We have five hens living in our back garden, though none of our close neighbours have them. We've had them for about 2 and a half years.
You are meant to register your flock, as fraggle mentioned, with the Dept of Agriculture. Apart from that, I don't think there is any legislation covering it.
Things you should bear in mind, though:
The hens will scratch and eat and roll around in dust, and generally wreck your garden. I don't mean this as a reason not to get them, but if you have particularly prized plants or if you're particular about your lawn, you'll have to make sure that you can confine them to a particular area of the garden which gives them sufficient space for scratching.
Noise can be an issue, especially if you're planning on getting a rooster. The hens tend to cluck a bit after laying an egg (you would too!), but don't make too much noise apart from that. Or maybe I've just got used to it by this stage.
If you don't get a rooster, you will still get eggs. However, the eggs won't be fertilised, so they will never hatch.
Sooner or later, foxes will figure out that you have hens and will start hanging around. This happened to us about 9 months after the hens first arrived, but fortunately, our neighbours took to throwing stuff out of their windows to scare away the foxes. (We repaid them with eggs.) It's worth considering how you're going to protect your hens. If you are at home for most of the day, it should be fine to let them roam around the garden, but if you tend to be out all day, it might be a good idea to put up a fence, which is submerged into the soil. This will stop the foxes tunnelling under hte fence.
There are probably hundreds of other bits of advice you might want, but I'll leave it at that for now.