It gets a lot more specific when it comes to the elevator buttons and signage that is required. All buttons must be easily reached by a person in a wheelchair, first of all. They should not be higher than 48", and that includes the two-way communication system, used in case of an emergency. The "open" and "close" buttons should be placed above the emergency alarm button, and additionally, must be accompanied by symbols that are easily identifiable. The buttons themselves must be 0.75 inches in diameter minimum, and are to be centered at 42" from the floor.
Braille should be in textile lettering, placed either below or next to the floor buttons on the control panel. In any set of numbers, they must be in ascending order, to correlate with the ascending elevator. There also must be clear indicators of when the elevator arrives at a floor, like a flashing green light, and a recorded announcement, as well. There must be two-way radio communication available in the ADA accessible elevator, and the emergency controls must be in a group at the bottom of the control panel. But wait! There's more… These emergency controls are to be installed with their centerlines no less than 35" from the finish floor. Are you getting the distinct impression that this is a bit much? Well, before you judge too harshly, keep in mind that because many of these patrons are blind or mentally challenged, the system must be completely uniform in design, and there mustn't be too much room for alternatives. If strictly done in this way, someone who is blind, for example, can count on right where to (look) feel for the Braille instructions, when necessary. And, since elevators are considered a key component of being able to navigate the building, as well as make it out in case of an emergency, especially for those with disabilities, it is crucial that you have ADA signage in place, as well.
Now that you have a pretty good understanding of what's required in an ADA elevator, what about ADA signs for locating it, navigating the rest of the building, finding restrooms, etc.? I'm so glad you asked! After all, if you've gotten this far, you're likely a qualified business owner, or about to be one, so do it right - right from the start, and make sure your business is fully ADA compliant. It's not just that it's a federal law, it's also simply the right thing to do. OK, moving on to ADA signage. ADA signs that identify a room or a space are to be posted on the latch side of the door, on the wall next to it. They should be posted no higher than 60" from the floor, and no lower than 48" from the floor. Do not mount them on the door itself. Just remember the name of a great sign company who specializes in ADA compliant signs, such as ADA Central. They will ask you some questions, and then be able to get you what you need. You won't have to worry about the signs being ADA compliant, since that's nearly exclusively all that they do at ADA Central. Hopefully your original question, when is an elevator required by ADA, has been satisfactorily addressed and answered.