If you do some research on Google you will find lots of information about how much space you need to provide for each bird and also perching space and height of perches, ventilation etc.
My husband built our henhouse (see picture attached) from bits of scrap wood. No plan was used, we simply built a box type shape with a roof and legs and a ladder for access. There is a small pop hole door which sits inside a much larger door at the front, the larger door comes off completely so makes cleaning very easy. A nest box "extension" with hinged lid was attached to one side of the box. The whole thing was then clad with wood.
If you have foxes about during the day time you need to consider a run as well. Our hen house is surrounded with a picket fence so that the hens don't destroy our garden. The fence was 4ft high but they jumped over (even with wings clipped) so it now has a trellis on the top making it 5ft.
