The ADA defines a covered disability as a physical or mental impairment that substantially limits one or more major life activities, a history of having such an impairment, or being regarded as having such an impairment. The Equal Employment Opportunity Commission (EEOC) was charged with interpreting the 1990 law with regard to discrimination in employment. The EEOC developed regulations limiting an individual's impairment to one that "severely or significantly restricts" a major life activity. The ADAAA directed the EEOC to amend its regulations and replace "severely or significantly" with "substantially limits", a more lenient standard.
UPDATE: Since writing this post in August 2017, several important changes have taken place in the laws regarding ADA compliance for websites. On December 26, 2017, the Department of Justice announced that they have withdrawn the Obama-era Advance Notice of Proposed Rulemaking mentioned in this article which intended to require ADA website compliance. The DOJ’s withdrawal announcement stated, “The Department will continue to assess whether specific technical standards are necessary and appropriate to assist covered entities with complying with the ADA.”
This is the reason why I think web design firms have to position themselves as solution providers and not just web guys/gals. A client doesn't need a website, they need sales. The website is just part of that solution but it's not the solution and unlike web design, service like email marketing automation, SEO, and PPC requires an experienced professional.
Maybe just like you, at first we didn't have a darn clue about how to build a website, nevermind write half a line of code if our life depended on it! We wanted to build a website to start a side business, and felt overwhelmed, confused & scared about how to actually do it, which builder to use, and making wrong decisions. After years of trials & errors using different website builders, we're here to share our experiences with you.
In 2008, the Americans with Disabilities Act Amendments Act (ADAAA) was signed into law and became effective on January 1, 2009. The ADAAA made a number of significant changes to the definition of “disability.” The changes in the definition of disability in the ADAAA apply to all titles of the ADA, including Title I (employment practices of private employers with 15 or more employees, state and local governments, employment agencies, labor unions, agents of the employer and joint management labor committees); Title II (programs and activities of state and local government entities); and Title III (private entities that are considered places of public accommodation).
While legal considerations might be your biggest worry, making your site more accessible is simply good customer service. More than 39 million Americans are blind and another 246 million have "low vision," Another one million are deaf in the U.S. Add to that people with mobility issues that prevent them from using their hands and that's a huge portion of the country's buying power.