Professional Price Estimator for Velux, Dormer, Hip-to-Gable & Mansard Conversions
Loft conversion cost calculators provide accurate estimates based on current UK market rates averaging £1,200-£2,800 per m². Velux conversions remain most economical at £15,000-£30,000 while complex hip-to-gable and mansard projects range £45,000-£100,000+. London/South East premiums of 25-40% reflect higher labour and scaffold costs.
Typical 40m² conversions add 15-25% property value with 8-12 week completion times. Building Regulations approval mandatory including structural calculations, fire escapes, and insulation to Part L standards. Permitted Development allows 40m² on terraced/50m² detached houses without planning permission.
| Type | 30m² Cost | 50m² Cost | Cost/m² | Timescale |
|---|---|---|---|---|
| Velux | £18,000-£25,000 | £25,000-£35,000 | £600-£900 | 4-6 weeks |
| Dormer | £30,000-£45,000 | £40,000-£60,000 | £1,000-£1,500 | 6-9 weeks |
| Hip-to-Gable | £40,000-£55,000 | £55,000-£75,000 | £1,400-£2,000 | 8-12 weeks |
| Mansard | £50,000-£70,000 | £70,000-£100,000 | £1,800-£2,500 | 10-14 weeks |
UK Specialists: London Loft Specialists | Homebuilding & Renovating | FMB Builders
Part A structural calculations mandatory for steel beams supporting roof loads. 30-minute fire resistance to escape routes with 0.33m² unobstructed opening per habitable room. Part L insulation minimum U-value 0.18 W/m²K walls/roof with 50mm clear air gap ventilation. Part F minimum 4 air changes per hour mechanical ventilation.
Electrical installations Part P certified with 100% LED lighting and TVR extractors. Sound insulation 40dB minimum between bedrooms and circulation spaces. Building control inspections required at: steelwork installation, insulation complete, plasterboard fixed, electrics first fix.
Structural steelwork 15-25% total cost with RSJs £300-£500 per metre supplied fixed. Scaffold £3,000-£8,000 for 8-12 weeks. Roofing felt/tile reinstatement £4,000-£7,000. Plastering/decoration £200-£350/m². En-suite bathrooms add £8,000-£15,000 including tiling and appliances.
Architectural fees 8-12% total build cost with full planning packs £2,500-£5,000. Party wall agreements £1,000-£2,500 where applicable. VAT reclaimable at 5% on professional fees for qualifying renovations. 10-year structural warranty standard on quality conversions.
40m² terraced/50m² detached houses. Max 2.5m eaves height. Materials match existing house. No balconies/verandas.
30-min fire doors to stairs. 0.33m² window opening per bedroom. Mains powered smoke alarms interconnected throughout.
U-value 0.18 W/m²K walls/roof. 50mm ventilated air gap. Mechanical ventilation 4 air changes/hour minimum.
203x133 UB typical for 4m spans. Engineer calculations mandatory. S275 grade steel standard specification.
15-25% property value uplift. ROI typically 60-80% of build cost. Most cost-effective space per m².
95% approval rate within Permitted Development. 4-8 weeks processing time. Pre-application advice recommended.
Generally no within Permitted Development limits (40/50m²). Confirm with LPA for side elevations or Article 4 restrictions. Pre-app advice £100-£250.
Full approval required: structure, fire, insulation, ventilation, electrics. 7-10 inspections typical over 8-12 week build period.
Minimum 2.2m ridge height, 2m clear floor height over 50% area. Structural engineer survey £500-£800 confirms feasibility.
Velux 4-6 weeks, Dormer 6-9 weeks, Hip-to-Gable 8-12 weeks. Weather dependent. Internal works continue during external delays.
Yes if within 3m of boundary. Surveyor fees £1,000-£2,500 shared. 2-month notice period required before works commence.
Full turnkey: steelwork, roof, windows, insulation, plastering, electrics, decoration. Extras: en-suite, velux lanterns, oak flooring.
Typically Band D to E (+25-33%). Check with local authority. Some offer council tax holidays during construction period.
UK Loft Resources: Planning Portal PD Rights | LABC Building Control | RIBA Architects