Power of revocation or modification ot permission to develop.

(1) If it appears to the Authority that it is expedient, having regard to development plan prepared or under preparation and to any other material consideration that any permission to develop land granted under this Act or any other law should be revoked or modified. the Authority after giving the person concerned an opportunity of being heard against such revocation or modification, may, by an order, revoke or modify the permission to such extent as appears to it to be necessary: Provided that- (a) where the permission relates to the carrying out of building or other operations, no such order :- (i) shall affect such of the operations as have been previously carried out; (ii) shall be passed after these operations have substantially progressed or have been completed; (b) where perrnisslon relates to a use of land, no such order shall be