I looked at this article and it asks under the "Trivia and Goofs" section how did the door and windows show up on SpongeBob's house in "Truth or Square". Well, after the pineapple smashed Squidward's private garden, Squidward cut out the door with a saw that popped out of nowhere (that's one of those 'things' with cartoons) then POOF! Instant doors and windows.
 His house
His bedroom is on the second floor on mulitpule episodes he is in his bed room in the second floor and also falls down the stairs with his alarm clock on his head. The second floor has a bathroom but the downstairs has the bathroom as well but the second floors has the library as well ( has a telescope in a episode) the third floor is the room on the top of the pine apple.