That's a great question Susan. I have three 2-flat conversions currently underway, one that is almost complete at 1660 W Carmen and another two in Old Irving Park and Lakewood-Balmoral respectively. The key is the condition and layout of the existing property. It's common knowledge that you can put as much $$ as you want into it and if you ask a contractor, they will spend your money happily. This is a long discussion so feel free to give me a call to discuss where to start. There are ways you can rule something in or out and at the end of the day, the market (and the bank) has to support the end product value. In other words, the sum of purchase and rehab cannot exceed 80% of the final market value.
2 flats, especially brownstone, greystone, and brick, make excellent homes and having a team including architect, designer and contractors who have experience in these conversions is critical. It also helps to have an experienced agent who can seek and find a viable subject property without guess work and risk.

Susan - I think you need a contractor - size and design will determine the cost estimate..let me know if you what some contractor referrals.