Class B – dormers and hip to gable expansion to a roof
The General Permitted Development Order (GPDO) allows for dormers or loft conversions to be built on houses without the need for any planning permission. This might include
Needless to say there are a few restrictions, which we will cover in this article. Despite not requiring planning (in many cases), you will require building regs. If you are looking to add skylights or Velux windows in addition, Class C allows for this on other parts of roofs. So maybe a dormer to the rear and velux windows to the front.
You can extend the roof space by up to 50 cubic metres on a detached or semi-detached property, or 40 cubic metres on a terraced property. The measurements are external and not the internal space. This would include hip to gable alterations as well as dormers. Further down this page, we have the calculations required for the two examples,
All these permissions are contained within Part 1 of the GPDO. Before starting work, check to see if there any conditions that have been placed on your property. These are quite common on new build properties or maybe as a result of a previous planning approval. In those cases full planning permission might be required relevant to that condition or restriction.
Once you have built your loft conversion enjoy it or you can choose to submit an optional application for a Lawful Development Certificate (LDC) which might be useful when you come to sell the property. Planning Geek can assist you with this – use this form.
What about the internal loft or attic space?
Any changes to the internal space is not normally classed as development, therefore you do not require any form of planning consent. This includes the loft or attic space. You will probably require building regulations.
If in doubt consult with your local planning authority. If altering the loft space of a flat, ensure that you own the roof void and that you also have the freeholders permission. Note that external alterations on a flat would normally require planning permission.
Calculate volume of a rear dormer
You are limited to either 40 cubic metres or 50 cubic metres depending upon the house.
To calculate a rear dormer, simply use the following calculation – it is the area of the triangle x its length. Note that all calculations should be external.
V = D x H/2 x W
V = Volume
D = Depth or Projection of rear dormer
H = Height of the rear dormer
W = Width or rear dormer
So for example:
Depth = 3.5m / Height = 2.6m / Width = 5m
Volume = 3.5m x 2.6m/2 x 5m = 22.75 cubic metres
Calculate a hip to gable roof expansion
This is a bit more complicated. But we have simplified the calculation.
You will need the following measurements.
V = Volume
D = Depth of main roof – from the front to rear walls.
H = Height of Main Roof
R = Ridge to eaves distance in the plans
V = R x H x D / 6
So for example:
Ridge = 3.3m / Height = 2.8m / Depth = 8.1m
V = 3.3 x 2.8 x 8.1 / 6 = 12.474 cubic metres
Where else can’t we benefit?
These permissions do not apply to flats or maisonettes. Nor do they apply to houses that have been converted from agricultural buildings, storage, light industrial, shops or casinos. They do however apply to houses (not flats) converted from offices under Class O. It is not possible to add a dormer under permitted development if the building is listed, in a conservation area, National Landscapes (AONB), Broads or Heritage site.
They might also have been removed as a result of an Article 4 or a condition on a previous planning application. Finally alterations to the roof are to the original building or as it was on 1st July 1948, so this means you can’t add on a dormer to an extension built under permitted development.
You have said where we can’t – so what can we do?
Glad you asked! There are a few limitations, but as long as you keep to the elements below, then enjoy your new loft or roof extension.
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Also see Permitted development rights for householders – Technical Guidance
Legislation for expanding a roof
Class B – additions etc to the roof of a dwellinghouse
Permitted development
B. The enlargement of a dwellinghouse consisting of an addition or alteration to its roof.
Development not permitted
B.1 Development is not permitted by Class B if—
(a) permission to use the dwellinghouse as a dwellinghouse has been granted only by virtue of Class G, M, MA, N, P, PA or Q of Part 3 of this Schedule (changes of use);
(b) any part of the dwellinghouse would, as a result of the works, exceed the height of the highest part of the existing roof;
(c) any part of the dwellinghouse would, as a result of the works, extend beyond the plane of any existing roof slope which forms the principal elevation of the dwellinghouse and fronts a highway;
(d) the cubic content of the resulting roof space would exceed the cubic content of the original roofspace by more than—
(i) 40 cubic metres in the case of a terrace house, or
(ii) 50 cubic metres in any other case;
(e) it would consist of or include—
(i) the construction or provision of a verandah, balcony or raised platform, or
(ii) the installation, alteration or replacement of a chimney, flue or soil and vent pipe;
(f) the dwellinghouse is on article 2(3) land;
(g) the dwellinghouse is built under Part 20 of this Schedule (construction of new dwellinghouses); or
(h) the existing dwellinghouse has been enlarged in reliance on the permission granted by Class AA (enlargement of a dwellinghouse by construction of additional storeys).
Conditions
B.2 Development is permitted by Class B subject to the following conditions—
(a) the materials used in any exterior work must be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse;
(b) the enlargement must be constructed so that—
(i) other than in the case of a hip-to-gable enlargement or an enlargement which joins the original roof to the roof of a rear or side extension—
(aa) the eaves of the original roof are maintained or reinstated; and
(bb) the edge of the enlargement closest to the eaves of the original roof is, so far as practicable, not less than 0.2 metres from the eaves, measured along the roof slope from the outside edge of the eaves; and
(ii) other than in the case of an enlargement which joins the original roof to the roof of a rear or side extension, no part of the enlargement extends beyond the outside face of any external wall of the original dwellinghouse; and
(c) any window inserted on a wall or roof slope forming a side elevation of the dwellinghouse must be—
(i) obscure-glazed, and
(ii) non-opening unless the parts of the window which can be opened are more than 1.7 metres above the floor of the room in which the window is installed.
Interpretation of Class B
B.3 For the purposes of Class B, “resulting roof space” means the roof space as enlarged, taking into account any enlargement to the original roof space, whether permitted by this Class or not.
B.4. For the purposes of paragraph B.2(b)(ii)—
(a) roof tiles, guttering, fascias, barge boards and other minor roof details overhanging the external wall of the original dwellinghouse are not to be considered part of the enlargement; and
(b) “rear or side extension” includes an original part of, or a subsequent extension of, the dwellinghouse that extends from the rear or side of the principal part of the original dwellinghouse.
Page Updated: 12rth November 2024