(1) Where a section 31A deduction is claimed, the following conditions apply.
(2) Where the building is, or the dwelling-house or dwelling-houses contained in the building, to which the claim relates, is or are—
(a) jointly owned or owned in common by the claimant and other persons, or
(b) subject to differing estates or interests, or
(c) separately owned by the claimant and other persons,
a just and reasonable apportionment of the section 31A deduction shall be allowed in respect of that part of the expenditure which is incurred by the claimant or claimants for the purposes of a Schedule A business, and, where the maximum amount is claimed, that amount shall be apportioned in the same way.
(3) If a claimant has received a contribution from any other person towards the expenditure incurred, the amount of that contribution shall be deducted from the amount which the claimant may claim in respect of a section 31A deduction, whether or not that other person is entitled to make a separate claim under that section.