800-£4,000), planning fees (£206-£600 per application), Building Control fees (£500-£1,500).

4. **Add client-supplied or excluded items**: If you are supplying kitchen appliances, bathroom sanitaryware, or flooring, these reduce the contractor's cost but are still a project cost.

5. **Add contingency**: For refurbishments or works to older London properties, add a 10-15% contingency for unforeseen works (hidden damp, buried drainage, asbestos, structural issues revealed on opening up).

6. **Sanity-check against quotes**: Once you have contractor quotes, divide the quoted sum by the floor area to get the implied cost per m2. If it is significantly below the market range, question what might be excluded. If it is significantly above, understand what premium specification is included — or whether the quote is simply overpriced.