I used to put all sims and horses together in one massive family. Well, that wasn't such a good idea of mine as it crashed my game and corrupted my save file.
From that point I decided to keep al my horses and sims saved in the sim bin and back them up frequently. For my stables I have a save file with the fully finished center and when I want to do something in game, I just drop a family in and take the horses/sims out of the bin which I need. For this I do make use of a horse friendly world but I might soon switch to a personal one.
So, I don't really play with my sims and horses the way it's meant to be. My facilities are basically just one big photo studio spread over different lots. As I have two stables, both of the facilities are on seperate save files.
As you can see in the pictures bellow (this is Iron Crest, my main stable) I spread my facilities over 4 lots. The main stable being located on the first lot with a secondary barn with small stallion section on the lot to the right of the main barn. The lot to the left has the training grounds with dressage arena and show jumping course and the lot on the back is my indoor arena.
My other stable has kind of the same idea. Though this isn't focused on the big three disciplines (Dressage, jumping and eventing) but more on Endurance. Again, these facilities have the main barn near the entrance, with a secondary stallion barn in the back together with and all purpose outdoor arena and lunging circle. To the right of the main barn we have an XC course and upper left has the indoor arena.
I share these pictures because there are many different ways to make your EC and I think everyone does differently/has their preffered style.
Sorry I haven't been working on these, uni is getting all my attention right now and just got arround to make my Secret Santa gifts but I haven't forgotten these! I will work on them very very soon and you all will have them before Xmas!