If you are designing it for horses you should slope a concrete floor normaly towards the back by about an inch and then there is a drain a there. Mind you with a half decent bed veryy little ends up in the drain.I have never kept horses in the UK, and I confess, keeping horses on concrete, even with matting on top, seems odd and counterintuitive to me. They only time I've ever seen stalls on concrete is in a converted dairy barn. Just doesn't seem to make sense to me if you're building a horse specific facility from the ground up, but that may be my American bias. Now, if you had permanent drains installed in the middle of each concrete floor, and then put matting on top of that, that would make tons of sense.
I do agree that you could probably mitigate any of the bad effects of the concrete flooring with matting and bedding, but it's not enough to persuade me to start with concrete by choice.
Why is it good to have horses lieing on years and years worth of old urine? Even if it is well drained it wont take away all of it. Doesnt the ammonia smell get to you? I personaly could muck out in my work clothes and then go to work and no one would know (if I managed to keep the hay out of my hair that is!)
My pony is on concrete with mats and hemp bedding but he a lot of horses are just straight on the concrete with a nice deep bed of straw/shavings/hemp/paper.
As I saidn, in the UK I have only once seen a non concrete floor for horses and it was a mess. Even the brand new multimillion ££ Equestrian centre at kingsbarn (£10million spent on he arenas alone) had the horses on concrete with rubber.