(1) An allowance or charge to which a person is entitled or liable under this Part is to be given effect in calculating the profits of that person's trade, by treating—

(a) the allowance as an expense of the trade, and

(b) the charge as a receipt of the trade.

(2) In the case of a person who—

(a) is entitled to an allowance or liable to a charge in respect of a qualifying building, and

(b) occupies that building in the course of a profession or vocation,

the references in subsection (1) to a trade are to be read as references to the profession or vocation.

(3) Subsection (1) is subject to the following provisions of this Chapter.