A dormer is a small structure that projects from a sloping roof, usually with a vertical window in its face and its own little roof above. It turns the sloping edge of a loft into a space you can stand in, which is why it is one of the most popular ways to add a bedroom or a study without extending the footprint of the house. It is also one of the most common sources of roof leaks, because it replaces a simple slope with several junctions where lead, tile and timber have to meet cleanly.
Two questions come up before any dormer is built. The first is planning: can it go up as permitted development, or does it need a planning application? The answer depends on a short list of tests in the General Permitted Development Order, and on whether the house sits in a conservation area or the South Downs National Park, where the answer is different. The second is the build itself, because the dormer is part of the roof and must be weathered like it.
This guide covers the Class B tests, the design points that keep a dormer looking right, and the repairs older dormers need. It is general information, and the council has the final say on your address.
Class B: what permitted development allows
Permitted development means works the law allows without a planning application, provided they stay inside set limits. For roof additions such as dormers on a house, the rules sit in Class B of Part 1 of Schedule 2 to the General Permitted Development Order 2015. They are a list of tests, and every one has to be passed.
Nothing may rise above the highest point of the existing roof. The added volume is capped at 40 cubic metres for a terrace house and 50 cubic metres for any other house. Verandahs, balconies and raised platforms are outside the rights, and so is any new or altered chimney, flue or soil pipe.
Reading the volume limit
Volume means the new space created, not the size of the window. Measuring it is a job for the plans, and the council can confirm how the allowance applies to your house. A wide box dormer spanning much of a slope can eat a large share of the allowance, while a pair of small gabled ones may sit comfortably inside it. Working it out properly takes a drawing.
Two conditions cover materials and position. The materials must look similar to those of the house, and the edge of the enlargement should, where practicable, stay at least 0.2 metres back from the eaves.
Fail any one test and the dormer needs a planning application. That is a different route, not a refusal. Our guide on pitched roof types helps you identify your roof shape, because a hipped roof and a gabled one take a dormer differently, and the pitched roofing page gives the wider picture.
The principal elevation test
One test concerns the front of the house. Under B.1(c), the enlargement must not go beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway. Put simply, a dormer pushing out of the road-facing slope falls outside the rights.
The principal elevation is normally the main face of the house, the side people see from the street. For most terraces and semis that means the front. On a corner plot, or a house set at an angle to the road, it is less clear, and this is where a conversation with the planning team pays off.
Why rear dormers dominate
This test explains why so many loft conversions put the dormer at the back. A rear slope does not front the highway, so a rear dormer can stay within permitted development provided the other tests are met. A front dormer needs an application and a design that suits the street: smaller, lined up with the windows below, set well back, and matching the roof covering.
Side slopes are a different matter. Side windows in a dormer face a particular condition, covered below.
Torn between a dormer and a roof window for the light? Our guide to roof window or dormer compares the two.
Conservation areas and the South Downs: the B.1(f) exclusion
This is the rule most often missed. Under B.1(f) the rights fall away if the house stands on article 2(3) land. The Order defines that land, and it takes in conservation areas, areas of outstanding natural beauty, the Broads, National Parks and World Heritage Sites.
The result is plain: a house in a conservation area or in the South Downs National Park has no Class B rights, so a dormer there always needs planning permission. Building one on the usual rules without an application means building without permission. The same holds for every other roof addition under Class B.
What this means around Worthing
Conservation areas are plentiful locally, and they matter more to dormers than to almost any other roof job. Worthing has 27, two of them (Castle Goring and Highdown) within the National Park for planning purposes, and Adur has seven, from Kingston Buci round to Southwick. The old centres of Steyning, Henfield and Storrington have appraisals of their own. If your house is inside one, or inside the Park, you apply.
Where the Park applies to your house, the application goes to the South Downs National Park Authority rather than the district council. Outside the Park, Adur and Worthing share one planning team at Worthing Town Hall, though they remain two planning authorities.
A listed building needs listed building consent for works touching its character, on top of planning permission. Standing outside a conservation area is no guarantee of freedom either: an Article 4 direction can remove rights across a defined area, and a condition attached to an earlier permission can remove them from a single house.
Design: materials, side windows and what looks right
A well-designed dormer looks as if it was always part of the roof. A badly designed one looks like a box dropped on a slope. The rules give a starting point, and good practice carries on from there.
The materials condition applies to the dormer's walls as well as its roof. Where the main roof is clay plain tile, the dormer roof and its cheeks, the two side walls, usually follow in tile or tile hanging. Mismatched grey cladding beside orange tiles is just what a planning officer notices.
Side windows
Under B.2, every side window in the enlargement has to be fitted with obscure glass; it must also be fixed shut unless the opening part is higher than 1.7 metres from the floor inside. It is a privacy rule, protecting the neighbour's garden from being overlooked.
Beyond the legal tests come design choices. A pitched or hipped dormer roof usually suits an older house better than a flat one. A flat-roofed box is easier to build and gives more headroom, but needs a good waterproof covering, as we explain below. Setting it back from the eaves and keeping its face no wider than the windows beneath help the house stay balanced. Slim window frames, a proportion of glass to wall that echoes the house, and a roof pitch that matches the main slope all make the finished dormer feel original. Inside, think about where the stair will come up, since a dormer placed over the stairwell gives headroom exactly where it is needed.
Light, headroom and regulations
A dormer is also a piece of building work, and the Building Regulations apply. Structural alterations bring in building control. On a two-storey house, a loft room needs a protected stair under the 4.5 m means-of-escape rule in Approved Document B, and new roof elements must meet the insulation limits in Approved Document L. Where the work is notifiable, the notification is ours to handle.
Weathering a dormer: lead, tiles and flat roofs
A dormer packs a lot of edges into a small space, and edges are where water gets in.
Start with the cheeks. They are usually tile-hung or clad in lead, and they meet the main roof at an abutment sealed by a flashing, a lead strip shaped over the joint and tucked into the wall. Leadworx names code 4 as the usual weight for general flashing work, with code 3 kept for soakers, the small pieces stepped under each course. MyJobQuote puts lead flashing at over 80 years, which is why it stays the standard material when dressed and fixed properly.
The roof of the dormer
Pitched dormer roofs are tiled like the main roof and need a valley or a soaker detail where they meet the slope. Flat dormer roofs are covered in EPDM, GRP, lead or felt. GreenSpec puts the service life of EPDM at 20 to 30 years, and Less puts GRP at 25 to 40 years. A flat dormer roof must also drain: the 2018 edition of BS 6229 (since revised), quoted by IKO, gives a design fall of 1:40 to achieve a finished minimum of 1:80, which leaves room for building tolerances.
The valleys and the tiles
Where a pitched dormer roof meets the main slope, the valley takes water from both sides, and a blocked or split lining will leak into the room below. Our guide to preparing a tiled roof for winter covers the autumn checks that include valleys and flashings.
A leaking dormer often stains the ceiling below the window while the entry point sits higher up. The cause is traced before any price is given. Ventilation matters in a converted loft too, and roof tile vents can supply the airflow.
Cost, consent and how we survey a dormer
A new dormer is priced on survey, because cost swings so widely with size, roof shape, structure and finish. A small pitched dormer on a bungalow is a very different job from a full-width box on a semi. The cost drivers are the structure (whether the roof needs steel or new timbers), access and scaffold, the covering, the windows and the amount of lead, and a written quote follows the visit.
Repairs are simpler. A lifted flashing or a blocked valley is a small job; a rotten cheek or a failed flat roof is bigger. Our guide to pitched roof cost shows typical national ranges, and we steer towards the smallest repair that settles the fault.
Getting consent right
Before any new dormer, three steps help. Check whether the house is in a conservation area, the National Park or a listed building, using the council's maps. Ask the planning team whether an Article 4 direction applies to the street. Then, if you believe it is permitted development, ask the planning team to confirm it in writing before work starts. Planning and Building Regulations run on separate tracks, so a green light from one says nothing about the other.
At the survey we look at the roof structure, the existing flashings, the cheeks, the flat covering and how the dormer reads from the street. You are given photographs of the fault with the quote.
If you are weighing up a new dormer against alternatives, a roof window is often simpler, and Class C roof windows can remain permitted development in a conservation area or the National Park, with conditions, even where a dormer cannot. Our skylight planning guide sets out those rules.
