Dormer rules under permitted development: eaves and ridge

A rear dormer usually keeps its permitted development rights only if three things hold at once: the edge nearest the eaves sits about 20cm back from them, no part of the dormer rises above the existing ridge, and any window in a side-facing wall or roof slope is obscure-glazed and fixed shut below 1.7 metres. Those are the dormer window rules permitted development turns on, and they sit in Class B of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Miss one and the whole dormer needs a householder planning application.

The volume allowance gets most of the attention. The set back and the ridge line cause more refusals, because they are geometric and a case officer can check them off your own drawings in under a minute.

The 20cm set back from the eaves

Condition B.2(c) says the edge of the enlargement closest to the eaves of the original roof must, so far as is practicable, be not less than 20cm from the eaves, measured along the roof slope from the outside edge of the eaves. Hip-to-gable enlargements are exempt from that condition.

Two phrases do the work. "Measured along the roof slope" means you measure up the pitch, not horizontally and not vertically, so on a shallow roof the same 20cm eats less height than on a steep one. "So far as is practicable" is the escape valve, and it is narrower than people hope. It covers the case where the existing roof structure will not allow the set back, not the case where you wanted a slightly taller dormer cheek.

In practice the set back reads as a strip of retained tile or slate running along the bottom of the dormer. Draw it. Dimension it. If your section shows the dormer cheek landing on the fascia, you have lost Class B whatever the volume figure says.

Why the dormer must stay below the ridge

Class B is not permitted where any part of the enlargement would exceed the highest part of the existing roof. No tolerance, no "so far as is practicable". A dormer flush with the ridge fails, and so does a dormer whose upstand or flashing pokes above it.

This bites hardest on shallow-pitched roofs and on houses where the head height inside the loft is already marginal. Builders sometimes suggest lifting the ridge by a course or two to win floor area. That is a separate enlargement, it breaches Class B, and it is the single most common reason a loft conversion that started as permitted development ends up as a retrospective application.

Measure from the original roof, not from a previous extension's roof. If a rear addition already sits lower than the main ridge, the dormer on that addition still has to stay below the highest part of the existing roof of the dwellinghouse.

Side facing windows, obscure glazing and 1.7 metres

Condition B.2(b) blocks any window in a wall or roof slope forming a side elevation unless it is obscure-glazed and, where it opens, the openable part is more than 1.7 metres above the floor of the room it serves. Both limbs apply together. A clear-glazed fixed light in a dormer cheek fails.

The Order says "obscure-glazed" and stops there. It sets no level on the usual 1 to 5 obscuration scale, so the safe move is to state your specification on the window schedule and in the covering description rather than leave the officer to assume. The same discipline helps if you later need to prove the works were lawful.

The 1.7 metre measurement is taken from the finished floor of the room, which on a loft conversion is often higher than the old joists. Check it against the built level, not the design intent.

Front dormers and designated land

Class B does not permit an enlargement that extends beyond the plane of any existing roof slope which forms the principal elevation and fronts a highway. A dormer on the front of a house facing the road is therefore an application, full stop. Side slopes fronting a highway are caught too, which matters on corner plots.

Designated land removes Class B altogether. That means conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites: no permitted dormers at all, front or rear. An Article 4 direction can do the same thing in a street that is not designated, and there are plenty of those on interwar suburbs. Check both before you spend money on drawings, and if the property is listed, listed building consent is a separate requirement that Class B has no bearing on. Where heritage is in play, the council will expect a Heritage Statement setting out the significance of the building and what the roof alteration does to it.

Flats and maisonettes have no Part 1 rights at all, which catches out owners of converted houses. The reasons that makes a difference are set out in our note on extending a flat or maisonette.

Proportion, the street scene and a full application

Once you are making an application, the numbers stop being the test and design judgement starts. Officers look at the dormer as a proportion of the roof slope: how much slate is left above, below and to each side, whether the cheeks line up with anything, whether the window heads align with those on the storey beneath.

The common criticisms are predictable. A box dormer that fills the slope corner to corner. Cheeks in white uPVC against a clay tile roof. A dormer window wider than the openings below it, which makes the top floor read as an add-on.

Volume figures include any earlier enlargement of the roof, so a 1990s side dormer counts against your allowance today.

Flat roof dormers against pitched forms

A flat-roofed box dormer is the least costly way to buy floor area and the hardest to defend on design grounds. A pitched or hipped dormer loses headroom at the edges and costs more in lead and carpentry, and it usually reads better on a pitched roof with visible eaves.

There is a middle route. Splitting one wide box into two or three smaller dormers, each aligned with a window below, keeps most of the volume and gives the elevation some rhythm. It costs more in flashing and it is worth the argument.

Materials matter as much as form. Condition B.2(a) requires materials similar in appearance to the existing house, and lead or zinc cheeks with a tile-hung or rendered face will get less resistance than plastic trim. For window design on an older house, the choices in our piece on replacement windows in an unlisted period house apply to dormer lights too.

Drawings that show compliance

Illustrative example: a 1930s mid-terrace with a 40 degree rear roof slope, a proposed box dormer 4.2 metres wide, and a cheek height of 2.1 metres. The dormer volume works out well inside 40 m3. The section, though, shows the dormer head 60mm below the ridge tile and the bottom edge 200mm up the slope from the fascia. Both are drawn and dimensioned, so the assessment takes minutes rather than an exchange of emails. Figures here are illustrative only.

What a set of drawings needs to carry:

1. An existing and proposed rear elevation at the same scale, with the ridge line dimensioned on both.
2. A section through the dormer showing the 200mm set back measured along the slope and the clearance below the ridge.
3. A roof plan showing which slopes are affected and which elevation fronts the highway.
4. A window schedule naming the glazing specification and the height of any openable part above finished floor level.
5. A materials note for cheeks, roof covering and window frames.

If you want certainty rather than a builder's opinion, apply for a lawful development certificate under section 192 of the Town and Country Planning Act 1990 for proposed works, or section 191 for works already done. It is the only formal way to confirm that a dormer is lawful, and it is worth the fee before a sale.

Where the dormer needs a full application, the supporting narrative is short: a Planning Statement that ties the design to the council's adopted policies on residential alterations and deals with neighbour amenity head on. That is the document PlanScope drafts from your postcode, the property you confirm on the map and a handful of answers about the work. Enter your postcode on PlanScope to see which reports a roof alteration at your address 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.