There is going to be tremendous variation in the size and types of chimneys - with some large and accommodating and some quite small. Some are going to allow easier access than others. Maybe some will be like the scene in the Santa Clause movie with Tim Allen when the chimney and fireplace opening - even for homes without one - would grow to allow Santa to enter and then shrink again to the way it was upon his departure. Maybe this is how it works.
It's not just the chimney either. The roof needs to be in good repair - can't have Santa sliding off the roof (also like the Tim Allen movie), slipping, or falling through a weak spot. That would certainly slow him down and affect the inconspicuous entrance he is trying to achieve.
Clearly, this is a special type of visitability because only Santa would visit our homes by coming down the chimney. Paying particular attention to its size and design would not be warranted when other areas of a home have a higher priority for visitability and accessibility concerns.
How about the front door? Homes without fireplaces rely on Santa visiting them by coming through the front door. Santa seems to be a large person - is the front door wide enough for him? It would need to be at least 36" to provide as much access as possible - the same as we would do for any visitor where we don't know how easy it is for them to pass through the doorway.
If Santa arrived by some other means than his reindeer and sleigh, where would he park, and how would gain access to the front door? We know that he doesn't have much time so we need to keep the number of steps he makes from his sleigh to the front door to a minimum - and have them on flat ground rather than actually ascending steps or stairs. We don't know if Santa can climb steps or how well he might do it. After a night of visiting so many homes, he likely gets tired so an easy entry is important.
Of course, an entry that is easy to reach from the driveway or sidewalk also is important. If we want Santa to visit us, we have to do our part and make our homes as visitable for him as possible.
Maybe it's not actually him that is visiting but some of his staff - maybe this is how he manages to visit every home. He has legions of helpers.
Not expecting a visit from Santa this year? No matter. Our homes still need to be visitable, and Santa's possible visit is a good reason to get started on making them more accessible and visitable to holiday visitors - such as Santa, should he choose to visit anyway. We don't always know in advance who will be ringing our bell or knocking on our door. That's the whole reason for visitability - to be ready for anyone, of any ability, to stop by and see us - and have an easy time of it.