Essentially, you wouldn't deny someone w/ a wheelchair to rent from you, and you wouldn't charge them a fee for said wheelchair.

A ground level (slab) addition, separate bedroom, living room and elder friendly bath.Then share the kitchen and dining room.You could make the doorways and bath ADA compliant for wheelchairs, and ground level because elders sometimes have issues with stairs.Then when you sell, demo the interior walls and make it into a nice patio.Basically build a patio and convert it into a demi-apartment with easily removable walls.Good Luck! 

Look, it's a fact that if you want to scale best, quickly and get too financial freedom before a wheel-chair your all but certainly going to have to figure out how to build in JV's and equity partners at some point, so why not start now, kill 2 birds with 1 purchase. 

A DISQUALIFYING HANDICAP (IE, 'STEPS THROUGHOUT NO WHEELCHAIRS PLEASE')9.

HUD guidance for ESA does not provide such guidance.A service animal or ESA is supposed to be treated the same as a medical device, like a wheelchair or walker.

Do I just use the word Accessible on it's own, or wheelchair accessible (or friendly, if it doesn't meet ADA standards)?

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.