First legalize the constructions by taking sale deeds or MOU or Joint Agreement Deed and Sharing Agreement with the builder. In the sharing agreement show your entitlement to each such particular house. Then get transfer the same in your names and then sell the property then only will fetch full market value otherwise prospective purchasers never believe you. The other answers are as follows:
1.Now what should I do to mutate my portion that is one flat??
Ans: Please follow above method or enter into partition deed between you and your uncle.
2.Do I have to make partition deed for that??
Ans: Yes, you can.
3.If so, what is the cost for partition deed??
Ans. Plesse see below
4.Can I sell my flat without partition deed??
Ans: Yes, provided you get purchaser and he is ready purchase on par with market value.
Stamp Duty for partition deed
0.5% of the market value of the separated share or shares of the property
Deed Of Partition Between Members Of A Joint Hindu Family
Get it online free on cost