Subject to the following provisions of this Part, if a person having a major interest in any agricultural land has incurred or incurs, after 31st March 1986 or, if it is expenditure under an existing contract, after 31st March 1987, any capital expenditure on the construction of farmhouses, farm buildings, cottages, fences or other works, there shall be made to him during a writing-down period of 25 years beginning on the first day of the chargeable period related to the incurring of the expenditure, writing-down allowances of an aggregate amount equal to that expenditure.