I assume that you did not buy the house yet. Before you decide to invest money in the house get an idea of the cost involved to do this project.
It might be more economical to buy a 4 bedroom house instead .
The problem with modifications is that the house layout can become funny .
If you build in the basement ,you might create safety issues as well as investment that have hardly any value when you sell the house.
Use a real estate agent that understands these issues and can help you finding you the right contractors for your project.