Laws control the use of accessory buildings, pools, trailer parking, and much more. Regulations set the rules for livestock activities and many other activities. Check with the local Planning Department, which administers zoning laws. If you are coming from the flatlands, then be careful with this one. The slope of the land can be real deceiving.
Steep slopes make driveways difficult and houses lopsided.
They make construction much more expensive. Thirty percent is about the very limit of what will work for roads, houses, and septic systems. Drive around and look for homesite construction on parcels with similar slopes for an idea of how the lot improvement will look. If you're not sure, consult a civil engineer, architect, or a qualified building designer or land use planner. They can assist you in dealing with topography.