Since you already talked about moving "someday" and he is on board with you getting a sewing room, I cannot imagine that your idea would be such a shock. Tell him that based on what you talked about you came up with this wonderful idea. It's logical.
Does he like to be included in things? Most men like to be needed and he can be a great partner in the implementation of it all.
Go slow, read the signs, and the forge ahead.