Where the building to be rebuilt or altered is the original building, the amount of gross floor space in the building as rebuilt or altered which may be used for any purpose shall not exceed by more than ten per cent. the amount of gross floor space which was last used for that purpose in the original building.

Where the building to be rebuilt or altered is not the original building, the amount of gross floor space in the building as rebuilt or altered which may be used for any purpose shall not exceed the amount of gross floor space which was last used for that purpose in the building before the rebuilding or alteration.

In determining under this Schedule the purpose for which floor space was last used in any building, no account shall be taken of any use in respect of which an effective enforcement notice has been or could be served or, in the case of a use which has been discontinued, could have been served immediately before the discontinuance.

For the purposes of this Schedule gross floor space shall be ascertained by external measurement; and where different parts of a building are used for different purposes, floor space common to those purposes shall be apportioned rateably.

In relation to a building erected after the appointed day, being a building resulting from the carrying out of any such works as are described in paragraph 1 of Schedule 8 to this Act, any reference in this Schedule to the original building is a reference to the building in relation to which those works were carried out and not to the building resulting from the carrying out of those works.