As others have said - keep foxes out. Ours have had the complete run of the garden (not any more) but they are locked away at night. Ours are now in a 6x4 shed as I find that so much easier to muck out. A traditional house (usually) involves lots of contorting to get the damn thing clean and my back/knees were suffering badly. I'm an old fart though. Lino floored (then covered in woodchip) shed floor with a snow shovel to clear. Chicken muck makes great compost. Seriously. It's fab.
Watch out for red mites in the spring/summer. They will make a bird very poorly and are quite common - esp if it's warm (look for a faded comb). You may not have this problem if it's a plastic house though. We've found Ficam W to be a super anti-bug solution but it's horrible stuff so protect yourself if you use it. (I mix with water then use a dustpan brush to apply/get in cracks whilst gloved, goggled and overall'ed)
Marmite on toast is a nice treat and it's fun to watch them all rugby-scrumming about to get the one piece. We mix oyster grit in with the regular pellets if the egg shells seem weak/thin and have a bag of corn mixed with sunflower seeds as a daily handful treat. I got my birds hand feeding (or palm pecking!) with sunflower seeds but they're expensive which is why I mix with regular corn.
They'll destroy anything precious. They'll also destroy weeds. they'll scrut soil onto your paths, make dust baths in your tree roots, crap everywhere, jump into hanging baskets and scoff your pansies. We had one that would come in through the cat flap and I had to build a gate for the patio doors to stop them coming in the house. A dedicated area for them is best unless you want an area clear of weeds. They love dandelions!
Dispatching is Not Fun. We use the broom handle method (Google it) as it's 100% effective and very quick. Signs to look out for are lethargy, not running for treats, weight loss, faded comb etc. BackyardChickens.com has been a fab resource for us. For me - it's gotten harder to do over time and unless you're prepared to pay vet fees it's part of keeping them. We have Baytril in the fridge which we always try first as we've found that to be a wonder drug, but if that doesn't work....
I've given our hens a "bus shelter" which is basically an open sided area (4ft square) with a roof that they all get under when it's raining. They prefer that to going in the house. I've also given them a dedicated dust bath area but they ignore it and go for the tree roots and in the bus shelter - as it's always dry. Dust bathing birds are happy birds. Also, they like to sunbathe and they look dead when they do it (laid down, a wing spread out) so don't panic.
We don't wing clip. Now that we've always got permanent residents, any newbies just follow the boss (Hilda - age 9) and go to bed where they should. But they can easily jump 4-6 feet with unclipped wings so be careful putting things like easy-to-get-on-top-of housing near the fence (for example). There can be up to a week or two of quite savage hen pecking when you get new birds all together as they establish who's in charge. It does settle down. My wife has tried everything to prevent it (including segregation) but nothing worked for us. Let nature run its course. We've had some birds join and be settled in a day, others have taken a couple of weeks. No set pattern. Watch out for feather pecking. There are a number of things that can cause it, but my wife got anti-feather pecking spray and we got vitamin supplements for the water and a powder for the food (forget what it was called now) which did the trick.
Watch the poly tunnel. If they see something interesting the other side, they may jump, scratch and tear. Tear becomes a rip, rip becomes a hole, then bye bye seedlings.
Sorry. Massive missive. Got a bit carried away
