Dormer loft conversions

What is a rear dormer loft conversion?

A dormer rises out of the existing roof slope, usually at the back of the house. Its vertical wall and flat or pitched top create usable floor space and standing room where the roof used to slope down.

Because it adds space without changing the whole roof, a dormer is one of the most common choices and works with many property types, creating a full-height room that suits a bedroom with an en-suite.

Which houses suit it

Dormers are the most common choice for terraced houses, because they add headroom without major structural changes. They also suit most semi-detached houses, and are often paired with a hip-to-gable conversion on houses with a hipped roof.

Rear, L-shaped and other dormers

A rear dormer sits on the back roof slope and is the most common version. An L-shaped dormer wraps around two roof slopes, usually the main roof and the roof over a rear wing, which creates more space on houses that have one. The planning rules specifically allow for enlargements that join the main roof to the roof of a rear or side extension.

Dormers can have a flat roof, which maximises headroom and floor space, or a pitched (sloping) roof, which can look more in keeping with the house but gives less internal space. Front dormers facing a road need planning permission.

How much does a rear dormer loft conversion cost?

In Berkshire, expect roughly £46,000 to £78,000 to build a typical 30m² rear dormer conversion, before VAT and fees. Elsewhere in the UK the typical range is £40,000 to £60,000. These are research-based estimates, not quotes.

Add VAT, drawings, a structural engineer, building control and any party wall costs to get a total. Use the calculator for your own size and finish, or read the cost guide.

Planning and building regulations

A rear dormer is usually permitted development, as long as it adds no more than 40m³ of roof space on a terraced house or 50m³ on other houses, sits no higher than the existing roof, uses similar-looking materials and is set back from the eaves.

You need planning permission for a dormer on the front of the house facing a road, in a conservation area, where an Article 4 direction applies, or on a flat.

Whatever the planning position, every loft conversion needs building regulations approval, covering structure, fire safety, stairs and insulation. See our planning guide and building regulations guide.

Pros and cons

Pros

• Full head height across most of the room
• Works on most terraced and semi-detached houses
• A rear dormer is often permitted development
• Good space for a bedroom and en-suite

Cons

• Costs more than a rooflight conversion
• Changes how the back of the roof looks
• Needs planning permission in conservation areas

How long it takes

Checkatrade's typical build time for a rear dormer conversion is 10 to 12 weeks on site, after design, approvals and any party wall process. See the process and timeline guide.

Rear dormers are often allowed under permitted development, within size limits. A lawful development certificate confirms it.

Where it's common in Berkshire

Based on the local mix of homes, this type is most likely to suit houses in Bracknell, Reading, Slough and Windsor:

• Bracknell: 29.7% of households in terraced houses
• Reading: 23.4% of households in terraced houses
• Slough: 26.4% of households in terraced houses
• Windsor: 25.7% of households in terraced houses

From 2021 Census and VOA figures for each town. A tendency, not a guarantee for any one house.