1. Make sure that there is a way that it can be a full basement without renters. You want to be able to sell the home and if the construction is done so that it will always be a separate dwelling unit, it will turn of many home buyers.
2. Consider this is a temporary thing. Don't plan on doing it like this forever. After a year or so of doing it like this, you could move into a different home and rent out the top unit and make it a duplex. Now your renters are paying the mortgage and some net cash flow to help you cover your mortgage on your next house.
Just some things to consider.