Years ago I added onto my garage; we'd originally had a 6' x the length of the garage added to the side of the garage; with an in wall heater/ac unit; after my husband passed away I had that ac unit removed and a wide doorway put in and a 14x14 foot room built in behind it; first off too many windows so very little wall space; the drawback was i had to go through the garage to get to the room; later I had the deck removed from the back and a 15 x 30 foot room added that adjoined the previously added room. I've since migrated into the bigger room; the original room - too small. I wish now i'd made it 25 x 25; so per others suggestions, yes plan, plan, plan - a lot. If you put in a built-in sewing cabinet/table; have the outlets just above the table tops; easy to get too; all mine are about 36-40 inches above the floor. I have carpet in mine; I think now I'd opt for some sort of wood laminate. But always as my mom said, too soon old, too late smart. I do wish I'd planned better. Good luck and how fun for you!