A hen house (or chicken Coop) is probably the biggest investment you will make. Assuming your hens will be able to come out of the chicken house into a run or free range in part of the garden, then their house will only normally be used for laying eggs during the day and sleeping at night. Sometimes hens will use the house in bad weather but most will prefer to shelter outside if they can.
A good guide for the amount of space needed for hens is to allow a meter square of floor space for every 4 large fowl, or 5 medium hens or bantams. Whilst it is possible to squeeze more hens into the coop than this, they will start to squabble more when encroaching on one another’s space, especially at roosting time. There should be 25cm of perch space per hen.
Chicken houses come in various shapes and sizes, suitable for different numbers and breeds.
If you are considering buying a hen house then you might want to read this article on Choosing a Chicken House before making up your mind.