To build a staircase for a person with bad knees and back.....?

The word "staircase" infers lots of steps.

More than a few, and I've never found an easy (or cheap) solution. And, I've contracted jobs for folks with disabilities for years.

I've built several stairs (6steps or so) for people who require the use of walkers.

You can custom design the height and depth of the steps, to the requirements of the person using it. You won't have a problem with bldg. code by making the steps shorter and deeper.

I make the riser as low as 3-5" tall, and as much depth as needed to allow the entire walker to be completely set on the next step. (as much as 36")

Even without walker, the person with bad knees needs room to bring both feet completely onto the next riser.

(and might plan ahead, in case a walker is needed later)

A hand rail, with the additional "interior railing", for extra grip on both sides. Allow a full 3' interior width. Or more.

I use "rolled edge" decking lumber. Less curling of lumber due to weather.

standard is about 7" rise (height) and 10" run (deep/depth).

I looked up a site, and they're calling for a 6" rise, and an 18" run, which sounds like a lot. It's the rise that usually gets people...but no matter what, I wouldn't do less than a 10" run...maybe 12"?

A rise per step of 6 inches with a tread 12 inches deep is very confortable to climb.

I have built many of these for my older customers and they were very pleased.

you build it an inch from the ground so that no one can get hurt