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.
Follow this link to see the page:
<A HREF="http://www.flexmls.com/link.html?u53rsaeq0f4,5,1">Click to view listing(s)</a>
This link is valid until 3/5/2011.