Loft Conversion Guide UK 2025
Types, Costs, and What You Need to Know
A loft conversion is one of the most effective ways to add space and value to your home. By converting unused roof space into a bedroom, office, or bathroom, you can add 10% to 20% to your property value, often exceeding the cost of the conversion itself.
This guide covers all loft conversion types from simple velux installations to full mansard conversions, with realistic costs, planning requirements, and building regulations you need to understand.
First Step: Check Your Loft Suitability
Before getting quotes, check three things:
- Headroom: Measure from floor joists to ridge. Need 2.4m+ for conversion.
- Pitch: Roof angle should be 30°+. Shallow pitches limit options.
- Access: Where will stairs go? Usually loses space from room below.
Loft Conversion Types
Each type offers different cost, space, and planning trade offs.
Cost Breakdown by Type
Where the money goes for each conversion type.
| Component | Velux | Rear Dormer | Hip to Gable | Mansard |
|---|---|---|---|---|
| Structural Steel | £500 to £1,500 | £1,500 to £3,000 | £2,500 to £4,000 | £3,000 to £5,000 |
| Roof/Dormer Construction | £3,000 to £6,000 | £12,000 to £20,000 | £15,000 to £25,000 | £20,000 to £35,000 |
| Stairs | £2,000 to £4,000 | £2,500 to £5,000 | £2,500 to £5,000 | £3,000 to £6,000 |
| Windows | £1,500 to £3,000 | £2,000 to £4,000 | £2,500 to £5,000 | £3,000 to £6,000 |
| Electrics | £1,500 to £2,500 | £2,000 to £3,500 | £2,500 to £4,000 | £3,000 to £5,000 |
| Plumbing (if bathroom) | £0 to £3,000 | £2,500 to £5,000 | £2,500 to £5,000 | £3,000 to £6,000 |
| Insulation | £1,000 to £2,000 | £1,500 to £3,000 | £2,000 to £3,500 | £2,500 to £4,000 |
| Plastering & Finishing | £2,000 to £4,000 | £3,500 to £6,000 | £4,000 to £7,000 | £5,000 to £9,000 |
| TOTAL | £15,000 to £30,000 | £35,000 to £55,000 | £45,000 to £65,000 | £55,000 to £80,000+ |
Building Regulations Requirements
All loft conversions need Building Regulations approval. Here are the key requirements.
Minimum Headroom
2.2m at centre, 1.8m at edges (for habitable room)
Note: Measure from finished floor to ceiling. Check before committing.
Stairs
Must comply with Part K. Usually paddle/space saver if tight.
Note: Stairs often the biggest challenge. May need to steal from bedroom below.
Fire Safety (Part B)
Protected stairway, fire doors, smoke alarms, escape window
Note: Creates 'protected escape route' through house. Existing doors may need upgrading.
Structural (Part A)
Floor must support intended load. New floor joists often needed.
Note: Structural engineer calculations required.
Insulation (Part L)
Roof insulation to current standards (U value 0.18)
Note: Significantly thicker than old installations. Affects internal headroom.
Sound (Part E)
Sound insulation if creating new dwelling/multiple rooms
Note: Between loft and existing rooms if applicable.
Ventilation (Part F)
Adequate ventilation to habitable rooms
Note: Windows must have trickle vents. Bathroom needs extractor.
Permitted Development Conditions
To qualify for Permitted Development (no planning application), your conversion must meet all these conditions:
- Volume does not exceed 40m³ (terraced) or 50m³ (detached/semi)
- No extension beyond the plane of the existing roof slope facing the highway
- Materials to be similar in appearance to the existing house
- No verandas, balconies, or raised platforms
- Side facing windows to be obscure glazed and non opening below 1.7m
- The roof extension does not exceed the highest part of the existing roof
- Must be set back at least 20cm from original eaves
- Property is not in conservation area, AONB, or listed
Lawful Development Certificate
Even if your conversion is Permitted Development, consider applying for a Lawful Development Certificate (£129). This provides legal proof that planning permission was not required, which is valuable when selling.
Key Takeaways
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