Raventiger, there are some tools/ cheats that are absolutely imperative when building or remodeling. Check out the cheats section to see what they are and how to use them. The building cheats are usually allowed in contests/challenges, such as "moveobjects on".
This particular cheat allows you to use the alt key to slide that wanted item into a space that the game may say it doesn't fit there!
Another thing you can do to enlarge your house without having to build a lot of exterior walls is go to build mode, select the hand tool, and drag the walls, whether inside or outside of the house to enlarge or shorten the space. You can make a room huge, then add a center wall to cut it into two or even three rooms that way. The nice thing about the dragging is you don't have to go back and decorate the walls and floor that were moved! A wall with counters on it cannot be moved, you have to move the counters away from the wall first and then replace them later. A wall that is not square, has angled walls somewhere, also cannot be moved. If you want to put in a second floor and have the space to drag a lower level exterior wall, that would give you space in which to place the stairs you need for the second floor. Then you merely have to move the view up, add floors and walls and you have your second story. Sometimes when you do this the roof gets all weird. Just use the hammer tool to remove the original roof, go to the roof section and put on a new one. For beginners I recommend using the automatic button. It sometimes gives you some strange looking roofs but it is an easy way to start.