Rear Dormer Loft Conversion
A rear dormer is the dormer built on the back of the house, facing the garden rather than the street, and it is by far the most common type of dormer loft conversion in the UK. The reason it is so popular comes down to planning. Because it faces away from the highway, a rear dormer usually qualifies for permitted development, while the same size dormer built on the front of the house facing the road would need full planning permission in almost every case.
There is one specific rule worth knowing before you start. Any rear dormer has to be set back at least 200 millimetres from the original eaves, the edge of the roof where it meets the wall. This keeps a visible strip of the original roof slope in place at the bottom, so the house still reads as having its original roofline from below, even with a large new structure built above it. This rule does not apply to a hip to gable conversion, which reshapes the roof rather than projecting a new structure from it, so it is specific to dormers.
Within that setback, a rear dormer can usually cover most of the width of the rear roof slope, which is why it is such an effective way to add space. A well designed rear dormer gives you a flat ceiling and full standing height through most of the new room, rather than the sloped ceiling you would be left with if you did nothing to the roof at all.
Who this for
Rear dormer conversions suit almost any house type, terraced, semi detached, detached and bungalow alike, provided the rear roof slope has enough width and the project stays within your permitted development volume allowance. It is usually the first option worth considering for anyone who wants a genuinely useful new room without the scale of work a mansard or full roof lift involves.
Rear Dormer Loft Conversion Cost
A rear dormer typically costs between £40,000 and £55,000 for a standard project, with a realistic overall range of £45,000 to £75,000 once you account for size, specification and any ensuite. Larger or higher end rear dormers can exceed £90,000. London and the South East generally run 20 to 35 percent above the UK average, largely due to labour rates and site access. If your rear dormer needs full planning permission rather than falling under permitted development, for example because the property is in a conservation area, the householder planning application fee in England currently stands at £258. Even where permitted development applies, it is worth applying for a Certificate of Lawful Development, which costs £103 and gives you official written confirmation that the work was legal, something that matters when you come to sell the house. As always, the figures above are a general guide. The real cost depends on the width of your rear roof slope, your volume allowance, and the finish you choose, so get in touch for a free survey and an accurate quote for your home.
Project Process Steps
We measure the rear roof slope and confirm how much of it can be used within the 200 millimetre setback from the eaves that permitted development requires.
We check your permitted development volume allowance, forty cubic metres for a terraced house or fifty for a semi detached or detached house, and confirm whether any previous work has already used part of it.
Our structural engineer designs the new dormer structure, including the steel beams needed to carry the load the roof slope used to carry, since a rear dormer effectively removes a large section of the original roof.
Once the structure is built and weathertight, we move on to insulation, plasterboard, plumbing, electrics, staircase and finishing, the same as any other loft conversion.
Frequently asked questions (FAQ)
Why is a rear dormer more likely to avoid planning permission than a front dormer?
Because it faces the garden rather than the street. Permitted development rules generally allow dormers on the rear or side of a house, but a dormer facing a highway almost always needs full planning permission, since it changes how the house looks from the road.
What is the 200 millimetre setback rule?
This is a permitted development requirement that any new dormer must be set back at least 200 millimetres from the original eaves, the point where the roof meets the wall. It keeps a strip of the original roofline visible from below, and it applies specifically to dormers rather than to a hip to gable conversion.
How much of the rear roof can a dormer cover?
Within the setback and your overall volume allowance, a rear dormer can often cover most of the width of the rear slope, which is part of why it is such an effective way to gain space compared with a smaller, centred dormer.
Do I still need Building Control approval if my rear dormer is permitted development?
Yes. Permitted development only removes the need for a planning application, it does not remove the need for Building Regulations approval, which applies to every loft conversion regardless of its planning status.
Can a rear dormer be combined with other techniques?
Yes. A rear dormer is often paired with velux windows on the front roof slope for extra light, or with a hip to gable conversion on a semi detached or detached house, to make the most of the available roof space.