Your cockatiel will be happiest in the kind of enclosure he is used to. If you're getting a cockatiel from another owner, hopefully his home tweet home comes along with the bird. You can upgrade after your new cockatiel becomes a member of the family. Your young chick is adaptable. He'll be most comfortable in the size cage he grew up in, but he'll learn to like any cage that is of adequate size.
You have some options in cockatiel enclosures. You can build an outside aviary or an inside aviary. Preconstructed versions of both are available. You can build a cockatiel habitat from an alcove or a room in your house. You can buy large, powder-coated, heavy parrot cages for your cockatiel. You can buy cockatiel enclosures that are as handsome as furniture or others that are utilitarian wire. You need to consider several factors when looking for that special abode for your cockatiel.
Cost
There is a practical side to cockatiel housing. Your price range may limit your choices to some extent. But try to think big even with a small budget. A cockatiel enclosure may cost from $55 for an adequate cage that on sale, about $120 for a roomy cage for one to two cockatiels, or over $300 for an aviary.
Comfort
Your cockatiel will go to the top of any cage you get. For that reason, long cages are preferable to tall cages. To your cockatiel, a cage is only as big as its length and width. Your cockatiel will spend most of his time at the top of any cage you provide, so height isn't important. Get a cage that is a minimum of two feet long. Your cockatiel deserves some room for play and exercise.
Bar spacing
Cockatiels know a lot about Murphy's Law and tempt it. Instead of "whatever can go wrong will go wrong," they chime in with "whatever trouble I can get into, I will." And the wrong type of cage can lead to trouble. Cockatiels are curious about everything, and they know the grass is greener, the feed is fresher, and there's more fun on the other side of the bars! Bar spacing on your cage should be close enough together that your cockatiel can't get his head through the bars. Bars that are 1/2 inch apart are great; bars that are 3/4 inch apart will probably be fine.
Check that the cage bars are made of something safe. Aviary wire is utilitarian, but if a bird chews on the wire, he could very well get zinc poisoning. You can do your best to prevent this by washing new cages or new wire with a vinegar solution before it is used.
Cast-iron cages may be welded with lead solder, which is not safe for a cockatiel. They may nibble or chew on the lead and get heavy metal poisoning. You can use a lead test kit (made for the safety of children) to find out if there is a problem with a cage. Most cast iron and large cages have a heavy-duty baked-on finish called powder coating. Occasionally this powder coating has been found to contain zinc. That's a problem only if your cockatiel chews off flakes of the baked-on finish. There aren't home tests for zinc, you would need to contact the manufacturer about the paint used (preferably before buying a cage) or send samples to a lab to ensure the safety of your cockatiel.
Zinc poisoning is also a heavy-metal poisoning. Your veterinarian can treat either lead or zinc poisoning can be treated by your veterinarian, but it's best not to expose your cockatiel to risk. Many cockatiel cages are made safely of cage wire, which becomes unsafe only if it gets rusty and the cockatiel eats flakes of metal. A rusty used cage isn't a good choice of home for your new cockatiel.
Cleaning
Keeping an accommodation clean probably isn't the first thing you think about when choosing a cage. If your new cage is a bear to clean, however, you'll be reminded on a daily basis. Rectangular cages clean much easier than round cages, simply because you can more easily fold newspaper into the bottom of them. A cage with a grate helps separate a cockatiel from droppings and used food, though you'll want to wipe off the grate often. A deep tray under the grate is easier to clean than a skimpy tray. You may or may not want the shields that extend out from some birdcages. Yes, they keep droppings from soiling the floor. But guess where the droppings land? On the shield. Then you need to wipe the shield clean frequently. Another feature that makes cleaning easier is a nice big door, so that you can reach in easily. Make sure that you can reach every corner of an enclosure. Some larger enclosures literally don't allow for cleaning every corner without a high-powered hose.
Access to bowls in a cage
If a relative or bird sitter is afraid of birds (what, your little sweetie?) or doesn't know how to handle them, they'll really appreciate the fact that your cockatiel's food and water bowls are accessible from outside the cage. You really need three or four bowls to serve your cockatiel feed, water, vegetables, and treats. For some reason, most cockatiel cages come equipped two bowls, but you can buy additional treat cups to place inside the cage. It is usually a good idea to have more bowls than are provided with most cages.
Open sesame
Look for a cage with a big front door. Cockatiels are fairly large for small birds, so they don't exit easily out of a smaller door meant for a parakeet. They also learn quickly to latch onto the entryway of a small doorway. Then you assume your cockatiel is coming out on your finger, but he will remain behind where he's grabbed onto the cage. When you need to reach into the cage, you too will appreciate the room you have to maneuver through a large cage door. Some doors convert to perches, as do the tops of some cages. With those features, you don't need to buy extra cage-top perch equipment.
Other openings in a cage are useful. If there's any chance you'd ever breed cockatiels, a pre-made opening high in the cage, where a nest box can be attached, is very useful. The alternative is cutting a hole in a perfectly good cage, a step that seems wasteful and isn't usually pretty.
Just for show
Safety should be your first concern when buying a cage. Curlicues and ornaments on a cage with no purpose can be hazards. A cockatiel could get his band or a wing wedged in a tight place. Similarly, if a round cage has wire bars that narrow as they come together on top that poses a danger. Look at a cage as though you're the embodiment of curiosity, and try to figure out what trouble you could get into if you lived in it.