I like the idea of an addition -- indoors, hopefully no stairs, safe & heated. I think if it's all girly & frilly with nice organization for all your beautiful fabrics, tools & notions, he's not really likely to bother you in that space anyways.
An addition adds square footage to your home, which in turn adds value. Appraisers typically do not add value for She Sheds & those don't appeal to most buyers. Of course, at least in Illinois, it is much cheaper to pull permits for a shed than for an addition, but I think the increase in value is worth it. If you're worried about noise, you can always use that acoustic insulation & then it's truly like you're in your own little world.
One other little note: if for some reason you end up staying over the garage, maybe you could have him install a dumb-waiter to transport your fabrics & such from inside the garage up into your craft room. I have seen a number of people installing those recently & they are really cool. Work great & then your hands are free to climb the stairs. Too bad they can't build you a little covered walkway out to either the garage or your She Shed. If you could get that, I'd go for the She Shed all the way! If you do go for the Shed, make sure to get double-pane windows & seriously consider spray insulation ... and maybe a heated floor (they're not too expensive to add to a slab of concrete & you can put tile or hardwood over it if you like). Radiant floor heating is one of the most efficient ways to heat a room since hot air rises & it really does keep the room warm.
Also, windows are wonderful, but not particularly warm in the wintertime. Obviously you want some windows to make it bright & airy, but you also need plenty of wall space so they can spray in insulation so you don't freeze. (and you probably want to locate it either close to the garage or house so you're not having to carry stuff a long way in bad weather, or having to run out in the rain/snow to use the washroom (oooohhhh! can you add a 2-piece washroom to a She Shed? ooohh! that would be fun! -- don't tell your hubby it was my idea
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