What Makes Removal of a Barrier "Readily Achievable"?

Are public accommodations required to retrofit existing buildings by adding elevators? A public accommodation generally would not be required to remove a barrier to physical access posed by a flight of steps, if removal would require extensive ramping or an elevator. The readily achievable standard does not require barrier removal that requires extensive restructuring or burdensome expense. Thus, where it is not readily achievable to do, the ADA would not require a public accommodation to provide access to an area reachable only by a flight of stairs.