Before you even begin, check with your local planning department to make sure that your property can be used as an income property by adding a basement apartment. Many areas do not allow apartments in certain zoning areas, and you could end up in a mess if you build one and rent it out and then have a problem with the "renter." The courts won't back you up unless it is a legal rental.
Once you've checked your zoning and permits needed, the planning department can also recommend architects to draw up the plans you need.