Not every website is required to adhere to the standards set forth by the Americans with Disabilities Act. However it is in the nature of the internet that sites should be accessible to as many people as possible, so even if not legally required to do so it is advisable to either make your site ADA compliant or offer a version that is.
All Federal agencies are required to adhere to the Section 508 Standards. The World Wide Web Consortium (W3C), the governing body for internet standards and protocols, also maintains a set of guidelines called the Web Content Accessibility Guidelines.
Private business are exempt from these regulations. Participation in ADA site compliance is strictly voluntary, but it’s just plain good business and a matter of civil rights. You wouldn’t think to bar entry to your store because someone is blind or in a wheelchair, but you might overlook making your website available to those same people.
It’s not as difficult as one may think. The W3C offers a wealth of resources to help web designers make sure their sites are accessible, including checklists and automatic HTML validators. They also have templates for evaluation reports so that your site’s accessibility is thoroughly and properly documents.
So there’s really no excuse: make sure your web designer can (and does) make your site accessible to people with disabilities.