Flooring - what do you plan to use? My advice is to make sure that it is able to be hosed out. Are you going to have drainage? Concrete can be alright as flooring, but maybe look around into rubbers.
I planned to lay a cement floor and put rubber floor mats in the stalls.
Are you going to have electricity and water? If so are you going to put lights in the stables or outside? Are you going to have a self filling water system, a tap somewhere outside the stable, or perhaps a tap near the roof to hose fill buckets while in stable?
We already have a water pump nearby (only a few feet away, actually), so that's not a problem, and there is electricity running to the garage, so running wiring out there to put in flood lights in the walkway/etc. isn't a huge deal either (that, and my mother is an electrician, so we wouldn't need to pay for one.)
Air flow, are you going to be putting windows at the back of the stable? If it is attached to a walkway and garage there may be no air flow going into the stables. At the same time you don't want it breezy. Remember to think about your locations temperature when selecting materials. Is it hotter or colder often, etc.
Maybe I didn't explain it well... the idea I have is basically a lean-to, except with stalls and a tack room set in the back. The stalls would get plenty of ventilation from the breezeway, which the stall fronts are open to.
Also, you don't want a feed room? Or are you using your tack room for that as well. Also when building it think about little things, like installing rings inside the stable to tie stuff to, rug racks. One place I kept my horse had lovely stables but when you closed yourself inside it was really difficult to get out again as the door was slightly to high and the latch a little too low. Check stuff like that on your doors. Also think if you want built in feed racks/hay racks.
Actually, we only keep hay on the property in winter, and then it is kept in a modified horse trailer. The only other feed we have is salt/mineral blocks and grain, which is bought as needed and doesn't take up much room, so that'd just stay in the tack room.
As for tying... we already have tie-rings on the backside of the garage where the shed is going. And for hardware in the tack room, we already have a makeshift tackroom outfitted with the basic hardware, so it's just a matter of moving things from one to the other.