Mistakes made in the vision phase cost nothing. Mistakes or changes made in the design phase cost you time. But carry those mistakes through to the building phase and they could cost you more than your budget can even consider.
Think about how you will use each space, the furnishings in the space, the circulation through and around the spaces and what is most important about each space. Next think about future use of that space as families grow such as converting a dining room into a den with french doors and a gas fireplace.