(1)The basic rule is that the writing-down allowance for a chargeable period is 4% of the qualifying expenditure attributable to the dwelling-house or (as the case may be) each dwelling-house falling within section 507(1)(d).
(2)The allowance is proportionately increased or reduced if the chargeable period is more or less than a year.
(3)The basic rule does not apply if section 509 applies.