Two words: lag bolts?
Samurai Carpenter has a quick tip for keeping stair stringers from rotting into the ground.
First, use pressure-treated wood. Second, screw a couple of lag bolts into the bottom of the stringers.
When screwed in, the lag bolts will hold the stringers off the landing 1/2 inch, and they also allow you to adjust the stringers to accommodate imperfections in the base that they sit on.
If your landing isn’t perfectly level, or if it slopes away from the house, you can unscrew the lag bolts until they all make solid contact with the landing.
You can also use this as a fail-safe for (slightly) miscut stringers. (Not that that would ever happen, but ...)
"It is super-easy, and it works like a hot-damn kind of trick ... makes you look like a Boss to your clients, too, so win-win."
Editor's Note: This also works with picnic tables