I'm a house designer, should I put a wifi router in a closet?

It is not the best place for router,because of the walls.

Ideally it should be situated in the centre of the house on the middle floor if there are 3 storeys or on the first or ground floor for a house with 2 storeys.

A even better solution would be to have the all the rooms (bar the bathroom) pre wired for ethernet with them all coming back to a central location with a nearby socket.

Wi-Fi routers like height -- on top of shelves or mounted high on a wall.  They don't like drawers or closets -- though many people like to hide their routers.