Do I Need Planning Permission for a Dormer? Class B Rules in Full

A dormer is usually permitted development on a house in England if it sits on a rear or side roof slope, adds no more than 40 cubic metres of roof volume on a terrace house or 50 on any other house, does not rise above the existing ridge, and the house is not in a conservation area, National Park or other article 2(3) land. Flats have no such rights at all. The rules are in Class B of Part 1 of Schedule 2 to the GPDO 2015.

Before you commit, look at what your council has approved and refused on nearby roofs: search planning applications near you free ->

Quick Answer

The Legal Basis

Dormers are not dealt with by name anywhere in planning law. They fall under Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, which grants permission for:

"The enlargement of a dwellinghouse consisting of an addition or alteration to its roof."

That single sentence is the whole grant. Everything else is subtraction: paragraph B.1 lists what takes the dormer outside the right, and paragraph B.2 lists the conditions that apply even when it is inside.

Two threshold points come before any of that.

It must be a dwellinghouse. Article 2(1) of the Order provides that "dwellinghouse", except in the parts of the Order dealing with changes of use, demolition, local authority development and new dwellings, "does not include a building containing one or more flats, or a flat contained within such a building". A top-floor flat has no Class B rights, however house-like the building looks.

The rights may have been removed. Class B rights can be withdrawn by an article 4 direction or by a condition on an earlier planning permission. Both are common on modern estates and in areas of consistent roofscape. Neither shows up on a survey — you have to check the planning history. See permitted development rights explained and how to check planning history of a property.

What Takes a Dormer Outside Class B

Paragraph B.1 says development is not permitted by Class B if:

• (a) permission to use the dwellinghouse as a dwellinghouse was granted only by virtue of Class G, M, MA, N, P, PA or Q of Part 3 of the Schedule (changes of use). A former office, shop, agricultural building or barn converted to a home under those classes has no Class B rights.
• (b) any part of the dwellinghouse would, as a result of the works, exceed the height of the highest part of the existing roof.
• (c) any part would extend beyond the plane of any existing roof slope which forms the principal elevation of the dwellinghouse and fronts a highway.
• (d) the cubic content of the resulting roof space would exceed that of the original roof space by more than 40 cubic metres in the case of a terrace house, or 50 cubic metres in any other case.
• (e) it would consist of or include (i) the construction or provision of a verandah, balcony or raised platform, or (ii) the installation, alteration or replacement of a chimney, flue or soil and vent pipe.
• (f) the dwellinghouse is on article 2(3) land.
• (g) the dwellinghouse is built under Part 20 of the Schedule (construction of new dwellinghouses).
• (h) the existing dwellinghouse has been enlarged in reliance on Class AA (enlargement by construction of additional storeys).

Four of these are worth unpacking, because they are where most dormer projects actually fail.

The principal elevation rule

B.1(c) is not a "front of the house" rule in ordinary language — it is a rule about the plane of an existing roof slope which both forms the principal elevation and fronts a highway. Paragraph I of Part 1 defines "highway" as including "an unadopted street or a private way". A private drive or an unadopted lane still counts.

A corner plot can therefore have two slopes engaged, and a house set behind another building may have none in the ordinary sense. A dormer that projects beyond a qualifying slope is outside Class B no matter how small it is.

The volume cap

B.1(d) is measured in volume, not width, height or number of dormers. There is no width limit in Class B, which is why full-width and L-shaped rear dormers are built as permitted development. The cap is:

• 40 cubic metres for a terrace house
• 50 cubic metres for any other house

Paragraph I defines a terrace house as a dwellinghouse situated in a row of three or more dwellinghouses used or designed for use as single dwellings, where it shares a party wall with, or has a main wall adjoining the main wall of, the dwellinghouse on either side; or, if it is at the end of a row, shares a party wall with or has a main wall adjoining the main wall of a dwellinghouse that meets that description. An end-of-terrace house is therefore a terrace house for this purpose, and is capped at 40 cubic metres — not 50. This single point catches out more projects than any other.

The cap is also cumulative and unforgiving about history. Paragraph B.3 defines "resulting roof space" as "the roof space as enlarged, taking into account any enlargement to the original roof space, whether permitted by this Class or not". A previous owner's rear dormer, a hip-to-gable done in 1998, a small side enlargement — all of it is counted against your 40 or 50 cubic metres. "Original" is defined in article 2(1) of the Order: for a building existing on 1 July 1948, the building as it existed on that date; for a building built on or after that date, the building as built.

Note also that "cubic content" is defined in article 2(1) of the Order as the cubic content measured externally.

Balconies, terraces and Juliet balconies

B.1(e)(i) removes the right where the works include the construction or provision of a "verandah, balcony or raised platform". Paragraph I defines "raised" in relation to a platform as a platform with a height greater than 0.3 metres. A dormer designed with a walk-out terrace on the flat roof of a rear extension is outside Class B. For what happens next, see do I need planning permission for a roof terrace.

Article 2(3) land

Part 1 of Schedule 1 to the Order defines article 2(3) land as land within:

• a conservation area designated under section 69 of the Planning (Listed Buildings and Conservation Areas) Act 1990;
• an area of outstanding natural beauty;
• an area specified by the Secretary of State for the purposes of section 41(3) of the Wildlife and Countryside Act 1981;
• the Broads;
• a National Park; and
• a World Heritage Site.

On any of those, Class B does not apply at all. Every dormer needs planning permission. See conservation areas and planning.

If the house is listed, planning permission is not the only issue. Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 prohibits works for the alteration or extension of a listed building in any manner which would affect its character as a building of special architectural or historic interest unless authorised — listed building consent is separate from, and additional to, planning permission. See planning permission for listed buildings.

The Conditions You Must Still Meet

Getting inside B.1 is not enough. Paragraph B.2 imposes conditions, and breaching one means the dormer was never permitted development at all:

• (a) the materials used in any exterior work must be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse;
• (b)(i) other than in the case of a hip-to-gable enlargement, or an enlargement which joins the original roof to the roof of a rear or side extension: (aa) the eaves of the original roof must be maintained or reinstated, and (bb) the edge of the enlargement closest to the eaves of the original roof must be, so far as practicable, not less than 0.2 metres from the eaves, measured along the roof slope from the outside edge of the eaves;
• (b)(ii) other than in the case of an enlargement which joins the original roof to the roof of a rear or side extension, no part of the enlargement may extend beyond the outside face of any external wall of the original dwellinghouse; and
• (c) any window inserted on a wall or roof slope forming a side elevation must be obscure-glazed and non-opening unless the openable parts are more than 1.7 metres above the floor of the room in which the window is installed.

Paragraph B.4 adds two useful clarifications to B.2(b)(ii): roof tiles, guttering, fascias, barge boards and other minor roof details overhanging the external wall of the original dwellinghouse are not treated as part of the enlargement; and "rear or side extension" includes an original part of, or a subsequent extension of, the dwellinghouse that extends from the rear or side of the principal part of the original dwellinghouse.

The 0.2 metre eaves setback in B.2(b)(i)(bb) is the condition most often missed on site. It is why a well-detailed rear dormer has a visible strip of original roof below it, and why a dormer built flush to the gutter line is a permitted development failure even if the volume is well inside the cap.

Rooflights: Class C, Not Class B

A rooflight that does not enlarge the roof falls under Class C — "any other alteration to the roof of a dwellinghouse". Development is not permitted by Class C if:

• (b) the alteration would protrude more than 0.15 metres beyond the plane of the slope of the original roof, measured from the perpendicular with the external surface of the original roof;
• (c) it would result in the highest part of the alteration being higher than the highest part of the original roof;
• (d) it would consist of or include the installation, alteration or replacement of a chimney, flue or soil and vent pipe, or of solar photovoltaics or solar thermal equipment.

Class C.1(a) and C.1(e) mirror Class B's exclusions for homes created by a Part 3 change of use and for houses built under Part 20. Condition C.2 applies the same obscure-glazing and 1.7 metre rule to any window on a roof slope forming a side elevation.

Unlike Class B, Class C is not excluded on article 2(3) land — but the 0.15 metre projection limit is tight, and a conservation-area rooflight still needs to be genuinely flush.

Worked Example

A mid-terrace Victorian house in a street that is not in a conservation area. The owner wants a full-width rear dormer with two windows facing the garden and one small window in the flank wall.

1. Is it a dwellinghouse? Yes — a single house, not a flat. Class B is available in principle.
2. Terrace house? Yes: three or more in a row, party walls both sides. The cap is 40 cubic metres, not 50.
3. History. The loft was part-converted in 2004 with a small rear box dormer, later removed. Under B.3 that earlier enlargement still counts toward the 40 cubic metres, because "resulting roof space" is measured against the original roof space.
4. Principal elevation. The dormer is on the rear slope, which does not front the highway. B.1(c) is not engaged.
5. Height. The dormer roof sits below the ridge. B.1(b) is satisfied.
6. Balcony. The owner drops the idea of a Juliet balcony with an outward-opening door onto a platform — B.1(e)(i) would have taken it outside Class B.
7. Eaves. The design is pulled back 0.25 metres from the eaves, satisfying B.2(b)(i)(bb), and the original eaves are retained.
8. Flank window. It is on a side elevation, so B.2(c) requires obscure glazing and either a fixed pane or openable parts above 1.7 metres.
9. Materials. Tile-hung cheeks to match the roof, satisfying B.2(a).

The volume calculation comes back at 37 cubic metres including the 2004 works. It is permitted development — with about three cubic metres of headroom. That is exactly the situation in which a certificate is worth buying.

When a Lawful Development Certificate Is Worth It

Permitted development gives you no document. A certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 does. If the council is satisfied that the operations described would be lawful, it must issue a certificate, and under section 192(4) the lawfulness of the operations is then conclusively presumed unless there is a material change before they are begun.

Get one when:

• the volume is anywhere near 40 or 50 cubic metres;
• the house has an enlargement history you did not carry out;
• the property is an end-of-terrace and the 40-cubic-metre cap is in play;
• the principal elevation is arguable — a corner plot, a private way, or a house set at an angle;
• you intend to sell within a few years, or the loft will be a bedroom that a buyer's solicitor will ask about.

Fees and timescales, from the primary sources. Under regulation 11 of the Town and Country Planning (Fees for Applications, Deemed Applications, Requests and Site Visits) (England) Regulations 2012, the fee for an application under section 192 is half the amount that would be payable for an application for planning permission to carry out the operations. Schedule 1, Part 2, category 6(1) of those Regulations sets the fee for "the enlargement, improvement or other alteration of existing dwellinghouses" at £528 where the application relates to a single dwellinghouse, as substituted with effect from 1 April 2025 by the 2025 amendment Regulations (S.I. 2025/342). A dormer planning application therefore costs £528, and a section 192 certificate for the same works costs half of that.

Under article 34(2)(b) of the Town and Country Planning (Development Management Procedure) (England) Order 2015, the council must give notice of its decision on an application for development which is not major development within 8 weeks beginning with the day immediately following receipt, unless an extended period is agreed in writing.

See lawful development certificate cost for the full picture.

Common Mistakes

1. Assuming an end-of-terrace gets 50 cubic metres. Paragraph I brings end-of-terrace houses inside the definition of "terrace house". The cap is 40.
2. Ignoring earlier roof works. B.3 counts every previous enlargement of the original roof space, "whether permitted by this Class or not".
3. Measuring width instead of volume. Class B has no width limit and no dormer-count limit. Volume is the constraint.
4. Building flush to the gutter. B.2(b)(i)(bb) requires a 0.2 metre setback from the eaves so far as practicable, except for hip-to-gable work or where the roof joins a rear or side extension roof.
5. Adding a balcony. B.1(e)(i) removes the right for a verandah, balcony or raised platform — and "raised" means over 0.3 metres.
6. Forgetting the side window condition. Obscure glazing and the 1.7 metre rule are conditions, not guidance.
7. Relying on a neighbour's dormer. Theirs may predate an article 4 direction, may have had permission, or may simply be unlawful. See permitted development loophole: the neighbour surprise.
8. Forgetting building regulations. Permitted development says nothing about structure, fire escape, stairs or insulation. A loft conversion is separately controlled under the Building Regulations.

Where This Sits

A dormer is one route to a loft conversion; the wider decision — rooflights, hip-to-gable, mansard, structural implications — is covered in do I need planning permission for a loft conversion. If the dormer falls outside Class B, you are into a householder planning application: see householder planning permission explained and how long does planning permission take.

Jurisdiction. This guide describes England only. The GPDO 2015 as described here is the England Order, and the Fees Regulations and Planning Practice Guidance cited are English. Wales, Scotland and Northern Ireland each have their own permitted development legislation with different limits, and this guide should not be relied on outside England.