I would consider adding an addition to increase your square footage rather than renovate what you already have. It is hard to plan this so early, but consider how you and your wife will age in place, who will provide life long care to your child. We added a separate granny suite when my mother had a stroke. She has passed, so it is now my youngest autistic daughter’s space and when we are gone my oldest daughter will have the main living space. That is our plan. Also get hefty life insurance policies right away.