Where a new dwelling is being or has been constructed (whether or not as part of a larger building) the earlier of the following days is a valuation date in the case of a single-dwelling interest in that dwelling—
the completion day;
the day on which the dwelling is first occupied.
The reference in subsection (1) to the construction of a new dwelling—
includes the production of a new dwelling by the alteration (whether structural or otherwise) of an existing building, but
does not include a case to which section 125 (dwellings produced from other dwellings) or section 128 (demolition and replacement: new dwellings) applies.
The reference in subsection (1) to the "completion day" is to the day on which the new dwelling is treated as having come into existence for the purposes of—
Part 1 of the Local Government Finance Act 1992 (council tax: England and Wales) (see section 17 of that Act), or
Part 2 of that Act (council tax: Scotland) (see section 83 of that Act), or
the Rates (Northern Ireland) Order 1977 (S.I. 1977/2157 (N.I. 28)) (see Article 25B of that Order).
In this section "building" includes a part of a building.