Paragraph 4
The right to acquire 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 acquire is served.