If this were me...addition, all the way. Maybe a shed for some extra storage to de-clutter the main room would be nice, but I want my main sewing space as part of the house.
I LIVE in my sewing room, and I think I would feel too isolated if it were completely separate from the house. DH's "man cave" is adjacent to my sewing room so we can still see & hear each other when we're in our own happy places. Plus the dogs...I don't let them IN my sewing room but they have beds right outside of it; in those beds they can usually see both me and DH. I think they'd be wanting to be let in and out constantly if it were a separate structure, going back and forth between me and DH.
I think my ideal sewing space would be a large loft space over the main part of the house. Very open so I can still be part of the household goings-on, but up high and with half walls so visitors can't see all my clutter.
