Guide to Dormer Loft Conversions

This type of addition will add space and value to your home. From design tips to rules and regs, Rebecca Foster explains the key factors to think about before going ahead
Rebecca Foster
by Rebecca Foster
10th May 2021

If you’re undertaking a loft conversion, incorporating a dormer extension as part of the works is a smart route to boosting the space and headroom. Positioned within the slope of the roof, this type of vertical addition is ideal for expanding urban properties without gobbling up precious garden amenity. Plus, it’s usually straightforward to achieve without having to apply for planning permission.

Design possibilities

Dormer windows come in an array of shapes or sizes, so selecting the style that works best with your home is important. “This type of extension is hard to categorise in terms of size – it can be anything from 1m to 10m wide, depending on the house,” says James Bernard, director at Plus Rooms.

Learn more: A Complete Guide to Loft Conversions

“A minimum internal head height of 2m is a good rule of thumb to justify a dormer addition. From there, you can incorporate glass doors, Juliet balconies or even a sliding entrance with a glass balustrade.”

Dormer loft

Inglis Badrashi Loddo Architects are the brains behind this plywood-lined loft conversion.

Try to pick a style that complements the aesthetic of original property. One way to achieve this is to specify finishing materials that complement the existing palette, or you could even go for something that offers a complete contrast to distinguish old from new.

In terms of shape, a gable-fronted dormer is the most common type, with a double pitched roof made from two sloping sides. A shed-style addition features a flat top that projects out from the existing roof at a shallower angle. Curved designs, such as eyebrow and segmental structures, are also possible.

Project routes

Engaging the services of an architect is one way to put together the initial plans for your extension, before bringing in a contractor to take care of construction. Your chosen designer or builder can be appointed to look after project management, if you wish, meaning they’ll oversee the individual trades and coordinate with building control.

Dormer loft on house

The large dormer window invites plenty of sunshine into the space, as well as opening up great views of the garden and beyond

Another route is to employ a dedicated loft conversion specialist firm who will handle the project from design to completion. Using a one-stop-shop provider does have its advantages, as you’re only dealing with one point of contact through the whole process. Some companies offer a fixed price contract before the build begins, so you’re armed with cost certainty from the start.

Whichever option you choose, always check contractors’ credentials before going ahead. “Ensure the company is fully insured and is a member of the relevant associations, such as TrustMark, the Federation of Master Builders, FairTrades and the Guild of Master Craftsmen,” says Rebecca Tibbert, director at Econoloft.

Is my loft suitable?

To determine whether your loft is an appropriate candidate for a dormer, head height is one of the first aspects to look at. With traditional roofs, built before the 1960s, you’ll need around 2.2m-2.4m of clear height for a straightforward conversion. Modern trussed roofs require more structural alterations (removing braces and reinforcing), so you may need more like 2.4m-2.6m to enable the works.

House with extension

A large double dormer extension to this end of terrace house has significantly expanded the available living space inside. The project was carried out by Econoloft, who took care of everything from design to construction

“The preferred amount of finished headroom for a habitable space is 2.3m+. However, this doesn’t need to be maintained across the whole floor area,” says Rob Wood, managing director at Simply Loft. A bigger dormer can work very well in a roof with minimal head height, for instance, while smaller additions are a good for lofts with a steep existing pitch where you can already stand up inside.

What’s involved?

First, your contractor will strip back the existing roof of the property. The dormer itself is usually constructed from timber, comprising a roof, side walls (cheeks) and a front wall that typically faces the back garden. Next, the structure is clad in your material of choice and the glazing installed to make it watertight.

An opening is cut into the existing rafters to make way for the new structure. The remaining rafters will often be supported by the dormer, though depending on the size of the opening that has been cut additional reinforcements may be needed.

“Next, a new floor is added internally, as well as first fix plumbing and electrics,” says Rob from Simply Loft. “We then create a new opening leading up from the first floor leading to the loft for the staircase to be installed. The final jobs are plastering, second fix plumbing and electrics, followed by fascias and gutters to the exterior.”

Rules & regs

If you’re installing a dormer extension at the back of your property, it’s likely that the works will fall under Permitted Development (PD) rules. If so, you won’t need to apply for permission. Bear in mind that any extensions that protrude beyond the existing plane at the front of the roof will, however, require formal consent.

Dormer loft

Building a dormer over the main frame of the house has transformed this 1930s property. The project, by Plus Rooms, has made space for a new bedroom and ensuite.

There are some stipulations regarding size, too: to be allowable under PD, additional volume created in the roof should not exceed 40m3 for terraced homes or 50m3 for detached houses. Any bigger means seeking permission. Check in with the council if you are unsure or obtain a Lawful Development Certificate for peace of mind. PD rights do not apply if your home is listed or in a designated zone, such as a conservation area.

For any loft conversion, including dormer extensions, your scheme is likely to be affected by (but not limited to) Building Regulations regarding thermal performance, ventilation and fire safety. If you’re adding an extra floor to a house that already has two storeys, providing a safe means of escape in the event of a fire is vital.

Your designer, structural engineer and trades should work with you to ensure everything is up to standard, and building control (or your appointed approved inspector) will carry out routine inspections as the work progresses.

Dormer conversion costs

Depending on where you are in the UK, a completed dormer loft conversion is likely to cost upwards of £30,000. “In London, this type of project typically starts at £38,000,” says Rob from Simply Loft.

“This usually includes the loft conversion shell, windows, plumbing and electrics, installing an ensuite and leaving the space with a plastered finish, ready for decoration.” Large schemes could cost £60,000 or more. A lot depends on the size of the addition, material spec and how much additional work is required inside (eg relocating the boiler as part of the scheme will add to the total sum).

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