Think about how you store or want to store your : fabric, thread, notions, rulers, rotary cutters, books, magazines, patterns, stabilizers, batting, yarns, paint, dyes, etc. Will your fabric be on open shelves or in drawers of some kind? What flooring will you have? I have a cork tile floor and love it. It's smooth, warm in the winter and cool in the summer. Also easy to keep clean - just vac. it. Where will your pressing area be? A large board or regular ironing board? Will you have a small pressing surface next to your sewing area to use while piecing? What kind of lighting will you have? Where will your design wall be and how large? Also will it be portable or stationary? Where will your cutting area be? Make sure it's a good height for you. What color paint on the walls? Make sure to put in enough electrical outlets, you might want some in the middle of the floor, instead of all on the walls. Will you have more than one sewing machine set up, a serger perhaps or a long arm? Don't forget to include some wall space to display some of your creations.
I'd use graph paper to plan out the design of the arrangement for the furniture in your new space. Cut out to scale shapes for the furniture and built-in cupboards/counter space. Move them around until you find one that you like and what will work for your needs. This is when you might figure out where the electrical outlets will go in the floor.