AM assuming those plans would include a layout of the house etc. Even with that "not very likely". OTOH if they were gone for quite a while he would have plenty of time to explore the house. Easy to tell a little girl's room with the decor.
in the dark?
the video of that basement - its a maze, the wine cellar, this room where the murder took place.
the only possible way Karr could have done this - if he had entered the house while the family was at the party, and was inside alone for house, and had a chance to figure out where everything was, find the paper, the location of the rooms, etc. then he just waited in the basement until everyone went to sleep, and made his move.