How big can a rear dormer be under Permitted Development?

There is no maximum width or depth stated in the GPDO. The size is set indirectly by four limits acting together: the 40m³/50m³ volume cap, the requirement that no part sits above the existing ridge, the 200mm setback from the eaves, and — on a hipped roof — the point at which the hip plane starts. On a typical terraced house those constraints usually produce a dormer running most of the rear roof width and stopping short of both ridge and eaves.

Can a dormer go right up to the ridge?

No. Class B.1(a) prevents any part of the enlargement exceeding the height of the highest part of the existing roof, measured to the outside of the finished roof. In practice the dormer's flat roof, including its build-up, has to be held demonstrably below the ridge line — designing it flush is a breach.

Does a dormer have to be set back 200mm from the eaves?

Yes, for a roof enlargement other than a hip-to-gable. Class B.2(b) requires the edge of the enlargement closest to the eaves of the original roof to be not less than 200mm from the eaves, measured along the roof slope, so far as practicable. Taking the dormer face down to the gutter line to gain floor area is the most common breach of Class B.

Can I have a flat roof on a dormer?

Yes. The GPDO says nothing about dormer roof form, and flat-roofed dormers are the standard because they maximise volume efficiency and headroom for a given ridge height. Local design guidance may prefer a pitched dormer, but that is a matter for a planning application, not for Permitted Development.

Conversion types

Rear dormer loft conversion rules

A rear dormer is Permitted Development under Class B if it adds no more than 40m³ (terraced) or 50m³ (semi-detached, detached) to the original roof, no part rises above the existing ridge, it sits at least 200mm back from the eaves, it does not extend beyond a roof slope fronting a highway, and the property is not on designated land.

Last reviewed 1 September 2026

What a dormer actually buys you

A rear dormer replaces part of the sloping roof plane with a vertical face and a flat roof. What that buys is full-height floor area — the wedge of space under a pitched roof that is too low to stand in becomes a room with a ceiling.

It is the difference between a loft you can use as a study and a loft you can use as a double bedroom with an en-suite. It is also the option that spends your entire volume allowance, costs the most, and is the one that designated land takes away.

The limits that shape the design

The GPDO sets no maximum width or depth for a dormer. Its size is decided by four constraints pushing against each other:

1. Nothing above the ridge (B.1(a))

The dormer’s flat roof — measured to the outside of the finished construction, not the ceiling — must sit below the highest part of the existing roof. Since a flat-roofed dormer’s crown height rises with its depth on a pitched roof, this is what caps how far back toward the ridge it can go.

2. 200mm back from the eaves (B.2(b))

The bottom edge has to stop at least 200mm short of the original eaves, measured along the roof slope. This is the limit most often ignored, because taking the dormer face straight down to the gutter is both structurally simpler and worth usable floor area.

3. Volume (B.1(d))

40m³ on a terraced or end-of-terrace house, 50m³ on a semi-detached or detached one, cumulative across every previous roof enlargement. On a narrow terrace this is rarely binding; on a wide semi it frequently is, especially alongside a hip-to-gable.

4. Highway frontage (B.1(c))

Nothing may extend beyond the plane of a roof slope that fronts a highway. On a mid-terrace with a road at the front only, this simply means “rear dormer”. On a corner plot, where the side elevation also fronts a highway, it can rule out the side slope too — and a highway includes public footpaths and bridleways, not just roads.

Conditions you must build to

Condition	Requirement
Materials (B.2(a))	Similar in appearance to the existing house. Tile-hung or slate-clad cheeks generally read better than render or lead-effect panels on a traditional roof.
Side windows (B.2(c))	Obscure-glazed, and non-opening unless the opening part is more than 1.7m above the floor of the room.
No balcony (B.1(e))	No balcony, veranda or raised platform. A Juliet guard set in the dormer face, with no platform to stand on outside the line of the wall, is generally accepted.

The case where a rear dormer does not apply

A standard rear dormer needs a rear roof slope to sit in — which means the ridge has to run parallel to the rear wall. On houses where the ridge runs front-to-back, there is no rear slope: the rear elevation is a gable. The options there are a side dormer or a hip-to-gable, both of which are usually still Class B development, and both of which need their own assessment against the highway frontage rule.

What the dormer means for Building Regulations

Nothing above is a Building Regulations matter, and all of it is separate from the approval you actually need to build. The dormer’s new flat roof and cheeks are new thermal elements with their own U-value targets, the floor has to carry domestic loading, and turning the house into three storeys triggers the fire-safety requirements set out in our guide to Building Regulations for loft conversions.

Common questions

How big can a rear dormer be under Permitted Development?: There is no maximum width or depth stated in the GPDO. The size is set indirectly by four limits acting together: the 40m³/50m³ volume cap, the requirement that no part sits above the existing ridge, the 200mm setback from the eaves, and — on a hipped roof — the point at which the hip plane starts. On a typical terraced house those constraints usually produce a dormer running most of the rear roof width and stopping short of both ridge and eaves.
Can a dormer go right up to the ridge?: No. Class B.1(a) prevents any part of the enlargement exceeding the height of the highest part of the existing roof, measured to the outside of the finished roof. In practice the dormer's flat roof, including its build-up, has to be held demonstrably below the ridge line — designing it flush is a breach.
Does a dormer have to be set back 200mm from the eaves?: Yes, for a roof enlargement other than a hip-to-gable. Class B.2(b) requires the edge of the enlargement closest to the eaves of the original roof to be not less than 200mm from the eaves, measured along the roof slope, so far as practicable. Taking the dormer face down to the gutter line to gain floor area is the most common breach of Class B.
Can I have a flat roof on a dormer?: Yes. The GPDO says nothing about dormer roof form, and flat-roofed dormers are the standard because they maximise volume efficiency and headroom for a given ridge height. Local design guidance may prefer a pitched dormer, but that is a matter for a planning application, not for Permitted Development.

Sources

GPDO 2015 — Schedule 2, Part 1, Class B — the roof-enlargement rules, including the 40m³/50m³ volume allowance
Permitted development rights for householders: technical guidance — the government's plain-English reading of Class B, with diagrams

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Related guides

The 40m³ / 50m³ volume allowance, explained

How much roof volume Permitted Development lets you add, what 'original' means, why the allowance is cumulative, and how a rear dormer's volume is actually calculated.

Hip-to-gable loft conversions

Squaring off a hipped roof end to a vertical gable. What it costs in Permitted Development volume, which houses can have one, and when to pair it with a rear dormer.

Rooflight loft conversions

A rooflight conversion adds no volume, sits under Class C, and survives designated land. The 150mm rule, what it costs you in headroom, and when it is the right call.

Permitted Development rules for loft conversions

Class B and Class C of the GPDO 2015, explained clause by clause: volume limits, ridge height, eaves setback, highway frontage, materials and side windows.

Written by the LoftNow team from the legislation and Approved Documents cited above, and reviewed against the rules our assessment engine implements. It is general guidance for properties in England, not advice on your particular property — Permitted Development is self-assessed, and the definitive answer for a specific house comes from your local planning authority, normally via a Lawful Development Certificate.