Yes it is possible. if the tenant has been in Adverse Possession of the house continuously for 12 years or more without any interference from the Landlord, and has been claiming to be the owner himself and denying the ownership of the landlord, then he may acquire the ownership of the house by prescripttion.
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If there is rent agreement or the tenant is paying rent to landlord then it is never possible that tenant can be owner by prescripttion; thatmeans in any way if landlord proves tenency the adverse possesion terms are not applicable.
Yes, if the owner wants to sell it and the tenant pays the agreed amount and stamps, executes and registers a sale deed to the house. All other options are illegal and subject to criminal and civil actions.
I think there is another possibility i.e. if the owner dies intestate leaving the tenant as the , sole occupier of the property under a valid agreement ,then in that case ,he can enter into the legal process seeking transfer of ownership.
Ownership of a house can only be transferred after paying the value of the property to the owner or his legal beneficiaries. This is not the platform for information of illegal means to usurp a property. Experts are requested not to entertain such queries.