Hi, hens are excellent pets and the eggs are fab
I have an Eglu which is perfect for a small garden, they are very good as they can be hosed out so are easy to keep clean! chickens produce an incomprehensible amount of poo.
Eglus are fairly expensive but I think they are worth it! I have four chickens in an Eglu with a 3m run to give you an idea of the space required
If converting or building a coop the hens will need somewhere dark to lay their eggs, a perch or slatted platform for roosting and something that has ventilation but is not draughty.