A pre-ordinance (built before July 8, 1957) residential building or building of mixed residential occupancy, not complying with the requirements in force and applicable to the building at the time of its conversion, may be altered so as to legalize one dwelling unit, in addition to the number of dwelling units originally authorized, providing that said unit was determined by the office of the zoning administrator or the zoning board of appeals to have existed prior to July 8, 1957, and provided such building complies with the other provisions of this chapter.

This is generally minor alterations. 

A: The ADA does not require covered entities to modify policies, practices, or procedures if it would “fundamentally alter” the nature of the goods, services, programs, or activities provided to the public.

@Matthew Keeney , it's nice you're concerned about their well-being but you're probably not going to be able to alter the path. 

Would you alter the underwriting to exclude these homes when determining the value?

Commercial facilities, such as office buildings, factories, warehouses, or other facilities that do not provide goods or services directly to the public are only subject to the ADA's requirements for new construction and alterations.

NOw of course they could alter or fudge but you hope people are being honest and that's why you want to use brokers in this scenario they have ficuciary and license to protect.

If you're not seeing smoking deals for similar properties, maybe they aren't all that common in your area and you need to alter your hopes/expectations.Good luck,

They may have some creative solutions to meet alter the stair configuration. 

Tenant shall not paint, paper or otherwise redecorate or make alterations to the Property without the prior written consent of the Landlord.