I'm assuming that you live in Mumbai in a housing system wherein you have not only tenancy rights, but also sale rights, right? For corroborations' sakes! However one thing must be clear, that tenancy rights under this system are not absolute. They are just like any rent rights.
Now coming to your query. No one can take away your right to have a toilet constructed within your 'pagdi' premises, as it is a basic necessity & a fundamental right. Please remember that under Clean India Green India (Swacch Bharat Abhiyan) scheme, you have a statutory right to construction of a toilet.
Send a legal notice from your lawyer to the landlord, intimating him about your intention to construct a toilet in your part of 'pagdi' land. Whether 'pagdi' system or otherwise, all of you, including your family's female members, have a constitutional right to access a toilet u/Art.21. Otherwise they stand many unspeakable risks, which I'm sure you can understand.
File an application with the Water and Sanitation Department, Maharashtra Government for construction of a private toilet in your house. Approach your local MLA for help. Or consult a property lawyer of the Maharashtra State Legal Services Authority for further help on your matter.
If I find out something that will be in your favour vis-a-vis your query, I will definitely revert back to you.