The complete guide to loft conversions

Almost every loft conversion is one of four types, and the type is largely decided by your existing roof rather than by preference.

Before anything else, measure from the top of the ceiling joist to the underside of the ridge. Below about 2.2m and a rooflight conversion will not give comfortable finished height once the new floor and insulation are in.

Then look at the roof structure. Cut roofs (purlins and rafters, generally pre-1965) convert easily. Trussed roofs — W-shaped webs, most post-1965 homes — need a new structural floor and steel beams before the trusses can be cut.

A loft conversion creates a third storey, which triggers escape route requirements: fire doors to habitable rooms off the stairwell, mains-linked smoke alarms, and a protected route to the final exit. In open-plan houses this can be the most disruptive part of the whole job.

The staircase must be permanent, compliant and land in a legal position with 1.9m head height over it. Fitting it often means reconfiguring a bedroom below — that cost belongs in your budget even though it is not in the loft.

Rear dormers and rooflight conversions are usually permitted development within volume limits (40m³ terrace, 50m³ semi or detached). Front dormers, mansards, flats and conservation areas are not. Apply for a Lawful Development Certificate regardless.

On semi-detached and terraced houses you will need Party Wall Act notices for beams bearing into the shared wall. Budget £1,000–£2,500 per neighbour for surveyor fees if they dissent.

• Assuming a loft 'looks big enough' without measuring head height at the ridge.
• Not checking where the staircase can legally land before agreeing a layout.
• Excluding the ensuite from the quote and discovering pumped waste is needed.
• Forgetting that fire door upgrades affect the entire house, not just the loft.
• Ignoring insulation depth — shallow rafters may need over-rafter insulation and a raised roof line.

• Does the price include the staircase, fire doors and building control certificate?
• What structural work is allowed for, and has an engineer designed it?
• Is party wall surveyor cost included or excluded?
• How will insulation meet building regulations with the existing rafter depth?
• Is scaffolding included and for how long?
• Is a Lawful Development Certificate or planning application included?
• Where will the ensuite waste run, and does it need a pump?
• How long will the roof be open, and how is the house protected?

• No structural engineer named anywhere in the documents.
• A quotation that omits the staircase.
• No mention of fire doors or alarm upgrades.
• Verbal claims that 'you don't need building control for a loft'.
• A start date offered before drawings exist.