Since I have been on a quest to update some of my cages, I thought I'd share what I learned in the process about housing rabbits over the years. First of all, let's talk about general housing. I know some folks have a lot of success with raising rabbits on the ground in a colony or warren set-up and I think it is a neat idea.. but I worry about internal and external parasites. Mites, worms, etc. live in the soil and come from droppings that other animals leave. This can result in rabbits that are weak, thin, in poor condition, and can even die if they have a significant infestation. Another important issue with parasites in livestock is that infected stock can be a potential vector to humans as well. Birds of prey, cats, dogs, snakes, fox, raccoons, rats can also kill your rabbits and/or their babies depending on how protected they are. Children may try to get into the enclosure to play with them and they may end up free if another human is so inclined to let them out. I also imagine they would be hard to catch at times, particularly when holed up.
I personally use wire cages. I've also used solid-bottom cages in the past with litter boxes. I don't like the litter box/solid bottom at all- some rabbits are clean and keep everything spotless, others pee and poop in their litter box, then dig it out all over the cage and take a nap in it, even when they have other clean boxes, toys, etc. Some are in-between, but I don't like that rabbits with this setup will have to step in their own waste unless you are planning to change the litter box contents several times a day. No thanks!
So, wire. Isn't it cruel to raise rabbits on metal? Well, it is for the most part self-cleaning (droppings and urine fall right through), though you still have to sanitize it from time to time and keep it free from fur that likes to stick during a molt. It also has great ventilation and rabbits have thick fur on the bottom of their feet that are perfectly suited to this kind of housing. A lot of people think rabbits develop sore hocks because of wire, but the truth is that some rabbits are lacking the thick fur (a fault) and get sore hocks even on solid flooring. Most folks also provide resting boards (myself included), but much of the time my rabbits still prefer the wire for most of their activities. Some of them are perpetually putting the resting mats up against the wall of their cage, out of the way of their toys, food, etc. :)
I've discovered through my search that there are about a zillion kinds of welded wire out there. The two most commonly used are galvanized before wire and galvanized after wire. Galvanized just means that a zinc or zinc and aluminum coating is applied to the steel to make it last longer and help keep it from rusting. Galvanized before wire is usually cheaper, but it rusts more easily because the heat from the welding damages the galvanized finish. Galvanized after usually feels thicker and stronger and the steel underneath may be of a higher quality as well. Most finishes I've seen just have zinc, but the zinc and aluminum combo is also reported to last longer than a standard galvanized finish. As a side note, you should never cut galvanized metal with power tools as breathing in the zinc fumes is dangerous (look up metal fume fever for more info).
I have both galvanized before and after wire here. I've found welded wire usually dulls in color over time and may even look powdery eventually. Galvanized before wire rusts much quicker in my experience and you should plan to replace it every few years when kept out of the elements and perhaps even more often if kept unprotected. If you order cages from vendors, check to make sure they use galvanized after wire. I was surprised that more expensive cage vendors don't specify which wire they use and unfortunately, these are the stackers I'm having to repair as the floor rusted out where the rabbits urinated most frequently.
As I am not very experienced doing cage repairs, I went with 25' of cheap galvanized before wire for $32 including shipping. That way if I screw up while cutting the wire or whatever, I am not out a significant amount of money. Galvanized after wire can be over $200 for 100' roll, to give you an idea of cost. I am going to use the extra wire after a repair to make a cage myself and see how it works over time. If all goes well with my new cage and my repairs, then next time I will go with the more expensive wire.
Another important factor in wire for rabbit cages is the size of the wire. Although it make sound backwards, a higher number gauge is a smaller/thinner wire. For example, a 14 gauge wire is thicker and stronger than a 19 gauge wire. I like 14 gauge wire for the walls and ceiling, though 16 gauge wire should work fine, too. I also like 14 gauge wire for the floor personally. I've found the 14 gauge lasts longer and helps keep bigger cages from sagging in the middle. I don't like 30" deep cages because it can be hard to clean the back and remove buns from that far back, so I tend to go for wider floorspace (such as 36" long and 24" deep). You can use 1" x 2" wire for the walls and ceiling of the cage (or you can use baby-saver wire with smaller gaps at the bottom if you are worried about kits falling out). I like 1/2" x 1" for the floor, but folks with smaller breeds may go for 1/2" x 1/2" for the floor, too. In my carriers, I noted that the droppings are too large for that size wire at times for the large breeds like English Lops, though it was fine for my Mini Lops and most of my Harlies. The best thing about do-it-yourself projects though is figuring out what works best for you. I hope this information is helpful and best of luck in your cage-building endeavors!