HOW TO DEFEAT THE MAGIC BURGLAR.
Make a 2 story house where the ground floor is empty - just a means of providing support for the 2nd floor.
Furnish everything on the 2nd floor as if it were just a 1-story house by itself.
Be sure to include an external stair going up to your front door on the 2nd floor.
When you're done, demolish the first floor (with the exception of the stairs).
Your sim now has a 1-story above-ground (literally) home. You can even plant a garden under it.
The trick here is to go in build mode at night and rotate the stairs so that they no longer lead to an external wall.
In order to do this, you will need a dummy platform floating off to the side to use as a nighttime anchor for the stairs.
Make it the same way as your floating house, but you'll only need 1 square of floor hovering at 90-degrees from the stair base.
Now, when the burglar comes he/she will wander around under your house, find nothing to steal, and then leave!