Wash down all of the walls and ceilings. If you ever cleaned a house with smokers in it the water turns yellow. Walls are dirtier than you think. Clean all dust magnets like ceiling fans. Wash out all cabinets, shelves and window sills – cats crawl everywhere. Next check the furnace filter, if the people were lazy and it failed the evaporator may be matted with a pad of dirt. Lastly pull the appliances and clean that mess up.