RenoPlan
Project Summary

Loft conversion Renovation Estimate

This estimate covers a complete dormer loft conversion with en-suite bathroom in London. It includes structural alterations, dormer construction, thermal and acoustic envelope, staircase, full MEP installation, and high-quality finishes. Pricing reflects London's premium labour rates, difficult access, and stringent building control requirements. The range accounts for specification choices, existing roof complexity, and potential unforeseen conditions.

Estimated Investment
£34.3K–53.1K

Includes 15% contingency

All estimates are for planning purposes only. Actual costs may vary based on location, materials, and contractor rates.

Project Parameters
Room Type
Loft conversion
Region
London, England, United Kingdom
Area
25 m²
Scope of Work
Dormer loft conversion with en-suite

Renovation Roadmap

Structural steelwork forms the backbone of the conversion, supporting the new floor and roof loads. Rates reflect the need for heavy lifting equipment, temporary works, and coordination with structural calculations in a dense urban setting.

  • Two RSJ beams
  • Padstones or bearing plates
  • Fire-resistant boarding to beams
  • Structural engineer's calculations
  • Building control inspections
Cost Breakdown
Structural steelwork (RSJs and supports)
Labor£3,000–5,000
2 item × £1,500–2,500/item
Materials£600–1,000
2 item × £300–500/item
Stage Total£3,600–6,000
Common Pitfalls

Undersized beams leading to deflection

Inadequate bearing on load-bearing walls

Missing fire protection to steelwork

A robust floor platform is essential for both structural integrity and sound insulation. Pricing accounts for working at height within the roof space and the precision needed to achieve a level surface.

  • C16/C24 graded joists at 400mm centres
  • 22mm P5 or OSB3 decking
  • Acoustic quilt between joists
  • Perimeter noggins and blocking
  • Vapour control layer
Cost Breakdown
New floor joists and decking
Labor£750–1,125
25 m2 × £30–45/m2
Materials£500–750
25 m2 × £20–30/m2
Stage Total£1,250–1,875
Common Pitfalls

Incorrect joist centres causing bounce

Insufficient insulation within floor zone

Lack of acoustic separation from rooms below

The dormer creates the usable headroom and natural light. Costs cover complex carpentry, roofing trades, and high-performance glazing suitable for a pitched roof environment.

  • Timber stud frame with sheathing
  • Breather membrane and tile/slate roof covering
  • Lead flashings to all junctions
  • Conservation rooflight or vertical window
  • Insulation to dormer cheeks and roof
Cost Breakdown
Dormer construction (frame, roof, window)
Labor£3,500–5,000
1 item × £3,500–5,000/item
Materials£2,000–3,500
1 item × £2,000–3,500/item
Stage Total£5,500–8,500
Common Pitfalls

Weatherproofing failures at dormer cheeks and apron

Inadequate structural connection to existing roof

Window specification not meeting fire escape requirements

Continuous thermal and acoustic envelope is critical for comfort and compliance. Rates reflect the labour-intensive fitting around irregular roof geometry and multiple service penetrations.

  • Mineral wool or PIR insulation to walls, floor, ceiling
  • Vapour control layer with taped joints
  • Plasterboard with staggered joints
  • Acoustic insulation to party walls
  • Fire-rated boarding where required
Cost Breakdown
Insulation and lining (walls, floor, ceiling)
Labor£2,000–2,800
80 m2 × £25–35/m2
Materials£1,200–2,000
80 m2 × £15–25/m2
Stage Total£3,200–4,800
Common Pitfalls

Thermal bridging at junctions

Vapour control layer punctured by services

Insufficient insulation depth to meet Part L

The staircase is a major structural and design element. Pricing includes bespoke joinery, structural modifications to the existing floor, and fire safety detailing.

  • Custom timber staircase with winders if needed
  • Handrails and balustrades to Part K
  • Fire-rated door at top or bottom
  • Trimming joists and newel post fixing
  • Decorative finish to match specification
Cost Breakdown
Staircase installation
Labor£2,000–3,000
1 item × £2,000–3,000/item
Materials£2,000–3,000
1 item × £2,000–3,000/item
Stage Total£4,000–6,000
Common Pitfalls

Headroom clearance less than 2m

Tread/riser dimensions non-compliant with Part K

Inadequate fire protection to stair enclosure

Adding a bathroom at roof level requires careful routing of soil pipes and ensuring adequate water pressure. Costs cover first and second fix plumbing, sanitaryware, and connection to existing drainage.

  • Shower tray, enclosure and thermostatic mixer
  • Wall-hung WC with concealed cistern
  • Basin with vanity unit
  • Soil and waste pipework to new stack
  • Water supply pipes in insulated runs
Cost Breakdown
En-suite plumbing and drainage
Labor£2,000–3,000
1 item × £2,000–3,000/item
Materials£2,000–3,000
1 item × £2,000–3,000/item
Stage Total£4,000–6,000
Common Pitfalls

Insufficient fall for waste pipes

Hot water pressure inadequate for shower

Missing isolation valves for maintenance

A full rewire of the new space with modern protection and energy-efficient lighting. Rates include certification, testing, and coordination with insulation and plastering trades.

  • Consumer unit with RCBOs
  • Lighting circuits with fire-rated downlights
  • Socket circuits for general and bathroom use
  • Towel rail and underfloor heating supplies
  • Smoke/heat detectors interlinked
Cost Breakdown
Electrical installation
Labor£1,500–2,500
1 item × £1,500–2,500/item
Materials£1,000–1,500
1 item × £1,000–1,500/item
Stage Total£2,500–4,000
Common Pitfalls

Insufficient circuits for future loads

Downlights compromising insulation continuity

Missing RCD protection for all circuits

Space heating and controlled ventilation are mandatory for habitable rooms. Pricing covers a wet radiator tied to the existing system and a continuous mechanical extract unit.

  • Panel radiator with TRV
  • Mechanical extract ventilation with heat recovery
  • Ducting insulated and routed to external grille
  • Programmable room thermostat
  • Pipework in insulated runs
Cost Breakdown
Heating and ventilation
Labor£800–1,200
1 item × £800–1,200/item
Materials£700–1,300
1 item × £700–1,300/item
Stage Total£1,500–2,500
Common Pitfalls

Radiator undersized for heat loss

Extractor fan not meeting Part F flow rates

Missing thermostatic radiator valves

Final finishes define the perceived quality. Rates include skilled plastering, floor preparation, wet-room tanking, and decorating to a high standard suitable for a principal bedroom suite.

  • Two-coat plaster to walls and ceiling
  • Engineered wood or laminate flooring
  • Porcelain tiles to en-suite floor and walls
  • Waterproof tanking to shower zone
  • Two coats emulsion and woodwork paint
Cost Breakdown
Finishes (plastering, flooring, tiling, decoration)
Labor£1,000–1,500
25 m2 × £40–60/m2
Materials£875–1,375
25 m2 × £35–55/m2
Stage Total£1,875–2,875
Common Pitfalls

Plaster shrinkage cracks at board joints

Floor finish incompatible with underfloor heating

Tankings failure in wet areas

Second-fix joinery completes the room's functionality and aesthetics. Costs reflect bespoke storage solutions and the precision needed for a professional installation in a converted roof space.

  • Fire-rated door to stair enclosure
  • Standard doors to en-suite and storage
  • Skirting and architraves throughout
  • Built-in wardrobe with sliding doors
  • Ironmongery (handles, hinges, latches)
Cost Breakdown
Joinery (doors, skirting, wardrobes)
Labor£1,200–1,800
1 item × £1,200–1,800/item
Materials£1,200–1,800
1 item × £1,200–1,800/item
Stage Total£2,400–3,600
Common Pitfalls

Doors not fire-rated where required

Skirting not scribed to uneven floors

Wardrobe internal fittings not adjustable