From a civil engineering perspective & American Disabilities Act (ADA) 2% is considered flat.
2% slope is for landings & change of direction, 8.33% for ramps with handrails. 4.99% for walkways without handrails. FHWA, AASHTO, USFS & WBDG has recommendations on road slopes, parking lots & airfields. You can also look at the International Building Code for slopes away from buildings, foundations. etc. The budget you have will inform you what the best solution for your development is. Terracing is one solution mentioned, retaining walls, split level building construction, etc. Drainage and soil type must be considered for global slope stability. Maintenance also has to be done and is key to prevent failures. USGS & GIS is helpful in getting approximate slopes but for construction detail plans get a good topographic survey, it will save you money later. Check with your local Authority Having Jurisdiction (AHJ) for zoning, setbacks, general soil type, frost depth, geological formation, hydrology, environmental constraints, landscaping, etc.
