We got a set of plans, but to be honest, haven't used them. Best to have a look round at as many examples as you can, and then built what you want, as big as you want. Ours turned out a tad bigger than expected, but better that than too small.
Loads of information and advice on Poultry Pages. Like Dizzy Lizzie, I got loads of advice and guidance from there.
Things we considered:-
To simplifying cleaning and avoid red mite. Keep interior surfaces as smooth as possible. We used marine ply. Built with as few internal joints as possible (posts on outside). Seal all joints with polyurethan wood glue, so no crevices.
Ventilation - we used corregated bitumen for roof (B&Q), and the corregulated gaps should provide the right sort of air flow at the right level. Though need to fill gaps with with mesh so rats can't get in.
Access - hens need their own door. We went mad and spluged on an automtic pop hole opener (horribly pricey). Need good wide doors for your own access for cleaning - we recycled a couple of cupboard doors. And access to nest boxes for egg collection - bit of ply with cut off of roofing felt.
Add perches, and the whole thing raised off the ground. A) so rats don't nest underneath, and B) so you don't have to bend to clean.
Trying to add a picture of ours (almost finished), but might not work, quite new to this.
