Let me put on my Exercise Physiologist (with an emphasis on gerontology) hat and offer another point of view. If you're knees are already bothering you, then by all means put your sewing room downstairs. However if they're not, unless you do A LOT of exercise already, going up and down those stairs is good for you. It also gets you up from the machine occasionally. If you're like me you know you should take more breaks than you do.
I have my sewing room upstairs. I mentally complain sometimes about going downstairs but then I think about the seniors I work with in a continuing care community and remind myself how important it is to be able to do steps at 80 if I want to live independently then.