I've done it all kinds of ways. We currently have a room built into the end of our building that is set up for chemical storage. Has explosion proof lighting, sealed floor, curb at the door and ventilation. Our other maintenance shop has a pre-fabricated, dedicated chemical storage building.
My other course down south of here has a pre-fab building I bought for them a few years back. I went through Grainger. This is what I bought them:
https://www.grainger.com/Grainger/DENIOS-Storage-Building-5NTR5?Pid=search. Grainger also has larger models available. I went through Grainger because they offered the best price and free shipping. You will need a large forklift or loader with forks to unload it though.
I've seen smaller courses, including some park facilities, that use the simple yellow safety cabinets for chemicals. You can get these pretty cheap and if you need more room, just buy another one.
Just check with any local laws before you buy.