I use half of 30x48 pole building for my breeder pens. The other half is my shop which is constantly covered with chicken feather dust even though I have a big blower pulling air out of the chicken area. If you can get good ventilation in the separate building use it. Both are plenty big for a 16 birds, huge for when you get down to 8. The big thing is ventilation and possibly cooling. I'm not sure what your max temps are but you want to keep the chickens out of the sun when it is hot and below 100 degrees in their shade. With all your space you could probably section part of it off to store your feed and supplies.
Sand floors are great, I don't think it matters what you have under the sand as long as you keep 3 or 4 inches of sand on top of it. If you sand gets too thin then the smoother the surface the easier it will be to run the manure fork/rake/pitch fork through the sand. I have packed gravel under mine and if I let the sand get too low I (or the chickens) dig up some gravel. I also have some chickens in what used to be a dog run. That has cement floor with sand over it. It is marginally easier to clean (when adjusting for it's short roof. So if you go with sand on the top, I don't think that the current floor really matters. Dry with good ventilation, but not drafty, is more important. Oh and predator proof if you don't have a good dog. I always forget that one because I do have a good dog. The neighbor feeds raccoons and possums 25 to 30 feet from the old dog run pen and I leave it open all night as my dog is good at his job (knock on wood).
Sand floors are great, I don't think it matters what you have under the sand as long as you keep 3 or 4 inches of sand on top of it. If you sand gets too thin then the smoother the surface the easier it will be to run the manure fork/rake/pitch fork through the sand. I have packed gravel under mine and if I let the sand get too low I (or the chickens) dig up some gravel. I also have some chickens in what used to be a dog run. That has cement floor with sand over it. It is marginally easier to clean (when adjusting for it's short roof. So if you go with sand on the top, I don't think that the current floor really matters. Dry with good ventilation, but not drafty, is more important. Oh and predator proof if you don't have a good dog. I always forget that one because I do have a good dog. The neighbor feeds raccoons and possums 25 to 30 feet from the old dog run pen and I leave it open all night as my dog is good at his job (knock on wood).
