I was recently able to move my sewing room to our finished basement which has lots of light and lots of room and I do love it but after a while I can see that it has been an adjustment being away from our main living area. Since I started sewing about 10 years ago I made do with a smallish bedroom/guest room which included a queen size bed. Then gave that room to DGS and moved to smaller bedroom/guest room with a twin size bed. My cutting table and ironing board were in the living room. Used the beds in the guest rooms to lay out blocks and stack "stuff". When our guests came, only a few times a year everything got put away. Worked just fine. After everyone moved out and I expanded to the basement, I realized that I was used to being able to cook, etc., easily in between sewing, pressing, cutting which was nice. I wouldn't change back, it is just different now. So, my point is you might want to think about how you like to sew and if you want to be able to integrate other things as you sew. As tempting as it sounds, I don't think a separate building would work for me. If you can manage it in a small space, I would go for either option inside your house.