The ultimate question is "What will the matket pay for it?" - not "What will a handicapped person pay for it?"
The "market" doesnt buy anything. Individual buyers do. Some, not insignificant portion of the market, is handicapped and will presumably pay more for a house adapted to their use. Provided the adaptation does not create a dis utility for regular users sufficient to drive down demand for the house there should be a net positive effect. Even for the non handicapped buyer there's still utility. Do you ever have house guests that are handicapped or old and infirm? I do. Have you ever seen a family that has all stick shift cars and somebody breaks their foot or tears something in their knee? That's why I've always tried to keep a automatic in the stable. Paying an extra percent or 2 over the price of a comparable property would seem to be a reasonable insurance against unforeseen disability. Think of it this way.... What's the unrecoverable costs you'd incur if you sold a 2 story and bought a comparable 1 story assuming the houses were the same price? You've got agent commissions, appraisal fees, loan processing fees, moving costs, and a 100 hours of your time at least. You're talking 5-10% at least.
If the immediate market area does not require elevators in SFR's, than you have a superadequacy, which is a subcategory of Functional Obsolescence.
Just because is not required does not make it a functional obsolete super adequacy. There's lots of not required luxury items that we regularly value.
As has been said above, you can not recognize it (adjust), or you can even penalize the property (probably a 'cost to cure')
I would probably do the former.