I'm in a fairly new building and the design of it drives me mad - why did no one think things through!!
Storage - every class needs a built-in cupboard full of shelves, big shelves that take topic boxes, books, equipment, that you can shut the door on. You also need additional storage rooms/big cupboards for shared equipment - science, art supplies, book supplies rather than it being piled in corners/corridors etc.
Cloakroom spaces and toilets that adequately fit the amount of children using them, with pegs/hooks designed to have space for coats and PE bags without everything ending up squashed and in a muddle on the floor.
Not much then :)
Work spaces - we need spaces away from the main classroom where interventions and small group work can take place - not a table squeezed into the corridor. Each classroom should have a smaller adjoining, but separate space big enough for 6-8 children and an adult to work in a focused, undisturbed way.
A resources room with space for a photocopier, scanner, paper storage, and additional computers/work spaces for adults doing PPA or preparing resources.
Outside shade and cover - if classrooms are shaped with alcoves into the playground, then give them a roof so children have outside shelter from rain and sunshine.
Some green outside space for growing things - raised beds etc