Do you have a valid and subsisting tenancy agreement with the land lord, if so, does the agreement state seeking permission to make alterations and putting up additional construction with the permission of the land lord, if so the same must be obtained in writing before doing so, then give a notice of your desire to put up a toilet at your own cost within the demised premises, if the landlord refuses, as he has done in your case, then proceed to file a civil case before the competent authority or civil court and seek permission from it.