Where a lease under which a rent is payable has been granted on terms which confer on the lessor a right to terminate it at a specified date or in specified circumstances, it shall be assumed, in determining the market value of the lease pursuant to article 18, that that right will not be exercised by the lessor:

Provided that, if the lease is terminated pursuant to that right, the market value shall be redetermined, as at the date on which the right is exercised by the lessor by reference to the actual term of the lease.

Where a lease under which a rent is payable has been granted on terms which confer on the lessee a right to renew or extend it at a specified date or in specified circumstances, it shall be assumed, in determining the market value of the lease pursuant to article 18, that that right will be exercised by the lessee:

Provided that, if the lease is not renewed or extended pursuant to that right, the market value shall be redetermined, as at the date on which such right lapses or becomes incapable of being exercised, by reference to the actual term of the lease.