Rear dormers and the 40 and 50 cubic metre allowances
Class B gives a terrace house 40 cubic metres of roof addition and other houses 50, measured against the original roof. The eaves set back, side glazing and article 2(3) land catch more dormers than the volume figure.
The dormer permitted development volume allowance is 40 cubic metres for a terrace house and 50 cubic metres for a detached or semi-detached house, and it is an allowance for the whole roof over the life of the building rather than for this one dormer. Class B of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 sets it, together with a set of limits that catch more loft conversions than the volume figure does.
Most refusals of a lawful development certificate for a rear dormer turn on one of three things: the house sits on article 2(3) land, a previous roof enlargement has already eaten the allowance, or the dormer sits flush with the eaves.
What Class B actually permits
Class B covers an addition or alteration to the roof of a dwellinghouse. A rear box dormer, a hip to gable enlargement, or both together all sit within it, provided every limit is met.
The limits that bite most often are these. No part of the works may go higher than the highest part of the existing roof, which rules out raising a ridge to gain headroom. Nothing may extend beyond the plane of a roof slope that forms the principal elevation and fronts a highway, which is why front dormers on a road-facing slope almost always need an application. The enlargement must not overhang the outer face of the wall of the original house. And the volume limit applies.
An end of terrace house counts as a terrace house for the volume figure. That single point costs householders 10 cubic metres more often than any other misreading of the Order.
The volume allowance and how it is consumed
The allowance is measured as the increase in the cubic content of the roof space over the original roof space. Original means as the house was built, or as it stood on 1 July 1948 for older properties, not as you bought it.
So a 1930s semi that gained a small side dormer in 1994 starts today with less than 50 cubic metres available. If the earlier work was itself carried out under permitted development, it still counts. If it was carried out with planning permission, it still counts. There is no reset on sale, and no allowance for works the current owner did not carry out. Our note on extending a house that has already been extended works through the same cumulative logic for ground floor additions.
Where the previous work is undocumented, the practical route is to reconstruct it from historic mapping, aerial imagery, the council's planning history and, if necessary, a measured survey of the existing roof against the original form. Building control records sometimes help. Guesswork does not.
Measuring the addition correctly
Cubic content is measured externally, which surprises people who have calculated the useful floor area inside and concluded they are well within the figure.
Take the external envelope of the added roof volume: the dormer cheeks, the flat roof above, the front face, and the void created behind where the original slope has been removed. A hip to gable enlargement adds the whole triangular wedge of new roof, and that wedge is usually larger than it looks on a section. On a typical three bedroom semi, a hip to gable plus a full width rear box dormer can approach or pass 50 cubic metres before anyone has thought about a rooflight.
Two practical habits keep this honest. Have the volume calculated and shown on the drawings, with the arithmetic visible, rather than stated as a conclusion. And model the original roof form on the same drawing, because the comparison is with the original, not with what is there now.
An illustrative example: on a mid-terrace house where a hip to gable enlargement takes 18 cubic metres, a full width rear dormer of 24 cubic metres leaves no room for any further roof addition under Class B, since the two together reach the 40 cubic metre terrace limit. The figures here are illustrative and any real scheme needs its own calculation.
The 200 millimetre set back from the eaves
Other than in the case of a hip to gable enlargement, the edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be at least 20 centimetres from those eaves.
That is 200 millimetres of original roof slope left visible below the dormer. It exists to stop a box dormer reading as a full extra storey, and it is the condition most often missed by builders working from a sketch rather than a drawing. Because the requirement is qualified by the words so far as practicable, there is a narrow argument where a particular eaves construction makes compliance impossible, but that argument is for a certificate application, not for a decision taken on site with the scaffolding up.
Measure it on the slope, not on plan. And check it against the finished tile line rather than the rafter.
Side windows and obscure glazing
Any window inserted in a wall or roof slope forming a side elevation of the enlargement must be obscure-glazed, and non-opening unless the opening part is more than 1.7 metres above the floor of the room in which it sits.
The 1.7 metre measurement is from the internal floor level of that room, which on a loft conversion is often higher than the surrounding roof level, so the practical effect varies between houses. Obscure glazing here means glass that prevents a clear view, not tinted or reflective glass. Where the same effect has been imposed by condition on a permission rather than by the Order, the enforcement position differs, and we cover that in our piece on obscure glazing conditions and how they are enforced.
Rear-facing windows in the dormer face are unaffected by this rule. Side cheeks are the issue.
Materials similar in appearance
The Order requires the materials used in any exterior work to be similar in appearance to those used in the construction of the exterior of the existing house.
Similar in appearance is not identical. Standing seam zinc on a rear box dormer has been accepted in plenty of places where the original roof is slate, because the test is about appearance in context rather than a material match. Fibre cement slate against natural slate is usually fine. A bright profiled metal cheek against a render and clay tile house is not. Councils do look at this when a certificate is applied for, and the drawings should name the products rather than saying to match existing.
Where the house is listed, none of this applies in the way described here: listed building consent is needed for works affecting character, and a Heritage Statement will be expected with the application.
Where the right disappears entirely
Class B does not apply at all on article 2(3) land, which covers conservation areas, National Parks, National Landscapes (formerly AONBs), World Heritage Sites and the Broads. A rear dormer in a conservation area needs planning permission, full stop, and the Planning Statement will need to address the character of the area rather than the volume figures.
The right is also removed where the building is a flat or maisonette rather than a house, where an article 4 direction has withdrawn it, and where a condition on an earlier permission has taken it away. New estates frequently carry such conditions. Check the decision notice for the house before the drawings start.
The only way to put the question beyond argument is a lawful development certificate under section 192 of the Town and Country Planning Act 1990, applied for before the work starts. The fee for a proposed use certificate is half the equivalent planning application fee, and fees are indexed annually, so check the current schedule. It is also the document a buyer's solicitor will ask for in eight years time.
If the dormer turns out to need permission, enter your postcode on PlanScope to see which supporting documents the council is likely to expect.
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.
Frequently asked questions
Is the volume allowance for one dormer or the whole roof?
The whole roof, over the life of the building. The figure measures the increase in cubic content against the original roof space, meaning the house as built or as it stood on 1 July 1948. Earlier roof enlargements count against the allowance whether they were built under permitted development or with planning permission, and selling the house does not reset it.
Does an end of terrace house get 40 or 50 cubic metres?
Forty. The Order treats an end of terrace house as a terrace house for the purposes of the volume limit, so the higher figure applies only to detached and semi-detached properties. This catches householders regularly, because an end of terrace can look and feel like a semi from the street while carrying the lower allowance.
Can I build a rear dormer in a conservation area?
Not under permitted development. Class B is withdrawn on article 2(3) land, which includes conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads, so a planning application is required. Many councils do grant permission for modest rear dormers in conservation areas, but the case has to be made on character and design rather than on volume.
What does the 200 millimetre eaves rule mean in practice?
Except for a hip to gable enlargement, the lowest edge of the dormer must sit at least 20 centimetres up the original roof slope from the eaves, so a strip of the original roof stays visible. Measure it along the slope against the finished tile line. Building the dormer flush with the fascia is one of the most common reasons a certificate is refused.
Do I need a lawful development certificate for a loft conversion?
It is not compulsory, but it is the only formal way to confirm the works are lawful. A certificate under section 192 of the Town and Country Planning Act 1990 is applied for before the work starts, and the fee is half the equivalent planning application fee. Conveyancing solicitors ask for it years later, and reconstructing the evidence then is far harder.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.