Documents for a loft conversion with a rear dormer

A rear dormer loft conversion on a house is often permitted development under Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015, which allows additions to the roof within a volume allowance of 40 cubic metres for a terraced house and 50 cubic metres for a detached or semi-detached house. The right disappears entirely on designated land, which includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. It never applied to flats. Before you draw anything, check the designation and work out the volume, because those two answers decide whether you are making a planning application at all.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works are lawful.

Whether the dormer is permitted development at all

Class B carries a set of conditions, and failing any one of them takes the whole thing out of permitted development. The ones that catch people:

• The volume allowance is cumulative. Any earlier roof enlargement, including one built by a previous owner, counts against it.
• No part of the enlargement may extend beyond the plane of any existing roof slope that fronts a highway. That rules out a front dormer on most streets.
• The enlargement must not be higher than the highest part of the existing roof.
• Materials must be similar in appearance to the existing house.
• Side facing windows must be obscure glazed and non-opening below 1.7 metres above the floor.
• Other than for a hip to gable enlargement, the extension must be set back at least 20 centimetres from the original eaves, measured along the roof slope.
• Verandas, balconies and raised platforms are excluded.

The word "original" does a lot of work here. It means the house as built, or as it stood on 1 July 1948, not as you bought it.

Two further traps. An Article 4 direction can remove Class B rights in a defined area, which many councils have done in terraced streets. And a house created through certain prior approval conversions may have had permitted development rights restricted by condition. Check both on the council's website before assuming.

Where the right exists, apply for a lawful development certificate anyway. It costs less than a planning application, it protects you at sale, and it forces the volume calculation to be done properly.

Drawings the council will insist on

Whether you are applying for a certificate or for planning permission, the drawing set is much the same.

1. A location plan at 1:1250 with the site edged red.
2. Existing and proposed floor plans, including the loft.
3. Existing and proposed elevations of every affected face, at a metric scale.
4. Existing and proposed roof plans.
5. A section through the roof showing the dormer, ridge height and eaves.
6. A volume calculation, set out so it can be checked, where permitted development is claimed.

The volume calculation is the item most often missing, and it is the one a lawful development certificate turns on. Show the method: the dimensions used, the shape broken into measurable solids, and any earlier enlargement deducted.

Scale bars and north points on every drawing. Councils return applications for less.

When a heritage statement is triggered

If the house is listed, any dormer will need listed building consent as well as planning permission, and a heritage statement describing the significance of the building and the effect of the works is a validation requirement. Internal alterations to form the stair are part of that assessment too.

If the house is in a conservation area but not listed, Class B does not apply, so you are making a planning application. The council will assess the effect on the character and appearance of the area, and it will usually want a short heritage statement even where the local list does not demand one.

A design and access statement is not normally required for a householder planning application, though it is for listed building consent. Check the council's local validation list, because some authorities ask for more.

Daylight to the neighbouring window

A rear dormer sits high and set back, so it rarely causes a daylight problem on its own. It becomes an issue where the dormer is wide, close to the boundary, and the neighbour has a rear facing habitable room window.

The test most councils apply informally is a 45 degree line drawn in plan and in section from the centre of the neighbour's window. Where the proposal crosses that line, expect a request for more information or a design change. Councils in denser areas apply their own householder design guidance, so read it before drawing.

The practical fixes are narrowing the dormer, setting it further from the boundary, or reducing its projection. None of them is expensive at sketch stage.

Structural and party wall matters outside planning

Planning and building control are separate regimes, and the party wall procedure is separate again.

The structural design, the new floor joists, the trimming around the stair opening and the support for the dormer cheeks are Building Regulations matters requiring calculations and a building control application.

Where the works affect a shared wall, the Party Wall etc. Act 1996 applies. That means serving notice on the adjoining owner, usually two months before starting work on a party wall, and appointing surveyors if the neighbour dissents. Planning permission does not remove that obligation, and the neighbour's agreement under the Party Wall Act is not consent for planning purposes either.

Start the party wall notices early. Two months is a long time when the scaffold is booked.

Fire escape from a third storey

Adding a habitable room in the roof usually creates a third storey, and that changes the fire strategy for the whole house. Approved Document B sets the requirements, and on a typical two storey house becoming three the answer is a protected stairway running from the loft to a final exit, with fire doors to habitable rooms and an interlinked alarm system.

This is the item that most often forces a redesign of the plan below. A stair that opens into an open plan kitchen at ground floor may need enclosing, or an alternative approach agreed with building control such as a sprinkler system. Neither is a planning matter, but both change the drawings you submit, so resolve them before the planning set is finalised.

Assembling the pack in order

A sequence that avoids rework:

1. Check the designation, any Article 4 direction and the planning history of the house.
2. Measure the existing roof and calculate the volume used and available.
3. Decide the route: lawful development certificate or planning application.
4. Resolve the fire escape strategy with building control in principle.
5. Draw the set, including the section and the volume calculation.
6. Serve party wall notices in parallel with the application, not after it.

A scheme that adds a bathroom over the stairwell in a semi-detached house with no previous roof works has plenty of volume headroom and usually falls comfortably within Class B. The same scheme in a conservation area terrace is a planning application with a heritage statement. That contrast is illustrative rather than drawn from a specific case.

For a larger project, the evidence base grows quickly, much as it does for a demolition and rebuild of a single house. Browse the report types to see what a fuller application involves, and enter your postcode on PlanScope to check which documents your property is likely to need.

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