Subject to any provision to the contrary, the indexation allowance is the aggregate of the indexed rise in each item of relevant allowable expenditure; and, in relation to any such item of expenditure, the indexed rise is a sum produced by multiplying the amount of that item by a figure expressed as a decimal and determined, subject to subsections (2) and (3) below, by the formula—
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The references in subsection (1) to an item of allowable expenditure do not include any item of expenditure incurred on or after 1 January 2018.
If, in relation to any item of expenditure—
the indexed rise in that item is nil.
If, in relation to any item of expenditure, the figure determined in accordance with the formula in subsection (1) above would, apart from this subsection, be a figure having more than 3 decimal places, it shall be rounded to the nearest third decimal place.
For the purposes of this section—