I assume you mean measuring. ANSI/BOMA Z-65.1 is the standard. This is from the BOMA site:
If a private stairway is built between two floors
occupied by one tenant, is that stairway part of
Rentable Area? Is it part of the Usable Area?
Yes. The Standard states specifically that vertical
penetrations built for the private use of a tenant
occupying Office Areas on more than one floor are
counted as Rentable. The stairway would also be part