@Greg Scott nailed it.I will also add that since an ESA is an "accommodation for disability" under HUD, the animal's owner is responsible for any modifications to the home in light of that accommodation (think wheelchair ramp). 

I am assuming making it ADA compliant (wheelchair ramps, etc.) would be a prerequisite. 

I have a property with a concrete wheelchair ramp. figured someone would love this. 

Built porch floor, wheelchair ramp for the future maybe.  

Each situation is unique, if they need to make modifications to the property, for example adding wheelchair ramps etc., I look at worst case of what would it cost to roll-back all changes and add that in. 

So the places we all encounter that don't even have a wheelchair ramp at the entrance (at the appropriate incline, etc), even though it would be relatively easy to put in, aren't compliant (envision a storefront set back 10 feet from the sidewalk).

Grandma & Grandpa could have their wheelchairs handy to get down the hall to see the grandkids.

I have a middle aged guy with a disabled son (wheelchair) who wants to move in our unit.

Also we plan to have the ADUs handicap/wheelchair accessible on the first floor.

The tenant while living in the home had one family member who became wheelchair bound during their occupancy.