8In the case of any building or other land which, at a material date, was in the occupation of a person by whom it was used as to part only for a particular purpose, the use for that purpose of any additional pan of the building or land not exceeding one-tenth of the cubic content of the part of the building used for that purpose on the appointed day, or on the day thereafter when the building began to be so used, or, as the case may be, one-tenth of the area of the land so used on that day.