This is a classic "Marginal Buyer" issue.
There is no single answer to this and unfortunately I imagine that your market evidence is not only extremely limited but also would be "all over the place." However, consider that a family without a wheelchair issue will probably find the attributes inconvenient. A 6' person may have to bend down to flick the light switch, and the lower counters located in the kitchen or master bath will be serious negatives to most buyers.
OTOH, considering the up-side (for higher pricing) there is a "Marginal Buyer" theory somewhere in the Appraisal Institute's Lum Library (I think) you might find useful in situations like this. When bidding on a property with special attributes with a limited market - like a H/C home - the wheelchair-buyer will not have to pay anywhere near "cost" since they only have to out-bid the non-wheelchair buyer (unless the seller gets lucky and there are several wheelchair buyers involved... not too likely). So, if there is only one wheelchair-buyer in the market at this time, they will only have to bid slightly higher than the typical buyer that would regard the improvements as functionally obsolescent. The "loss" isn't a matter of waiting for that unique buyer to come along - it's negotiation. Purchasing the home knowing that it probably doesn't have good re-sale prospects would not be prudent for the wheelchair buyer, and so the pressure is on the seller to reduce the price.
I think you'll conclude that the improvements reflect functional obsolescence and your adjustment will reflect the cost to cure the low counters, lights and other "problem" features - plus incentive/profit. You will have to use judgement to discern what would need to be raised depending on "what and where" in your particular case.
