There are various open-source or "donationware" stair makers available that provide basic functionality without the cost. 3D Warehouse:
Create solid stairs, stairs with sloped bottoms, or "steps-only" designs by defining riser heights and total overall height. instant stair plugin sketchup free download
This is the most famous plugin with "Instant" in the name. It creates stairs, roads, and bridges along a path. There are various open-source or "donationware" stair makers
Lines raced across the screen, stitching themselves together. In seconds, a perfect, spiralling sweep of mahogany and steel appeared, precisely bridging the two floors. The "impossible" geometry was suddenly there, solid and graceful. stairs with sloped bottoms