4E+WThe right to buy does not arise unless the landlord owns the freehold or has an interest sufficient to grant a lease in pursuance of this Part for—
(a)where the dwelling-house is a house, a term exceeding 21 years, or
(b)where the dwelling-house is a flat, a term of not less than 50 years,
commencing, in either case, with the date on which the tenant’s notice claiming to exercise the right to buy is served.