Dear chitra,
Welcome to this forum.
All the best for your house hunting. you did not mention whether you are looking at first/second hand. both are available at present.
Define a good flat

??
For a budget of 20/25 L, your down payment will be 15% and the loan amount will be 85%. If you are resident indian, you have very competitive rates in housing loans right now..with some of the public sector banks givng at 8.5 to 9 % which may go up, if the government decides to boost the infrastructure and the other companies related.
We have discussed a lot of projects, areas, and also requirements when buying property. do check them at leisure and post your questions there. will be moving this thread in a day or two to the main thread on housing.
Buying property ??? Current Sq Ft rates at chennai