Typical cost in 2025
Most homeowners pay around £35,000 for a dormer loft conversion with en-suite, including stairs
What affects the price?
Conversion type
Velux (roof lights only): £20,000–£30,000. Dormer (box extension from roof): £30,000–£50,000. Hip-to-gable: £35,000–£55,000. Mansard (near-vertical walls): £45,000–£70,000.
Size of loft space
A minimum head height of 2.2m is needed. Larger floor areas and higher pitches cost more in materials and structural work but deliver more usable space.
En-suite bathroom
Adding an en-suite adds £4,000–£8,000 to the conversion cost — but significantly increases the value and appeal of the room, especially for a master bedroom.
Staircase
A new staircase is required for building regulations compliance. Expect £1,500–£5,000 depending on design. Space-saving alternating tread stairs are cheaper but less convenient.
Building regulations
All loft conversions require building regulations approval. Structural calculations, insulation, fire doors, and a completion certificate all add to cost and time.
Typical cost breakdown
Estimates are based on UK average rates in 2025. Prices in London and the South East are typically 20–30% higher.
Frequently asked questions
Do I need planning permission for a loft conversion?
Most loft conversions fall under Permitted Development and don't need planning permission. Exceptions include properties in conservation areas, listed buildings, and conversions that significantly alter the roofline.
How much value does a loft conversion add?
A bedroom loft conversion typically adds 15–20% to the value of a property. In London and the South East, this can be significantly more, often exceeding the cost of the conversion.
How long does a loft conversion take?
A standard dormer conversion takes 8–12 weeks from start to finish. Velux conversions can be done in 4–6 weeks. Mansard conversions are the most complex and can take 3–4 months.
Will I need to move out during the conversion?
Generally no — most of the work happens at roof level and in the loft space. You'll experience disruption when the staircase is fitted and during plastering, but living in the property is usually manageable.
What's the minimum head height for a loft conversion?
Building regulations require a minimum 2.2m head height at the centre of the space. Your builder should check this before work starts — if it's marginal, a dormer can increase the usable height.