Subject to the provisions of this section, in this Part, “the relevant interest” means, in relation to any expenditure incurred on the construction of a building or structure, the interest in that building or structure to which the person who incurred the expenditure was entitled when he incurred it.

Where, when he incurs expenditure on the construction of a building or structure, a person is entitled to two or more interests in the building or structure, and one of those interests is an interest which is reversionary on all the others, that interest shall be the relevant interest for the purposes of this Part.

An interest shall not cease to be the relevant interest for the purposes of this Part by reason of the creation of any lease or other interest to which that interest is subject, and where the relevant interest is a leasehold interest and is extinguished by reason of the surrender thereof or on the person entitled thereto acquiring the interest which is reversionary thereon, the interest into which that leasehold interest merges shall thereupon become the relevant interest.

An interest which by virtue of section 11(4) of the 1968 Act was immediately before the commencement of this section treated as the relevant interest shall continue to be so treated for the purposes of the provisions of this Part in so far as they relate to writing-down allowances, balancing allowances and balancing charges.