What accessibility standards for buildings and facilities must be followed by commercial facilities and places of public accommodation?

Both places of public accommodations and commercial facilities are subject to the Title III requirements for new construction and alterations. This part of Title III is the only part that commercial facilities must follow. 

However, places of public accommodation must follow all parts of the Title III requirements. This includes nondiscriminatory eligibility criteria; reasonable modifications in policies, practices, and procedures; provision of auxiliary aids; and removal of barriers in existing facilities.

See the ADA Standards for more information: https://www.access-board.gov/ada/

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

FAQ: What does the term “readily achievable” mean?

FAQ: Where can I find a complete set of ADA standards for accessible design?