The building work usually takes 4 to 6 weeks for a rooflight conversion, around 10 to 12 weeks for a dormer and 12 to 16 weeks or more for a mansard. Add time before that for design and approvals: a planning decision can take up to 8 weeks, and party wall notice must be given at least 2 months before work starts. Many of these can run at the same time.
Last reviewed 22 September 2026. Based on our research into official and published sources, listed at the end of this page.
Step by step
Check your loft. Measure the head height and see what kind of roof structure you have. Our suitability checker walks you through it.
Design and drawings. An architect or designer draws the plans, and a structural engineer does calculations for the new floor and any steel.
Planning. If the design is permitted development, you can apply for a lawful development certificate (optional). If it needs planning permission, the council's statutory time limit for deciding a householder application is 8 weeks (GOV.UK). See the planning guide.
Party wall notice, if needed. Notice must be given between 2 months and a year before work starts, and your neighbour has 14 days to respond (GOV.UK). See party wall agreements.
Building regulations. With a full plans application, you can expect a decision within 5 weeks, or 2 months with your consent (GOV.UK). See the building regulations guide.
The build. Scaffolding goes up, the roof is altered and the new floor, stairs, walls and services go in, followed by plastering and finishing. Typical times are below.
Sign-off. After the final inspection, a full plans application leads to a completion certificate within 8 weeks of the work finishing (GOV.UK). Keep it safe for when you sell.
Steps 4 to 6 can run side by side once the design is settled. Where a party wall is involved, the 2-month notice period often sets the earliest start date.
Build times by type
Type of conversion
Typical time on site
Velux or rooflight
4 to 6 weeks
Rear dormer
10 to 12 weeks
Hip to gable
10 to 14 weeks
Mansard
12 to 16 weeks or more
Bungalow
10 to 12 weeks
Checkatrade (August 2026); bungalow from Checkatrade's cost guide (March 2026 (table), August 2026 (breakdown)). Typical times, not guarantees: older roofs, weather and design changes can add time.
Keeping it on track
Settle the design before applying for anything, because changes later mean new drawings and approvals.
Serve party wall notice as early as the rules allow if your design affects a shared wall.
Ask each specialist for a written programme with the main stages and when building control will inspect.
Build in a contingency for time as well as money: Checkatrade notes that older roofs can need more strengthening once they are opened up.
Frequently asked questions
How long does a loft conversion take?
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The building work itself typically takes 4 to 6 weeks for a rooflight conversion, 10 to 12 weeks for a dormer, 10 to 14 weeks for a hip-to-gable and 12 to 16 weeks or more for a mansard (Checkatrade). Before that, allow for design, any planning decision (8 weeks for a householder application) and, if a party wall is involved, at least 2 months' notice to your neighbour.
What is the first step in a loft conversion?
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Check whether your loft is likely to be suitable: measure the head height and look at the roof structure. Then get a specialist to survey it and quote. Our suitability checker helps with the first part.
Can planning and party wall notice run at the same time?
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Yes. Once the design is settled, you can apply for planning permission or a lawful development certificate, submit building regulations plans and serve party wall notice in parallel, which saves time.
This guide is general information to help you plan. It is not legal or professional advice: check your own project with your council or a qualified professional.
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