How long a loft conversion takes comes down to two separate stretches of time. The first is the lead-in: design, drawings and the approvals that have to be in place before anyone opens the roof. The second is the build itself. Homeowners often only think about the second, but the lead-in can take as long as the build, and sometimes longer.
This guide walks through both, names the things that speed a project up or slow it down, and explains which parts sit with your builder and which sit with you. If you are already weighing up a project, our loft conversion services across West Sussex page explains how we work.
What decides how long a loft conversion takes?
A loft conversion’s timeline depends on seven things: the type of conversion, how long design and drawings take, whether planning permission or a Lawful Development Certificate is needed, Building Control, any party wall matters, the lead time on structural steel, and the weather while the roof is open.
Some of these are fixed by law. A council has a set period to decide a planning application, and the Party Wall etc. Act 1996 sets minimum notice periods. Others depend on your choices, such as how quickly the design is signed off and how settled the finishes are before work starts. The rest depend on the house: its roof shape, what is in the loft now, and whether a wall is shared with a neighbour.
How long is the lead-in before building work starts?
The lead-in runs from the first site visit to the day the scaffolding goes up. It covers the survey, design drawings, structural calculations, any planning or Lawful Development Certificate application, Building Control approval and party wall matters. Several of these can run side by side, which is the single biggest way to shorten it.
Survey and design drawings
Every project starts with a survey of the loft and the roof. Apex works with trusted architectural partners who produce the design drawings, so the layout, window positions and staircase route are agreed before anything is submitted. The quicker the design is signed off, the sooner everything that depends on it can start.
Planning permission or a Lawful Development Certificate
Many loft conversions in England are permitted development, which means they do not need a planning application. Permitted development has limits on volume, height and what can face the road, and our guide to planning permission for loft conversions covers how those rules work.
Where a project is permitted development, a Lawful Development Certificate confirms it in writing. It is not a legal requirement, but it gives you a record that the work was lawful, which helps when you come to sell. Councils generally aim to decide a certificate within 8 weeks of a valid application.
Where a project needs full householder planning permission, the council has a statutory period of 8 weeks to decide it. Mansard conversions and anything that changes the front roof slope usually fall into this group. So does any loft conversion inside the South Downs National Park or a conservation area, because permitted development rights for loft conversions do not apply on that designated land. Article 4 directions can remove them in other places too.
Apex can prepare and submit a planning application or a Lawful Development Certificate application on the homeowner’s behalf. Which council decides it depends on where you live: Adur & Worthing, Arun, Brighton & Hove, Horsham or Mid Sussex, or the South Downs National Park Authority inside the Park.
Building Control
Every loft conversion needs Building Regulations approval, whether or not it needs planning. The homeowner makes the application, either to the council’s building control service or to a registered building control approver.
There are two routes with a council. A full plans application has to be decided within 5 weeks, or up to 2 months if you agree to an extension. A building notice lets work start sooner, but nothing is approved in advance, so any work found not to comply has to be put right. Whichever route you take, Building Control inspects the work at key stages during the build and issues a completion certificate at the end.
Party wall matters
If your house shares a wall with a neighbour, a loft conversion often involves cutting into that wall to take the bearing of a steel beam. That is work the Party Wall etc. Act 1996 covers. The GOV.UK explanatory booklet sets the notice for work to an existing party wall at at least two months before the planned start.
Your neighbour then has 14 days to reply. If they agree in writing, work can go ahead once the notice period has run. If they do not reply, or they dissent, a dispute is treated as having arisen and surveyors are appointed to agree a party wall award, which adds time.
Party wall matters are the homeowner’s responsibility. The notice has to be served by you, or by a surveyor you have authorised in writing. A builder cannot serve it for you, so talk to a party wall surveyor early, because this period cannot be shortened once it has started.
Structural calculations and steel
A structural engineer works out the steel beams and new floor joists the conversion needs. The steel can only be ordered once those calculations are done, and it is made to measure, so its lead time sits on the critical path. Getting the calculations finished while the approvals are running means the steel can be ready for the first week on site.
How does the type of loft conversion change the build time?
The type of loft conversion has the biggest effect on build time, because it decides how much of the roof is rebuilt. A rooflight conversion keeps the roof as it is and is the quickest to build. A mansard or L-shape rebuilds the most and takes the longest. For the differences between them in more detail, see our guide to the types of loft conversion.
| Type | What changes on the roof | Effect on build time |
|---|---|---|
| Rooflight (Velux) | The roof shape stays the same. Windows are fitted into the existing slope. | The shortest build, because the roof is only opened for the windows. |
| Dormer | A box-shaped structure is built out from the rear slope to add head height and floor space. | Longer, because part of the roof is removed and a new structure is framed and weathered. |
| Hip-to-gable | The sloping side of a hipped roof is rebuilt as a vertical gable wall, often with a rear dormer as well. | Longer again, because a new wall is built up from the existing one and the roof is extended over it. |
| Mansard | Most of the rear slope is rebuilt at a steep angle with a flat roof above. | One of the longest builds, and it usually needs planning permission first. |
| L-shape | Two dormers are joined, one over the main roof and one over a rear wing. | One of the longest builds, because two roof sections are rebuilt and tied together. |
Within each type, the size of the room, the number of windows, whether a bathroom goes in, and the condition of the existing roof timbers all add or take away time.
What happens during the build, stage by stage?
A loft conversion build follows the same order whatever the type. The roof is opened and made weathertight first, then the new floor and structure go in, and the inside is finished last. Building Control inspects at key stages along the way.
- Scaffolding and protection. Scaffolding goes up and most early access is from outside, which keeps disruption inside the house low.
- Steel and floor joists. The steel beams and the new floor structure go in, as set out in the structural calculations.
- Opening the roof and building the new structure. The dormer, gable or mansard is framed, then roofed and made weathertight. This is the stretch where the weather matters most.
- First fix. Electrics, plumbing and heating pipework are run, and the walls, floor and roof are insulated.
- The staircase. The new stair is fitted from the floor below. This is usually the point where the work comes into the rest of the house.
- Fire safety and finishes. Plasterboard, fire doors and mains-wired smoke alarms go in, followed by second fix, flooring and decoration. Our guide to fire safety regulations for loft conversions explains what a protected escape route involves.
- Completion. Building Control makes its final inspection and issues the completion certificate, and the scaffolding comes down.
Can the weather hold up a loft conversion?
Yes. While the roof is open, rain and strong wind can stop work on the new structure, and wet conditions can slow roofing and leadwork. Once the roof is weathertight, the inside work carries on whatever the weather.
The open-roof stage is protected with sheeting or a temporary roof so the rooms below stay dry. Even so, a run of wet or windy days during that stage can push the whole programme back. That is one reason the build is planned so the roof is opened for as short a time as possible.
How can you keep a loft conversion on schedule?
Most delays come from decisions and approvals rather than from the build itself. A few things make the biggest difference:
- Start party wall matters early. The notice period cannot be shortened once served, so speak to a surveyor as soon as the design is settled.
- Run the approvals side by side. Planning or a Lawful Development Certificate, Building Control and structural calculations do not have to wait for each other.
- Choose finishes before work starts. Bathroom fittings, windows, flooring and the staircase style all have their own lead times.
- Agree changes in writing. A change to the layout once the structure is up can mean revised calculations or a revised Building Control submission.
Please note: planning rules vary from council to council, and some homes have had their permitted development rights removed. Before any work starts, the homeowner should confirm the planning position with their local council and discuss any party wall matters with a party wall surveyor. Apex can prepare planning and Lawful Development Certificate applications, but it does not serve party wall notices or submit Building Control applications.
Thinking about a loft conversion?
Every loft is different, so the best way to find out how long yours will take is a site visit. Get in touch with Apex Lofts & Construction and we will look at your loft, talk through the type of conversion that suits it, and explain what needs to happen before work can start.
Frequently Asked Questions
Can we stay at home during a loft conversion?
Many homeowners stay at home throughout a loft conversion. Much of the early work is done from the scaffolding, so the rest of the house is largely unaffected until the staircase goes in. There will be noise and some dust, and the floor below the loft is busiest when the stair is fitted. Your builder should explain which weeks are most disruptive.
Does Apex Lofts & Construction serve party wall notices?
No. Under the Party Wall etc. Act 1996, the notice has to be served by the homeowner or by a surveyor they have authorised in writing, so a builder cannot serve it. We recommend speaking to a party wall surveyor as soon as the design is settled, because notice for work to an existing party wall must be given at least two months before the work starts.
Can work start before planning permission or a Lawful Development Certificate comes back?
If the project needs planning permission, work must not start until permission is granted. A Lawful Development Certificate is not a legal requirement for permitted development, but starting before it is issued means building without written confirmation that the work is lawful. Building Regulations approval, or a valid building notice, also needs to be in place before work begins.
Does Apex Lofts & Construction help with planning applications?
Yes. We work with trusted architectural partners on the design drawings, and we can prepare and submit a planning application or a Lawful Development Certificate application for the homeowner. The homeowner should still confirm the planning position with their council, and Building Control applications and party wall notices remain the homeowner’s responsibility.
Why does a mansard take longer than a rooflight conversion?
A rooflight conversion keeps the existing roof and fits windows into the slope, so the roof is only opened in a few places. A mansard rebuilds most of the rear slope at a steep angle with a flat roof above, so far more of the roof is removed and rebuilt. Mansards also usually need full planning permission, which adds to the lead-in.
When should I start planning a loft conversion?
Start as early as you can. Design, structural calculations, planning or a Lawful Development Certificate, Building Control and any party wall notices all have to be in place before the build, and some carry fixed periods in law. Starting the conversation well before you want work to begin gives each of these time to run.