Town and Country Planning Act 1990

7 In the case of any building or other land which at a material date was in the occupation of a person by whom part only of it was used for a particular purpose, the use for that purpose of any additional part of the building or land not exceeding—

(a) one-tenth of the cubic content of the part of the building used for that purpose on 1st July 1948 or, if later, the date when the building began to be so used, or

(b) one-tenth of the area of the land so used on that date,

as the case may be.