Rear dormer proportions and the 20cm eaves set back

The rear dormer permitted development rules are mostly conditions rather than limits, and the one that shapes the drawing is the 20 centimetre set back. Class B of Schedule 2 Part 1 to the General Permitted Development Order 2015 requires the edge of a roof enlargement nearest the eaves to sit at least 20 centimetres back from the eaves of the original roof, measured along the roof slope, so far as practicable. That small dimension changes the proportions of the dormer, the internal headroom at the back wall, and the detailing of the gutter.

Get it right on the drawings and a lawful development certificate is straightforward. Get it wrong and you have built something outside Class B, which is a problem that surfaces when you sell.

The conditions that shape a rear dormer

Class B carries limits, which decide whether the work is permitted at all, and conditions, which decide how it has to be built. The conditions are the ones builders overlook.

• Materials must be of similar appearance to those used on the exterior of the existing house.
• The enlargement must be set back at least 20 centimetres from the eaves of the original roof, measured along the roof slope, so far as practicable. This condition does not apply to a hip to gable enlargement, or to an enlargement joining the original roof to the roof of a rear or side extension.
• Any window in a wall or roof slope forming a side elevation must be obscure-glazed and non-opening, unless the opening parts are more than 1.7 metres above the floor of the room.

Alongside those, the limits: no part above the height of the highest part of the existing roof, nothing beyond the plane of a roof slope forming the principal elevation and fronting a highway, no veranda, balcony or raised platform, a volume allowance of 40 cubic metres for a terrace house and 50 otherwise, and no Class B rights at all on designated land.

Measuring the 20 centimetre set back

Three features of the wording decide how the dimension is taken, and all three are easy to misread.

It is measured along the roof slope from the outside edge of the eaves, not horizontally. On a 40 degree pitch, 20 centimetres along the slope is about 15 centimetres measured on plan, which is why a drawing that dimensions the set back horizontally can look compliant and be wrong, or look non-compliant and be fine. Dimension it on the section, along the slope, and label it as such.

It runs from the eaves of the original roof. If the roof has been altered, or if an earlier extension changed the eaves line, the measurement is taken from where the eaves were when the house was built, or as they stood on 1 July 1948 for older houses.

And it is qualified by "so far as practicable". That phrase exists because there are junctions where a set back cannot physically be achieved, typically where the dormer meets the roof of a rear outrigger. It is not a general escape clause. A council will accept an explanation about a specific junction and will not accept the claim that a set back was inconvenient.

The practical consequence inside the loft is a sloping zone at the back of the room. Twenty centimetres along the slope plus the roof build-up leaves a low triangle at the base of the dormer wall, which is where the gutter and the structural detail live. Plan the room around it rather than discovering it on site.

Staying below the ridge, and what that costs in headroom

No part of the enlargement may exceed the height of the highest part of the existing roof. On a dormer that means the flat roof sits below the ridge line, usually with a modest step.

That single limit is what makes shallow-pitched roofs unsuitable for loft conversions. Working backwards from the ridge, you need the dormer roof build-up, the ceiling zone, the headroom and the floor build-up to fit in the space available.

Building Regulations give a little relief on the stair. Approved Document K allows reduced headroom over a stair in a loft conversion, with less than the usual two metres acceptable at the centre of the stair width and less again at the side. That concession applies to the stair only, not to the room, so a conversion that relies on it still needs full headroom where people stand.

Side-facing windows and rooflights

The obscure glazing condition catches more than people expect, because it applies to a window in a wall or roof slope forming a side elevation, rather than to a dormer cheek window alone.

So a rooflight set into the side slope of a hipped roof is caught. A small window in the side cheek of a rear dormer is caught. A window in the rear face of the dormer is not, because that is a rear elevation.

Obscure-glazed means the glass itself obscures the view, not a film applied afterwards and not a blind. Non-opening below 1.7 metres means the opening parts have to be above that height measured from the floor of the room, which usually means a top-hung vent above a fixed obscure pane.

Where the dormer faces a neighbour at close range, the condition does nothing for the rear elevation, and overlooking becomes a planning question rather than a permitted development one. The same tension shows up with Juliet balconies, terraces and overlooking on extensions generally.

Balconies, terraces and the raised platform rule

Class B excludes a veranda, balcony or raised platform. A raised platform means a platform more than 300 millimetres above the ground.

The line that matters in practice is between an opening with a guard across it and a usable external space. A set of doors in the rear face of a dormer with a guarding rail immediately behind them, with no floor outside, stays within Class B. Cutting the roof back to form a terrace outside the dormer does not, and that is a full application.

Materials are the other condition that bites on a dormer, because the usual finishes are not the ones on the existing house. Tile hanging, slate, lead, zinc, render and timber boarding all appear on dormer cheeks, and whether a given finish is of similar appearance to the existing exterior is a judgement your council may take differently from your builder. The same question arises when rendering or cladding a house, and the safe route where there is doubt is a lawful development certificate before the scaffolding goes up.

Where the council's design guide does and does not apply

This is the point that confuses homeowners most. If the dormer is permitted development, the council's householder design guide does not apply to it. There is no application, so there is no design assessment, and a dormer that the guide would have resisted is still lawful.

If the dormer needs an application, the guide governs. Most English design guides ask for dormers to be set in from the party walls, kept below the ridge by a clear margin, subordinate in width to the roof slope, and detailed with a pitched rather than flat roof in sensitive streets. A full-width flat-roofed box that was acceptable as permitted development will often be refused when it comes forward as an application, which is why checking the volume allowance first matters so much.

A dormer needs a full application where the house is on designated land, where an article 4 direction removes Class B, where the volume allowance has been used up by earlier work, where the dormer would break the ridge line or the principal elevation plane, where the building is a flat, or where a terrace is proposed. Set out the design reasoning in the Planning Statement, and where the property is listed or in a conservation area a Design and Access Statement will also be expected.

Drawing it so compliance is obvious

Five annotations turn a drawing into evidence.

1. A section through the dormer at a stated scale, with the 20 centimetre set back dimensioned along the roof slope and labelled as measured from the original eaves.
2. The existing ridge level shown as a datum line, with the dormer roof level dimensioned below it.
3. A volume calculation panel showing the original roof volume, the proposed roof volume and the difference, with any earlier enlargement listed.
4. Window annotations stating obscure glazing and opening heights on every side elevation opening.
5. A materials note naming the finishes and comparing them with the existing exterior.

Enter your postcode on PlanScope to see which reports a loft conversion on your house 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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.