I think this depends on the brand you have and how it was installed. I've seen some ingenious installations before in a plenum that involved custom work. I've also seen some scary setups that resulted in condensate running back into the furnace instead of out the condensate hose. Some brands use a coil box that screws right to the furnace itself, leaving very little gap between the coil and the furnace. Condensate trays can vary as well based on the shape of the coil.