The part of the Americans with Disabilities Act that applies to businesses open to the public - shops, restaurants, hotels, clinics, theatres, and in many courts their websites - requiring access, policy changes and communication aids, with the cost limits that shape what can be demanded.
Title III of the ADA prohibits discrimination on the basis of disability by "public accommodations" - private businesses and non-profits whose operations affect commerce and fall within twelve listed categories that between them cover nearly every place a member of the public can enter, from a corner shop to a hospital. It also covers commercial facilities and the entities that offer examinations and courses. Government buildings and services are covered by Title II instead, and employment by Title I. Religious organisations and private clubs are exempt.
The obligations are of three kinds. A public accommodation must make reasonable modifications to its policies and practices for a person with a disability - admitting a service animal, allowing a companion in, adjusting a no-outside-food rule for a medical need - unless the change would fundamentally alter what it offers. It must provide auxiliary aids and services so that communication with a person who is deaf, blind or has another communication disability is as effective as with anyone else, unless doing so would be an undue burden or a fundamental alteration. And it must remove physical barriers in existing buildings where removal is "readily achievable" - easily accomplished without much difficulty or expense, judged against the business's resources - while new construction and alterations must meet the federal accessibility standards in full. Whether a website or app is a place of public accommodation is decided differently by different federal courts; the Department of Justice takes the position that the law applies to the online offerings of covered businesses.
A private individual may sue for an injunction ordering the business to comply and, if they prevail, for attorney's fees, but not for damages under the federal statute; damages are available only in a suit by the Attorney General, and under a number of state accessibility laws that run in parallel. That remedy structure is why Title III suits are typically brought to change something rather than to be compensated, and why serial litigation over technical violations has led some states and courts to impose pre-suit notice requirements.
For a person refused access, the practical first step is a written request to the business naming the modification or aid needed, because most Title III claims are won or lost on whether a reasonable request was made and refused. A lawyer is worth consulting when the refusal persists, when the barrier is physical and the business claims removal is not readily achievable, or when a state law offers damages the federal statute does not. Businesses that receive a demand letter should treat it as the start of a compliance question, not a nuisance claim, and get advice before responding.
Choose your state. Each link opens the directory page for the city in that state with the most currently published law firms in this practice area; a +n beside the city is how many other cities in the state also have one. The list is generated when this page loads, so a state whose listings have lapsed drops out rather than becoming a dead link.