Class B – additions etc to the roof of a dwellinghouse

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What does the legislation say?

Permitted development

B.1 Development not permitted

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 M, 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 roof space 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) or the dwellinghouse is on article 2(3) land.; or

(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).


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—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

(c) 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

(d) 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.

Our Views on Class B

Along with Class A, Class B Development is the cornerstone of householder Permitted Development providing the basis for many extensions and other alterations to houses in England.

Class B is not available to buildings that are within Conservation Areas but is available to smaller HMO’s so long as they remain a single dwelling house. This allowance is not available to Listed Buildings.

When combined with Class A it is Class B that must be carried out first as the cubic additions can include any works carried out under a planning permission or any form of extension to the property that has a roof. This only works in one direction however effecting Class B only and not development in Class A. It is often best practice however to seek a certificate of lawful development in order to ensure that your proposals comply in full. This is especially important where Council’s have sought to resist development to houses such as hip-to-gable build up and larger rear dormer windows.

Class B is immune from the imposition of the National Floor-space Standards as the development does not result in a new dwelling.

The ‘existing dwelling’ provision is set in law as the day the property was constructed or the 1st July 1948 (whichever the sooner).

How to Use Class B

Class B permitted development should be explored first before engaging with a planning permission and as with other permitted development allowances it is best practice to seek a lawful development certificate.

*Correct at time of writing.

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