The cost for building a wheel chair ramp would be carried by the tenant.

I’ve heard that section 8 houses that are all one level for someone in a wheelchair or an older person are really hard to find and you can get someone that will stay a long time because of that so maybe something to look for if buying for section 8. 

Senior friendly bathrooms and kitchens with enough room to move around a walker or a wheelchair. 

If the applicant or tenant provides the proper documentation, he can keep the animal and you cannot charge a pet fee, an additional security deposit, a pet deposit, pet rent, or anything additional because a documented service animal or emotional support animal is considered a “device” like a cane or a wheelchair.

They'll haul out all the wheelchairs and one-legged war veterans and the crying mothers with babies sobbing about losing some benefit, and the moron media will cover it with relish.

The veteran was wheelchair bound, and was mostly paralyzed in his legs.

While I'm expecting to do walk throughs on draw days, it's not practical for me to wander around construction sites since Old House + Construction Site + Wheelchair = PITA. 

A prospective tenant is in a wheelchair and wants to look at a unit.

In my earlier response, I did not intend to imply there is any reason to eschew stacked duplexes, triplexes - we have a couple and, as @Matt Geerts points out, no stairs on the ground level provides the opportunity to create an accessible unit for those in wheelchairs or other mobility issues {Matt, might consider this ... designing with wider hallways and doorways (36/38") and a more open bathroom layout (perhaps with walk/roll-in shower) is simple when building new or doing a major renovation.} and makes your unit appealing to new categories of tenants (those with disability and aging Boomers are but two).That said, when looking at an older residential building {i.e. without an elevator}, my preference is still for side-by-side units over stacked.

Negative, you may be required to have someone with a ESA/Glasses/a Wheelchair/Seeing Eye Dog/4 day a week nurse etc.