Class B: Crown building extensions and alterations

Class B permits the Crown to extend or alter an operational Crown building, within strict size, height, appearance, boundary and parking limits. The finished building must continue to serve Crown purposes or employee facilities.

England only. Welsh permitted development rules are separate.

How large can the extension be?

The extended or altered building must not exceed the original height. On article 2(3) land, the original cubic content may increase by no more than 10% and floor space by no more than 500 square metres. Elsewhere, the limits are 25% and 1,000 square metres.

The right also excludes a material change to the original building’s external appearance, development within 5 metres of its curtilage boundary, and a reduction in vehicle parking or turning space. Related buildings within one curtilage may have to be measured together under B.2.

Official source and further checks

The authoritative wording is Schedule 2 Part 19 of the GPDO 2015. Apply the general provisions in Article 3 as well as the class-specific rules. These pages describe England; Wales has its own GPDO.

Related: Crown land enforcement and the separate Crown development application route.

Wording of Part 19 Class B

The text below is reproduced for convenience. The current version linked above remains the source for a proposal.

Class B – extension or alteration of an operational Crown building

Permitted development

B. The extension or alteration by or on behalf of the Crown of an operational Crown building.

Development not permitted

B.1 Development is not permitted by Class B if—

(a) the building as extended or altered is to be used for purposes other than those of—
(i) the Crown; or
(ii) the provision of employee facilities;

(b) the height of the building as extended or altered would exceed the height of the original building;

(c) the cubic content of the original building would be exceeded by more than—
(i) 10%, in respect of development on any article 2(3) land; or
(ii) 25%, in any other case;

(d) the floor space of the original building would be exceeded by more than—
(i) 500 square metres in respect of development on any article 2(3) land; or
(ii) 1,000 square metres in any other case;

(e) the external appearance of the original building would be materially affected;

(f) any part of the building as extended or altered would be within 5 metres of any boundary of the curtilage of the original building; or

(g) the development would lead to a reduction in the space available for the parking or turning of vehicles.

Interpretation of Class B

B.2 For the purposes of Class B—

(a) the erection of any additional building within the curtilage of another building (whether by virtue of Class B or otherwise) and used in connection with it is to be treated as the extension of that building, and the additional building is not to be treated as an original building;

(b) where 2 or more original buildings are within the same curtilage and are used for the same operational purposes, they are to be treated as a single original building in making any measurement; and

(c) “employee facilities” means social, care or recreational facilities provided for employees or servants of the Crown, including crèche facilities provided for the children of such employees or servants.

Explore GPDO Part 19

Page first published: 16th August 2023. Page updated: 24th September 2026.