Outrigger dormer permitted development: Class B or Class A?

An outrigger dormer can be permitted development under Class B even where it meets the rear wall of the house, joins roofs at different levels or extends over a flat-roofed rear projection. Contact with a wall does not, by itself, make Class A apply.

The starting point is the nature of the operation: is it an addition or alteration to a roof, or does it include a separate enlargement of the house? Several Inspectors have rejected attempts to apply Class A automatically to a joined roof enlargement. Other appeals show that a new vertical storey can fall within Class A. The drawings and the facts matter.

A joining roof enlargement can meet the main rear wall without automatically engaging Class A. Blue shows the roof enlargement; grey shows the existing building.

Why an outrigger dormer can fall within Class B

An outrigger is a projecting wing at the rear of a house, often narrower and lower than the main building. It may be original or a later addition. An L-shaped dormer joins a dormer on the main roof to a roof enlargement over that rear wing.

Class A is not an additional test that every roof enlargement must pass. Paragraph A.1(k)(iv) excludes an alteration to any part of the roof from the development permitted by Class A. We must first identify what the proposed works actually do: calling something a dormer does not decide which class applies.

Class B permits the enlargement of a dwellinghouse consisting of an addition or alteration to its roof, subject to its limitations and conditions. A scheme does not become a Class A extension merely because it raises part of an outrigger wall or meets a rear wall below the main eaves.

Condition B.2(b) contains specific exceptions for an enlargement joining the original roof to the roof of a rear or side extension. These qualify the requirements to maintain or reinstate the original eaves, provide the usual 0.2 metre setback and keep the enlargement within the outside face of an external wall. They do not remove the remaining Class B tests.

The Government’s householder technical guidance expressly discusses L-shaped dormers joining a main roof and an outrigger or back addition. It recognises that the roofs can have different shapes, pitches or levels and that the rear projection can be original or a later enlargement.

For the outside-face rule in B.2(b)(ii), the statutory definition in B.4(b) expressly includes an original projecting part of the house as well as a subsequent rear or side extension. This definition was introduced with effect from 1st August 2020.

Touching the rear wall does not automatically engage Class A

At 78 Cowper Street, Hove, the council argued that the amount of rear wall engaging with the dormer brought Class A into play. The Inspector rejected that approach and assessed the roof enlargement solely under Class B. The existing main dormer and the proposed lower outrigger element would form one larger L-shaped enlargement. See paragraphs 7 to 15 of APP/Q1445/X/23/3327291.

At 61 Belgrave Road, Slough, the Inspector likewise concluded that Class A was not relevant to the joined roof works. The lower outrigger was extended upwards, but the proposal was an alteration and enlargement of the roofs. The dormer was assessed under Class B, with separate rooflights under Class C. See paragraphs 8 to 16 of APP/J0350/X/25/3371511.

Raising a wall is not an automatic Class A trigger

At 162 West Street, Dunstable, sections of wall would be raised by up to 2.7 metres above part of a low outrigger. The Inspector found that this did not, in itself, prevent the works being an addition to the roof. The lower element was physically and functionally connected with the main dormer, while about half the outrigger remained at its existing height. See paragraphs 6 to 12 of APP/P0240/X/20/3263086.

The drawing below illustrates that distinction. It does not establish a permitted height for raising an outrigger or suggest that every connected structure is Class B. The scope and substance of the proposed operation must be assessed.

A roof addition can involve raising a wall while remaining part of a joined roof enlargement. This schematic shows part of the lower roof retained.

What about a flat-roofed outrigger?

A flat roof is not excluded from Class B. At 26 Merivale Road, Harrow, the proposal extended from the main rear roof slope over a two-storey flat-roofed outrigger. The Inspector found no requirement for the enlargement to remain within a pitched roof slope and held that Class A was not relevant to that roof operation. See paragraphs 8 to 11 of APP/M5450/X/22/3301565.

Compare 90 Cromwell Road, Hounslow. The disputed element added a new storey above the existing single-storey flat-roofed section of the outrigger, with vertical walls and an entirely new roof. The Inspector treated that as a vertical house extension under Class A, which failed the boundary/eaves limitation. See paragraphs 11 to 13 of APP/F5540/X/22/3308927.

These outcomes do not create a universal test based on whether the existing roof is flat or whether a wall is raised. They illustrate why the complete proposed operation must be identified before applying the limits of the relevant class.

The left example illustrates a joined roof enlargement; the right illustrates an additional storey above a lower flat-roofed part. These are schematic comparisons, not drawings of the appeal sites.

If the main dormer already exists

Construction sequence is relevant evidence, but it is not an automatic pass or fail rule. Cowper Street succeeded even though the main dormer already existed. In Balvernie Grove, the Inspector reached a different conclusion about the later outrigger works and their relationship to the earlier dormer. Explain exactly which roof structures are retained, altered, replaced or joined and identify the complete operation for which the certificate is sought.

Appeals addressing both Class A and Class B

These decisions discuss the Class A/Class B issue within the same appeal. They are persuasive examples, not binding court judgments or guarantees for another scheme. The application date, the applicable GPDO wording and the actual drawings must be compared.

Swipe left or right to read the whole table.

The other Class B limits still apply

Classifying the work as a roof enlargement is only the first step. For a terrace house the resulting roof space must not exceed the original roof space by more than 40 cubic metres; the limit is 50 cubic metres in other cases. Measure externally and account for previous enlargements to the original roof space. The allowance is not a separate entitlement to build any shape of box within that volume.

• Confirm that the property is an eligible dwellinghouse and that rights have not been removed by a planning condition or an Article 4 direction
• Check the article 2(3) land restriction and the exclusions for houses built under Part 20 or enlarged under Class AA
• Check the highest-roof limit and the restriction on enlarging a principal-elevation roof slope which fronts a highway
• Apply the eaves, setback and outside-face conditions, including only the exceptions relevant to the actual proposal
• Use exterior materials of a similar appearance to the existing dwellinghouse
• Check obscure glazing and opening restrictions for windows on a side wall or roof slope
• Check every remaining restriction, including balconies, raised platforms and chimneys, and assess separate rooflights under Class C

The main Class B roof enlargement guide explains the full rules. A photograph or a schematic drawing cannot establish that a particular proposal is permitted development.

What if the drawings also show a ground-floor extension?

The same set of drawings can show an outrigger dormer and a separate ground-floor extension. The roof enlargement may fall within Class B while the ground-floor work needs its own Class A assessment, including the larger-home-extension prior approval procedure where applicable.

A separate Class A extension does not automatically make the roof enlargement subject to Class A as well. Equally, we cannot avoid the relevant limits by artificially dividing one integrated operation into convenient labels. Show the existing building, each proposed operation and how the elements relate to one another.

Evidence for a lawful development certificate

Provide accurate existing and proposed roof plans, elevations, sections, construction history and external volume calculations. Mark the original roof, the outrigger roof, the main rear wall and the proposed junction. Where the outrigger is lower, show how the roof addition relates physically and functionally to the main loft conversion.

If the council considers Class A relevant, ask it to identify the operation beyond the roof addition or alteration and explain why the actual works fall within that class. Address any materially comparable contrary appeal as well as the decisions supporting your position. A certificate is determined on lawfulness and evidence, not the planning merits of the design.

Planning Geek is a planning consultancy and can help with permitted development assessments, lawful development certificate applications and appeals. See our lawful development certificate appeal guide for the appeal process.

Relevant legislation and guidance for an outrigger dormer

• Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 1, Class B
• Town and Country Planning (Permitted Development and Miscellaneous Amendments) (England) (Coronavirus) Regulations 2020, regulation 6
• Permitted development rights for householders: technical guidance
• Town and Country Planning Act 1990, section 192: certificates of lawfulness for proposed development

Outrigger Dormer Page Updated: 30th September 2026