2010 ADA Standards for Accessible Design requires all new restrooms to be wheelchair accessible, even if no accessible route is required or provided.
"Advisory 206.2.3 Multi-Story Buildings and Facilities. Spaces and elements located on a level not required to be served by an accessible route must fully comply with this document. " An ADA rep explained at a seminar long ago that they need to be accessible in case an accessible route is provided in the future.
The only exception is in 202.5 and is pretty much limited to State or National Landmarks Register buildings, and not to any old building of local interest.
"Where the State Historic Preservation Officer or Advisory Council on Historic Preservation determines that compliance with the requirements for accessible routes, entrances, or toilet facilities would threaten or destroy the historic significance of the building or facility, the exceptions for alterations to qualified historic buildings or facilities for that element shall be permitted."
to apply.