People are often very confused about how much space they need for their chickens. DEFRA regulations defining the marketing terms 'battery', 'barn eggs' and 'free range' are set out here. In my opinion, the 'free range' classification should be taken as an absolute minimum.
As a rule of thumb for barnevelders, which are classed as a heavy breed, I would allow 2 square feet per bird inside a house, with 10 inches of perch space each. In a run attached to the house, I would allow at least 1 square yard per bird. If you are going to keep them in the run all the time, it would be good if you could allow more than that; and in my opinion, the run would need to be floored with something like concrete slabs and have a pretty thick layer of straw in it that would need to be changed regularly.
If you are going to let the chickens out for much of the time, then it's not necessary to have such a large run - although it is convenient if, for example, you are going to be out at work all day and don't want to let them out to free-range whilst you're not there.
For example, we have a 7' by 5' shed that we have converted to a chicken house. It has two perches, that total 10' in length. The perches give enough roof for 12 fully grown birds, and the floor space is about right for 17 - so if we wanted, we could stick another perch in there and have another half dozen. However, I prefer not to, as I'd rather they had extra space to scratch about inside when it's wet and be able to put their food and water in there permanently. Because we only keep half a dozen fully grown birds in there, it also gives room for young ones to go in there for a couple of months as we grow them on.
From the photo you can see that the perches are over a 'droppings board'. This is because chickens pass two thirds of their faecaes at night - you can scrape the droppings board frequently and not need to change their litter so often. We do the droppings boards weekly and the litter every 2 to 4 weeks - it needs changing more often if it's wet.
Outside we have a pen of about 20 square yards that is partially roofed. According to DEFRA, this would technically allow us to keep 20 birds in there and still call the eggs they produce 'free range' - but, in my opinion, this would be very cramped. They only stay in the pen when it's very wet or snowy or we are out all day. Otherwise, they are free to roam around.
In a smaller example, this is a four foot wide rabbit hutch with a slide out floor. A perch has been added and an extra piece of plywood over the wire front, a next box in the corner; and a little ladder for them. Four pekins were happily living in here, with space underneath for their food to stay dry. You can also see an automatic water drinker in the bottom right of the picture. We also have a have a 4' by 3' house with a 4' perch, with no droppings board. This has six fully grown birds living in it. There is no room for their feed and water inside - but they have those under a covered area just outside and the feed is taken in at night. They have a pen of about 9 square yards.
Chicken houses can be purpose built like this Omlet Cube; converted sheds or even converted wardrobes. They can be made out of ply, pallets or timber. John Seymour talks about making a small house out of wire, offcuts of wood and old plastic feed bags. So long as it is predator-proof (we are talking 'fox', here) and keeps the rain and wind out, then it will do the job.
When you are constructing your pen, remember that CHICKENS CAN FLY. Not far and not fast - but if it's a windy day and they get excited, there's a possibility that they will be over a six foot high wire fence. Also, foxes can climb - I've seen one go up and over a fence that high, too. Consider roofing any kind of pen with either wire, or nylon netting.
You should make sure that they have access to shade in hot weather - chickens suffer far more from the heat than from the cold. Their house should be well ventilated - in my opinion, many of the small houses on the market at the moment are not only overpriced but are also badly designed, with very little in the way of ventilation. If you can manage it and their house isn't that large, try to roof part of the run to keep the rain off. When it's warm, our hens like scratching about in the bushes and trees at the top of the garden and when it's cold or wet, they stay in their house, scratching around on the floor.
The other thing that you should try to provide them with is a dust-bath of some kind. Dust-bathing helps keep them free of mites, gets rid of irritating chicken-itches and is generally something that they love to do. Dry soil or sand is ideal. If you have a particularly favourite plant or bed of vegetables, you can be almost certain that they will uproot it and turn the area in to a dust bath. The very dry, warm soil of a greenhouse is also favourite. They can look very peculiar when dust-bathing - almost as if they are having some kind of seizure. And then they get up, shake themselves off and are perfectly fine.