A while back, we took in a friend for half a year. We rented a storage locker to make room for her, and she handled that cost. The contract there did say no firearms, and there was language about flammable goods that could have been interpreted to mean no ammunition. I debated ignoring the language, as the risk of getting caught seemed negligible and the consequences insignificant, but in the end decided i didn't like being 15 minutes away from my babies.
However, as has been noted, the security of that place really was superior to the security of my house. Plus, it was climate controlled! I spent a lot of time, and still do, thinking about making one of those into a "gun room." Obviously I'd keep my SD and frequently-used guns at home, but a walk-in, air-conditioned gun closet would be SO cherry! I'd be able to start growing my collection again, triple or quadruple my stockpile of ammo, and stop stubbing my toe on cases of shotgun shells that almost fit under the bed.
I would be perfectly happy to build a cage inside the unit to up security. Maybe even a cleaning station. Hell, sounds like a good place for a reloading bench!
But, as I mentioned, the one closest to our house has (or had) a no firearms policy. I'm certainly not willing to drive more than 15 minutes for this!