Unlike multiple floor layouts, the visitable concept does not need to extend beyond the main floor anyway. It reads that we should create a design that is the minimum space to accommodate a person in a wheelchair using the main floor of a home. That's a great concept, but if the main floor is the only floor, visitability takes on more significance because the whole home then becomes a visitable design.
With multiple floors (including basements), vertical access becomes important. Accommodating people going up and down stairs, and for the times when climbing stairs is difficult, impeded, or not physically possible, is often necessary in a two-story home or one with a basement. When assisting people going between floors is not a viable alternative due to the configuration of the home or the physical ability of the occupants, they are more limited in the use of their home. Because stair glides (also referred to by other names such as chair lifts, stair lifts, lift chairs, and chair glides) are not particularly expensive and can even be obtained on a refurbished basis, budget is not as much of a reason as the home and the individuals for deciding whether to install one.
Just the interior stairs in general - necessary in a two or more floor home or one with a basement - are not a factor in a single-level home. Their size, location, design, safety aspects, tread materials and coverings, railings, landings, lighting, access, and other parts of stairway construction and utilization are not something to consider.
Elevators are a much more universal design solution when stairs are present, but again, there needs to be more than one floor for this to work. Design strategies include creating two closets of the same size in a public area (such as a hallway) and stacking one closet over the other on each floor with the ceiling of the lower one (which at the same time is the floor of the upper one) being installed so that it can be removed at a future date to create a vertical shaft in which to install an elevator and car. The doors to each closet would become the elevator access.
As desirable as elevators are for movement of people and goods within the home - for visitors as well as full-time residents - they can be cost prohibitive for many people, and they only work in multiple-story homes of two or more floors or ones with a basement. They add value to a home because anyone of any ability can use them, but they clearly are not a consideration for one-story homes.
Bedrooms are necessarily all located on the main floor when it is a one-story dwelling. They may be split with the master suite (also called the owner's suite or owner's retreat) being on one side of the house and the secondary bedrooms and bathrooms being located apart from the master area - completely on the other side of the home or strategically located away from the master. In two-story construction, it is common for all of the bedrooms to be located on the second floor although increasingly at least one en-suite bedroom (if not the master itself) is being located on the main floor. Seniors are definitely requesting and looking for a first-floor master when they shop for a home or have theirs remodeled. Of course, the concept of a livable home includes a first-floor bedroom in addition to any that might be on additional floors.
Bedroom locations, stairs, and elevators and other vertical access can be considerations in multiple story homes or ones with basements, but for those single-level homes, they do not apply.