Setting a rear dormer back from the eaves

A rear dormer built as permitted development has to sit at least 0.2 metres back from the eaves of the original roof, measured along the roof slope from the outside edge of the eaves, so far as practicable. That condition in Class B of Part 1 of the General Permitted Development Order 2015 is the one most often ignored on site and the one a planning enforcement officer can check from a neighbour's garden with a tape. Nothing in the Order requires a set back from the party wall, and almost every council design guide asks for one anyway.

Two other limits decide whether your dormer is permitted development at all: it cannot rise above the highest part of the existing roof, and the roof enlargement has a cubic content allowance that previous work eats into.

The 0.2 metre set back from the original eaves

The condition applies to the edge of the enlargement closest to the eaves of the original roof. Measure along the slope, from the outside edge of the eaves, and keep 200 millimetres of original roof covering below the dormer.

Three points of detail matter.

"Original" means the roof as built, or as it stood on 1 July 1948 for older houses. If a previous owner rebuilt the eaves, the original line is what counts, and evidencing it may mean an old photograph or an earlier planning drawing.

"So far as practicable" is a real qualification, and it exists because on some roofs the structure makes 200 millimetres impossible without a step that leaks. It is not a general excuse. If you rely on it, record why in writing before you build, with a note from the designer.

The hip to gable case is excluded from the condition, because a hip to gable enlargement necessarily reaches the eaves. A dormer combined with a hip to gable on the same roof still needs the set back on the dormer element.

Why the party wall edge matters even though the order is silent

Class B says nothing about distance from a party wall or boundary. Three other things do.

Council design guides. Most authorities' householder guidance asks for a dormer to be set in from the party wall by half a metre or a metre, so the roof reads as a roof with a dormer in it rather than as a new upper floor. That guidance does not bind a permitted development dormer, and it binds you completely once you need planning permission.

The Party Wall etc. Act 1996. Cutting into a party wall to build the dormer cheek, inserting a beam, or raising the wall all require notice to the adjoining owner, and the notice periods run to one or two months. Setting the dormer in from the wall by enough to avoid touching it removes that process entirely, which is often the cheapest reason to do it.

Building Regulations. The party wall has to maintain fire separation up to the underside of the roof covering, and a dormer built hard against it makes that detail awkward and the inspection harder. Approved Document B is the reference, and a loft conversion also brings in means of escape, which usually means a protected stairway or an escape window.

Set the dormer in and all three problems shrink. Build it tight to the wall and you have a planning risk, a party wall procedure and a fire detail to resolve at the same time.

Staying below the existing ridge

No part of the enlargement may exceed the height of the highest part of the existing roof. Chimneys are not part of the roof for this purpose, so a dormer must sit below the ridge line itself.

On a shallow pitch this is the binding constraint. A flat roof box dormer designed for maximum headroom will want its top at or near the ridge, and the drawing has to show a clear set down. Fifty to a hundred millimetres of visible original roof above the dormer is the usual arrangement, and it also looks better than a dormer that merges into the ridge.

Beware the reference point. It is the existing roof, so where the house has already been extended with a higher roof, that higher ridge is the datum. Where a previous dormer exists, its height does not raise the datum.

The cubic content allowance and what counts against it

Class B allows the resulting roof space to exceed the cubic content of the original roof space by no more than 40 cubic metres on a terrace house and 50 cubic metres on any other house.

That allowance is cumulative. Every earlier roof enlargement since the original roof counts against it, including a previous dormer, a hip to gable conversion and a rear roof extension carried out by a previous owner. A mid-terrace house with an existing hip to gable has often used most of its 40 cubic metres already.

Calculate it properly rather than estimating. The figure is the volume enclosed by the new roof space over and above the original, and the calculation should appear on the drawings with the dimensions it was derived from. A dormer of 3.5 metres width, 2.2 metres depth and 2.1 metres average height is already over 16 cubic metres before you add the cheeks and the roof structure. The same arithmetic discipline applies to ground floor work, where wrap around extensions and the Class A limits fail for much the same reason.

If you are over the allowance, you need planning permission, and at that point the design guide applies in full.

Cheeks, materials and windows in a side elevation

The materials condition requires materials of similar appearance to the exterior of the existing house. In practice that is satisfied by tile hanging or slate hanging matched to the roof, or by lead, zinc or a dark standing seam finish that authorities accept as traditional on dormer cheeks. Bright white uPVC cheeks on a slate roof are the classic dispute.

Any window inserted in a wall or roof slope forming a side elevation of the house must be obscure glazed, and it must be non-opening unless the openable part is more than 1.7 metres above the floor of the room it serves. Rear-facing windows in the dormer face are unaffected.

A balcony, verandah or raised platform is excluded from Class B altogether. A set of doors in the dormer face with a railing flush in the opening is normally acceptable. A walk-on terrace formed on a flat roof is not, and neither is a dormer with a recessed standing area.

Where a chimney is in the way, resist the temptation to take it out without checking. Removing a chimney and when planning permission applies is a separate question from the dormer, and it can matter in a conservation area or on a building group where the stacks are part of the character.

Where the council's design guide is stricter

Once the dormer falls outside Class B, for any reason, you are in a planning application and the local design guidance governs. Common requirements, beyond the party wall set in:

• A set back from the ridge, the eaves and both verges, often 0.5 metres or more from each.
• A maximum width expressed as a proportion of the roof, commonly two thirds.
• A pitched roof to the dormer rather than a flat one, in some areas.
• Window proportions and subdivision matching the elevation below.
• A prohibition on full width box dormers in particular streets.

A short Planning Statement that takes each requirement in turn and says how the design responds is worth more than a page of general description, and PlanScope drafts one from the answers you give about the property, flagging the facts it cannot confirm.

Dormers in a conservation area

Class B is not available on designated land, which includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. So in a conservation area every rear dormer needs planning permission, however modest.

Expect a harder test. Conservation officers assess the contribution the roofscape makes to the character of the area, and a rear roof visible from a public vantage point is treated as a public elevation even though it faces the garden. Traditional proportions, a set back on all sides, lead or slate cheeks, timber or slim metal windows and no plastic are the usual expectations. A Heritage Statement explaining the significance of the roofscape and why this dormer causes least harm is the document that decides these applications.

Where the building is listed, listed building consent is needed as well, and a rear dormer on a listed roof is a serious proposition rather than a routine one.

Drawing it so the conditions can be checked

Six items on the drawings and the officer or building inspector can verify compliance without asking:

1. An annotated section through the dormer showing the 0.2 metre dimension measured along the slope from the outside edge of the eaves.
2. A section showing the dormer top below the highest part of the existing roof, dimensioned.
3. The cubic content calculation, with the dimensions used and any earlier roof enlargements listed and deducted.
4. A rear elevation showing the dormer position relative to the party walls and the verges, dimensioned.
5. A materials note for the cheeks, the roof, the windows and the flashings.
6. A note on the side elevation windows confirming obscure glazing and the 1.7 metre restriction.

Those six items take an afternoon and they are what keeps a permitted development dormer permitted. Enter your postcode on PlanScope to see which documents a loft conversion or dormer at your property is likely to need.

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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.