The 2010 ADA Standards states that only stairs that are part of a means of egress must comply with the ADA sections 504 and 505. That means that grand stairs that are typically found in a lobby of an office building, for example, will not have to comply because typically are not part of a means of egress according to the building code.
ADA 210.1 General. Interior and exterior stairs that are part of a means of egress shall comply with 504
What happens when a stair that is part of a means of egress is getting modified or altered?
According to ADA Section 210 Exception #2, in alterations, existing stairs between levels that are already connected by an accessible route (like an elevator or ramp) is not required to comply with 504, except for the handrails. The handrails will have to be brought up to compliance with ADA section 505.
What if you add a new adjoining stair between floors?
A typical scenario that has been occurring in office buildings is that a tenant will take two floors and create a connection within their suite with an adjoining stair.
If the stair did not exist in the space before, and even if the building core has an elevator, an accessible route will be required in the same area. That means that an elevator or wheelchair lift would also would have to be installed in the suite.
It would not be acceptable to make a person in a wheelchair exit the space and find the elevator in the core and then re-enter the space on the upper floor.
The Texas Department of Licensing and regulation wrote a technical memo to explain their position. Click here for the memo