A few questions. First do you have an alternative entry? Chances are you need to have access to the basement for furnace maintenance, plumbing or just inspections. Have you considered building a deck on the back with a hatch access to the lower level? I did this on a house and it worked out great. Basically it was just a 4 x 6 section of deck that could be lifted up to allow access to the basement. It integrated in to a larger deck and allowed an nice spot to sit and enjoy the backyard.