Loft Conversion: Planning Permission, Types and Costs

Everything you need to know about loft conversions - types, planning rules, building regulations, typical costs and how to get started.

A loft conversion is one of the most popular and cost-effective ways to add an extra bedroom, bathroom or home office to your home. Here is everything you need to know.

Types of Loft Conversion

Velux / Rooflight Conversion

The simplest and cheapest option. No changes to the roof structure - just roof windows (Velux or similar) installed into the existing roof slope. Works best when there is already sufficient headroom in the loft.

• Cost: £20,000-£35,000
• Planning permission: rarely needed

Dormer Conversion

The most common type. A box-shaped structure is built out from the rear roof slope, creating additional headroom and floor space. Available as a single dormer or full-width dormer.

• Cost: £35,000-£60,000
• Planning permission: usually permitted development (rear dormers)

Hip-to-Gable Conversion

The sloping side of a hipped roof is extended vertically to create a gable end, increasing the usable loft space. Common on semi-detached and detached houses with hipped roofs.

• Cost: £40,000-£65,000
• Planning permission: may be permitted development on detached houses; usually needs permission in conservation areas

Mansard Conversion

The roof slope is rebuilt at a near-vertical angle (72 degrees), maximising internal space. Creates the most room but involves the most structural work. Popular in London terraces.

• Cost: £50,000-£80,000
• Planning permission: almost always required (changes the roof shape significantly)

Planning Permission

Many loft conversions fall under permitted development, meaning no planning permission is needed. However, you will need to apply if:

• The additional volume exceeds 40 cubic metres (terraced/semi) or 50 cubic metres (detached)
• You want a front-facing dormer on a highway-facing elevation
• Your property is in a conservation area (restrictions on dormers and hip-to-gable)
• Your home is a listed building or a flat
• You want to raise the ridge height

Building Regulations

All loft conversions require building regulations approval. Key areas inspected:

• Structural design - floor joists, beams, load paths
• Fire safety - protected escape route (fire doors, intumescent strips), mains-wired smoke alarms on every floor
• Staircase - minimum headroom 2m, maximum pitch 42 degrees
• Insulation - walls, roof and floor must meet current thermal standards
• Sound insulation - between the loft and rooms below
• Windows - minimum opening area for fire escape

How Long Does a Loft Conversion Take?

• Velux conversion: 3-4 weeks
• Dormer conversion: 5-7 weeks
• Hip-to-gable: 6-8 weeks
• Mansard: 7-10 weeks

Add 2-4 months for design, planning (if needed), and building regulations before construction starts.

Does a Loft Conversion Add Value?

A well-executed loft conversion typically adds 15-20% to your property's value, often more than the cost of the work. Adding a bedroom with an en-suite bathroom delivers the best return on investment.

Getting Started

1. Check your loft's suitability - minimum 2.2m head height at the ridge is ideal
2. Appoint an architect or architectural technologist for a feasibility study
3. Confirm whether you need planning permission
4. Get building regulations drawings prepared
5. Obtain 3 quotes from experienced loft conversion specialists
6. Check party wall requirements if you share a wall with a neighbour