You will need to check over all appliances for holes and areas that the ferret could get inside of. Ferrets can get up inside of some refrigerators and get hurt on the fan. Ovens have insulation inside that can be ripped out, if it is a gas oven, there will be a pilot light. Dryers have tubing that looks like a ferret tube, ferrets can tear a hole, get inside, and even get outside through the exit vent. Cabinets may have holes inside that will lead inside of the walls, and ferrets can open cabinets.
Couches can be very dangerous to ferrets.
Most couches have a thin fabric lining underneath that ferrets will easily get through, and then up inside of the couch and the springs. To prevent this, you can either heavy duty staple hardware cloth over the bottom, or nail plywood cut to fit. Depending on your type of couch, you can also just take off the legs, so that the ferrets can not get underneath. Many long term ferret owners simply give up and get a futon. Recliners and sofa beds can be deathtraps for ferrets.