I vote to plan for the future. If you plan to grow old in your present residence, an addition makes the most sense. How long are you going to feel safe walking out to a she-shed? Add to that carrying things and especially in winter or wet weather. I have had knee problems and know that walking outside and climbing steps would render outside space totally unusable during flare ups and healing. Knee problems with replacements are not uncommon. Indoor space would be usable even with a wheelchair. Too often we fail to plan for future needs and circumstances. As a negotiating chip with hubby, I would certainly use the stair falls and almost falls.