You assume you remember it all off the top of your head. But when you put pen to paper and lay it all out in black and white, you notice holes in some places and you remember other things that just might surprise you. I, for instance, remembered upon sketching my plan that my meager little studio actually had an entryway of sorts. Problem was, it wasn't at the front door, but a few steps to the side of it. But it was a perfect place for kicking off your shoes when you stepped inside and hanging up your coat.
Go ahead - try to make a quick sketch of your first apartment's floorplan. Any surprises rise to the surface of your memory?