Loft Conversion Cost Calculator London 2026: How to Estimate Your Project
- LCCL Construction
- Jul 15
- 3 min read
Estimating your loft conversion cost accurately before contacting builders lets you budget realistically, understand which conversion type suits your property and compare quotes with confidence. This step-by-step calculator method walks through the four factors that determine any loft conversion cost — type, size, specification and location — and works through examples for typical London properties. LCCL Construction also offers an instant quote calculator on our website for a personalised estimate in under 2 minutes.
The Four Factors That Determine Any Loft Conversion Cost
Every loft conversion cost is determined by four things: 1) type of conversion, 2) size in square metres, 3) specification level, 4) location within London. Get these four right and you can estimate any project to within 15 per cent of the actual final cost. Here is how each factor works.
Factor 1: Conversion Type — Base Cost Ranges
Start with your conversion type.
Velux (rooflight): £27,000 to £42,000.
Dormer: £47,000 to £67,000.
L-shaped dormer: £49,000 to £82,000.
Hip to gable: £50,000 to £75,000.
Mansard: £64,000 to £124,000.
Pick the type that matches your property.
Factor 2: Size — Adjust for Your Square Metres
Loft conversion cost per square metre in London is typically £2,000 to £3,500 depending on type and specification. A small conversion (15-18 sqm) sits at the lower end of the range. A large conversion (28-40 sqm) sits at the upper end. Measure the internal floor area you will have once the conversion is built — remember to exclude areas below 1.5 metres ceiling height which cannot be counted as habitable floor space under building regulations.
Factor 3: Specification — Standard vs Premium
Standard specification includes: standard Velux rooflights, standard skirting, painted MDF joinery, basic tiled ensuite, oak or engineered wood flooring. Premium specification adds: Cor-ten steel-framed rooflights, oak joinery, marble or porcelain tiled ensuite with underfloor heating, hardwood flooring, bespoke fitted wardrobes and premium ironmongery. Premium specification typically adds 15 to 30 per cent to the base cost.
Factor 4: Location — London Postcode Adjustment
Outer London (Barnet, Enfield, Croydon, Havering, Bromley, Bexley, Sutton, Kingston): baseline pricing. Inner London (Camden, Islington, Hackney, Wandsworth, Lambeth, Southwark, Tower Hamlets, Newham, Hammersmith and Fulham, Lewisham, Greenwich, Brent, Ealing, Merton, Redbridge, Waltham Forest, Hounslow, Richmond, Haringey): add 10-15 per cent. Central London (Westminster, Kensington and Chelsea, City of London): add 20-25 per cent.
Worked Example 1: Standard Dormer, Standard Spec, Outer London
Property: Victorian terrace in Barnet EN5.
Conversion: standard dormer with one bedroom and ensuite (20 sqm).
Specification: standard.
Location: outer London (baseline).
Base cost: £47,000 to £67,000.
Middle of the range for a standard 20 sqm Dormer: approximately £55,000 to £60,000.
Standard specification: no adjustment.
Outer London: no adjustment. Estimated total: £55,000 to £60,000.
Worked Example 2: L-Shaped Dormer, Premium Spec, Inner London
Property: Victorian terrace in Islington N1.
Conversion: L-shaped dormer with master bedroom and ensuite (26 sqm).
Specification: premium. Location: inner London (+15 per cent).
Base cost: £49,000 to £82,000.
Upper middle for 26 sqm: approximately £70,000.
Add 20 per cent for premium specification: £84,000.
Add 15 per cent for inner London: £97,000.
Estimated total: £90,000 to £100,000.
Worked Example 3: Mansard, Premium Spec, Central London
Property: Victorian terrace in Kensington W8.
Conversion: mansard creating 2 bedrooms and 2 ensuites (34 sqm).
Specification: premium. Location: Central London (+25 per cent).
Base cost: £64,000 to £124,000.
Upper end for 34 sqm mansard: approximately £110,000.
Add 25 per cent for premium specification: £137,500.
Add 25 per cent for Central London: £171,875.
Estimated total: £165,000 to £180,000.
What the Calculator Does Not Include
This estimation method covers the core construction cost. Additional items to budget for: planning permission complexity — full planning applications add 8-12 weeks to timeline. Party wall agreements — £2,000 to £15,000+ depending on adjoining owners. Structural warranty — £1,500 to £3,500 for 10-year LABC warranty. Furnishings and specialist items — bespoke fitted wardrobes, blinds and window treatments typically add £8,000 to £25,000. Consult LCCL Construction for a fully itemised quote covering all of these.
Get an Instant Loft Conversion Quote Online
For a personalised estimate use the LCCL Construction instant quote calculator. Enter your conversion type, size, specification and postcode and receive an instant estimated cost range in under 2 minutes. For a detailed quote including planning, party wall and specification consultation, call LCCL Construction on 020 8037 7332.
Frequently Asked Questions About Loft Conversion Cost Calculation
How accurate is a loft conversion cost calculator?
A well-designed calculator using type, size, specification and location should be accurate to within 10-15 per cent of the final construction cost. Complex projects with structural challenges or unusual site conditions may require detailed on-site survey for full accuracy.
See our full loft conversion cost guide for the complete 2026 price breakdown.
