as far as the hen herself goes. It doesnt matter how high. If she wants a raised next box, she'll get in. Now for the chicks, they aint going back in the nest after they hatch, especially off the ground, no breed will til they can fly, but I have never had or heard of any that needed to. Just keep a good clean coop, maybe move other birds out so she can have it to her self and that's good enough. Some good bedding on the ground will be plenty for her to "bed up" with them in at night. You could try the ladder thing, but I highly doubt if any would try to go up.
I always just let my broodies raise them on the ground in the breeding pens and they do just fine. They nest on the ground here too.
That's the same with my birds. I been working with the breed for years now and have tried nest boxes on and off the ground and mine do much better on the ground. I just put one or two 5 gal. buckets in the pen and that is where mine lay and set. After the chicks hatch I just pick them up and put them running around with there mom in the pen. That way I can get a good look at the chicks legs for color and length and over all health of the chick. I even have hens that will just lay on the ground of the pen and will not lay in a nest box at all on or off the ground.