As far as the law goes, it would depend on the jurisdiction as to whether or not there is a law. The ADA has nothing to do with this question unless they are talking about the leasing office because the Americans with Disabilities Act, Title 3, covers places of public accommodation. Every place in your community, the apartment homes, the amenities, the parking lot, the swimming pool, is covered by the Fair Housing Act. The only place that’s an area of public accommodation is your leasing center. Everything else is private property covered by the Fair Housing Act. Your local jurisdiction may have a requirement that animals be on a leash, that animals be licensed, or that animals be vaccinated, or it may have all three. Whether it’s a service animal, a working animal, a companion animal, an emotional support animal, whatever it is, it’s still an animal. It is not unreasonable to expect an animal to be put on a leash, and if your jurisdiction has a leash law, it’s go beyond being reasonable, it’s a requirement. So find out what your local jurisdiction, and absent any leash law, if it were me, I would be require all animals to be on a leash, period.