Where a landlord has at any time, before the commencement of this Act with or without the approval of the tenant or after the commencement of this Act with the written approval of the tenant incurred expenditure for any improvement, addition or structural alteration in the premises, not being expenditure on decoration or tenantable repairs necessary or usual for such premises, and the cost of that improvement, addition or alteration has not been taken into account in determining the rent of the premises, the landlord may lawfully increase the rent per year by an amount not exceeding ten per cent. of such cost.
     (2) Where, after the rent of a premises has been fixed under this Act, or agreed upon, as the case may be, there has been a decrease, diminution or deterioration of accommodation in such premises, the tenant may claim a reduction in the rent.