The Americans with Disabilities Act states that no business shall deny someone services if they have a service animal with them. Therapy dogs are under another guideline altogether and don't have the same protections. There is a difference between the two. For service animals, you should know you are only allowed to ask 2 questions to service animal owners: 1) is this animal YOUR service animal and 2) what service does your animal provide. After that, you are expected to provide them with the same service as you would anyone else. With that said, your guest with the service animal should definitely be letting the host know of the service animal before hand, and, if your specific place cannot accommodate a service animal, they would have the opportunity to find another airbnb host that does, therefore airbnb hasn't violated any laws. If they just "show up" and you cannot accommodate, I would notify airbnb immediately and it would be their responsibility to re-place them at a location that can accommodate. If you decide to allow the service animal, and they didn't let you know before hand, I would definitely put that in the review. They should be letting people know. Just like people with service animals do when they fly, etc.