You can only ask for proof that it was prescribed by a physician.As a medical device (similar to a wheelchair) you cannot require extra pet deposit or extra pet rent.

Just ensure you are applying these standards equally, in all units.Applied to the Fair Housing Act, reasonable accommodations for handicap accessibility (wheel chair ramp, lowered counters, etc) can require a deposit, ensuring any property changes are reformed to previous state. 

Does R&D, tweaks, always improves.See... take the wheelchair for example.Every researcher out there is trying to improve the wheelchair.

Assistance animals perform many disability-related functions, including but not limited to, guiding individuals who are blind or have low vision, alerting individuals who are deaf or hard of hearing to sounds, providing protection or rescue assistance, pulling a wheelchair, fetching items, alerting persons to impending seizures, or providing emotional support to persons with disabilities who have a disability-related need for such support.

Just like saying no because they need a wheelchair.   

My wife being permanently disabled and now requires a wheelchair from being struck from behind.  

An emotional support animal has to be treated like any service animal, which means you can't discriminate against them any more than you could a wheelchair or crutches.You could say, "I'm sorry but we don't allow pets."

It's a ranch home with 3 bedrooms and 1 bath, no close neighbors, and currently has wheelchair ramps into home.

We have a renter asking for a wheelchair ramp. 

Not only was there a paved walkway through the yard of 1 to the neighboring house there was also a wheelchair ramp that connected the houses together.