Is my building "grandfathered in" under the older 1991 ADA Standards for Accessible Design or do I need to comply with the 2010 ADA Standards?

The ADA does not have a provision to "grandfather" a facility but it does have a provision called “safe harbor” in the 2010 ADA regulations for businesses and state and local governments. A "safe harbor" means that you do not have to make modifications to elements in a building that comply with the 1991 ADA Standards, even if the 2010 ADA Standards have different requirements for them. For example the 1991 ADA Standards permitted controls, such as a light switch, to be 54 inches high maximum for a side reach. The 2010 ADA Standards lowered that to 48 inches maximum. If the light switch was installed before March 15, 2012 (the date the 2010 ADA Standards went into effect) it does not need to be lowered to 48 inches. This provision is applied on an element-by-element basis.  However, if you choose to alter elements that were in compliance with the 1991 ADA Standards, the altered elements must comply with the 2010 ADA Standards.

Please note that “safe harbor” does not apply to elements that were NOT addressed in the original 1991 ADA Standards but ARE addressed in the 2010 ADA Standards. These elements include recreation facilities such as swimming pools, play areas, exercise machines, miniature golf facilities, fishing piers, boating facilities, and bowling alleys. Public accommodations must remove architectural barriers to these elements when it is readily achievable to do so. State and local governments must ensure program accessibility at these recreational areas. 

For additional information, take a look at the following resources:

FAQ: Must an employer modify existing facilities to make them accessible?

FAQ: If I am altering or doing an addition to my building, can I still follow the older 1991 ADA Standards for Accessible Design or must I use the new 2010 ADA Standards?

Architectural Accessibility Laws