The importance of overdesigning
At the outset of the design process, make your spaces about 10 percent larger than they need to be to meet the assigned program. During the design process, additional spatial requirements will arise—for mechanical rooms, structural columns, storage, circulation space, wall thicknesses, and a hundred other things not anticipated when the building program was created.
The point of overdesigning is not to design a larger building than is necessary but to design one that is ultimately the right size. In the unlikely event the extra space turns out to be unnecessary, you will find it easier to shrink an overlarge building than to create more space where it doesn’t already exist.