Ferrets love to worm their way into any little hole (as small as 2 X 2 inches, or smaller for kits and some adults), which can be very bad if the hole in question is under or behind a refrigerator or other appliance (with exposed wires, fans, insulation, and other dangers), into a wall, or outside.
Crawl around on your stomach to look for holes near the floor and under cabinets, especially in the kitchen and laundry area. Even holes inside cabinets (which are particularly common in apartments, where plumbers are often rather sloppy) should be blocked, just in case. Ferrets can open cabinets and drawers, which can be dangerous or just annoying depending on what’s inside them. Also watch out for heaters or furnace ducts. You can block openings with wood or wire mesh; be sure to leave ventilation around appliances. For doorways, try a smooth piece of plywood or Plexiglas slid into slots attached to the sides of the doorway.
Recliners and sofa-beds are very dangerous; many ferrets have gotten crushed in the levers and springs underneath. They’re difficult to ferret-proof, except by putting them in a forbidden room. Even regular couches and beds can be dangerous if the ferret digs or crawls his way into the springs or stuffing. Next, look around the area your ferret will be playing. Remove anything spongy from reach, and put fragile items out of the way. Keep in mind that many ferrets are good climbers and jumpers, and they excel at finding complicated routes to places you never thought they could reach. They can get onto a sofa, into a trash can, onto the third shelf of a set of bookcases, into a bathtub or toilet (from which they might not be able to jump out), and into the opening on the back of a stereo speaker. They can also open cabinets and drawers, unzip backpacks, and climb up drawers from underneath or behind to get onto the desk or kitchen counter.
Apart from obvious dangers such as bottles of household cleaners, which ferrets do sometimes like to drink, be particularly careful with sponges, erasers, shoe insoles, foam earplugs, Silly Putty, foam rubber (even inside a cushion or mattress), Styrofoam, insulation, rubber door stoppers, and anything else spongy or springy. Ferrets love to chew on that kind of thing, and swallowed bits can cause intestinal blockages. For some reason, many ferrets like to eat soap, so you’ll have to keep that away from them. (A little lick won’t hurt your ferret, just give her a bit of diarrhea, but large amounts can be a problem.) Human foods should also be kept out of reach, since even the ones which aren’t dangerous to ferrets aren’t good for them in large quantities.
Be careful about full bathtubs, where your ferret might possibly drown, and consider keeping your toilet lid closed for the same reason. Buckets of water, paint, etc. can also be drowning or poisoning hazards, or might just be tipped over. Toilet paper and paper towel rolls are a problem because ferrets get their heads stuck in them and can choke or suffocate, and if you let your ferret play with plastic bags, you may want to cut off the handles and cut a slit in the bottom. Certain ferrets may also have special ferret-proofing needs; for example, some like to eat paper, cloth, or plastic bags, which can easily cause a life-threatening intestinal blockage. A few ferrets like to chew on electrical cords or plants, and some common plants are quite poisonous. Liberal application of Bitter Apple paste to the cord or plant can help persuade your pet to stop gnawing on it.
Finally, once your home is done, it’s important to keep it safe. Watch your ferret’s toys to make sure they’re not beginning to crack or break apart, and keep in mind that you can be dangerous to your ferret, too. Always double-check your dishwasher, refrigerator, clothes washer and dryer (even top-loading models) before closing them or turning them on, and watch where you sit and walk: that chair, throw rug, or pile of laundry might be hiding a napping ferret.
Many ferrets dig at the carpet, especially near doors that are closed. It’s very difficult to teach them not to do it. You’re better off protecting your carpet by putting down a piece of plastic carpet protector from an office-supply store. Chances are your ferret will get bored with digging when she sees she’s not getting anywhere, though it might take a while for that to happen. A carpet scrap or sample from a carpet store might work, too, although your pet will be able to shred it, so she might not give up as quickly. For out-of-the-way places, wire mesh can be nailed to the floor through the carpet; be sure to protect any sharp corners or points. Also be aware that ferrets like to dig in and possibly chew on houseplants, and some common ones are quite poisonous.
Plants can be protected from digging (but not chewing) by putting large rocks or metal mesh over the tops of their pots. Many ferrets like to rip the cloth on the bottom of a box spring and climb into it, where they can easily get crushed or caught. To prevent that, try putting a fitted sheet on the bottom of the bed, anchored in place with small nails or brads, or attach wire mesh or a thin piece of wood to the underside of the box spring. You may need to drill air holes in the wood so the box spring can still compress. Depending on how your cabinets and drawers are constructed and how determined your ferrets are, you might be able to keep them closed using strong tape, rubber bands around a pair of handles, a nail or wooden dowel through the handles, or a strip of strong Velcro-type tape on the door and frame.
The magnetic latch-and-key system works best for many people; they’re available at many hardware or children’s’ stores. The kind that lock around two handles at once, available from baby stores, have also gotten a good report. If your ferret scratches at the underside of your couch to get through the fabric into the bottom, try taking off the couch’s legs, if it has them. Heavy cloth or plywood stapled or nailed to the bottom can work, too, though ferrets can often rip cloth loose.
Sometimes ferrets try to get into the bottom or arms of the couch by burrowing between the cushions and the back or sides. This is much harder to prevent, but some people have had good luck blocking the area with cloth or wood, stapled, nailed, taped or sewn to the couch.
You can also give in and remove the bottom fabric and lower stuffing from your couch, putting a piece of plywood on the springs and the cushions on that. Then it doesn’t matter as much if your ferrets get into the bottom, as long as they don’t get caught between the cushions and the springs. Many ferret owners find it simpler to give up and get a futon or a "suspended" couch that doesn’t have an inside in the first place.
Attaching eye hooks (screws with a ring shape at the top) to the door and cabinet and putting a nail through them both has worked for some people, and the latches with a pair of rollers on one piece and a mushroom-shaped catch are said to be strong enough for most ferrets. Some kinds of child-proof locks also work very well, though others are too weak or open wide enough to let a ferret through.