Hi, I've had three hens in a tiny urban backyard since springtime (when we moved them outdoors--we started them as chicks inside the house). They range freely through a tiny roughly 15 by 30 foot backyard that is filled with perennial flower beds, two small raised vegetable beds, a patio, pond and pea gravel path. We recently built them a small 6 by 4 grass patch under a lilac bush in case they wanted some lawn.
So far, they have been destroying the perennial beds--they stomp over everything and eat leaves, and take dustbaths in the soil on sunny days. They also dig plants up to get at bugs. They ignore the pond, and they love foraging under the rose bush and Japanese maple tree. They like to sit on their grassy knoll and nibble a bit on the grass, and rest in the shade, but are not that destructive with the grass, actually. (Although I've read that typically they are.)
My tip--and I am a newbie to this--would be that once you commit to letting them run free-range, you can't really go back to caging them. I wanted them to be in their 5 by 8 indoor coop or 2 1/2 by 10 outdoor run for most of the day and only go free range from about 5pm to 8 pm to minimize the damage they could do to my plants, but once my husband started letting them out at noon, they wouldn't have it any other way. So now they are free range.
They do NOT fly over the fence (although they can fly), but they might if they got frightened by a cat, dog or stranger.
Issues: they leave poo everywhere which is an issue if you like to spend time in your yard (i have to clean daily), this will attract more flies to your area, and finally, you may think you can raise them on the down-low, but they will vocalize at some time and your neighbours WILL find out about them.
Right now I am terrified my obnoxious neighbour next door is about to turn us in to the bylaw police.
These are all things to consider when thinking about chickens.
Also, they live a long time, so when I think back to my post-university days, I don't think it would have been a good idea to get one back then. They require a certain routine and stability that I know I could not provide until I bought a house and had a kid. It's not necessarily a money issue, but being home or hiring a sitter when you go on vacation, not being able to suddenly sleep over at someone else's house b/c the hens need feeding the next morningetc etc.