For any tax year the cost of providing living accommodation is given by the formula—
A + 1 - P
where—
A is any expenditure incurred in acquiring the estate or interest in the property held by a person involved in providing the accommodation,
I is any expenditure incurred on improvements to the property which has been incurred before the tax year in question by a person involved in providing the accommodation, and
P is so much of any payment or payments made by the employee to a person involved in providing the accommodation as represents—
reimbursement of A or I, or
consideration for the grant to the employee of a tenancy or sub-tenancy of the property.