Since ADA standards do not apply to residential construction, except where local codes require their use, we are free to create solutions for our clients that are specific to them rather than including elements that may not be necessary for their needs - and would take up part of the budget. Instead of spending money on grab bars around the toilets, a higher toilet than normal, or a ramp at the entrance - when none of these are required to meet the needs of a particular client - their budget can go farther by providing for what is necessary for them.
It's not a case of items called for in ADA standards not ever being necessary. It's one of having the flexibility of using what is required for a client because their needs call for it but not for others when there is no specific indication that this will help them.
Whereas, the standards tell us what must be done to be in compliance or what we shall do, we generally are free to use what is client-specific and in a manner we deem most appropriate for them. This could mean that we would include a grab or assist bar near the toilet, but maybe just one rather than two or possibly two but located at a different height, size, or location than what is called for to be compliant with the ADA standards.
Hallway widths, minimum door sizes, location of switches, countertop heights, and similar aspects of the home design generally are not regulated either. There might be some minimum standards such as the smallest a door opening can be or the narrowest a hallway can be, but we are free to make them larger or to design a plan with no hallways at all (an open plan, for instance) and with cased openings or large spaces instead of traditional doorways into rooms and not have to adhere to any specific requirements of what must be done.
There are conventional heights for countertops, upper cabinets, and range hoods, but they can be higher or lower according to the needs and personal tastes of the client unless the local building codes require them to done in a certain way.
There are some things that we must do such as include outlets on each wall, but the height, color, and style of them generally are not regulated and can vary tremendously by which room they are in, the intended purpose, and the preference of the contractor or the client. We must follow lead paint and asbestos protocols when faced with the possibility that either is present, but we don't need to worry about either in newer homes.
Door and cabinet hardware, the style of cabinetry and whether there are doors or drawers in them, and the types of faucets and plumbing fixtures are subject to the needs and desires of the client rather than being mandated by any set of requirements.
While it might seem that they are many regulations than we need to comply with and many guidelines suggesting how we are supposed to complete a renovation of someone's home, we actually have a tremendous amount of freedom and flexibility to meet our client's needs rather than just doing or including something because it is required.