Dear Sir,
My answers are as follows:
1) Can I make house on all 216 Sq Yd ?? will I be owner of 16 Sq Yd as well?
Ans: No, you cannot purchase under a unregistered document (Notary agreement). Section 17(1)(b) says any purchase of immovable property worth more than Rs.100/- must be evidenced by a Sale Deed
Section 17 in The Registration Act, 1908
17. Documents of which registration is compulsory.—(l) The following documents shall be registered, if the property to which they relate is situate in a district in which, and if they have been executed on or after the date on which, Act No. XVI of 1864, or the Indian Registration Act, 1866, or the Indian Registration Act, 1871, or the Indian Registration Act, 1877, or this Act came or comes into force, namely:—
(a) instruments of gift of immovable property;
(b) other non-testamentary instruments which purport or operate to create, declare, assign, limit or extinguish, whether in present or in future, any right, title or interest, whether vested or contingent, of the value of one hundred rupees and upwards, to or in immovable property;
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2) If in future another part owner ( Person X) changes his mind or any malign intention , Can he challenge this agreement in court to claim his 16 Sq yd land back as it is not registered? It is only on notary agreement.
Ans: Yes, it is illegal and irregular transaction.
3) An stamped agreement in this case is valid for lifetime. ( I mean whether his legal heirs can claim it back in future? )
Ans: Yes, definitely