Stepped footings
On flat building sites (or sites without slopes) a footing excavation can be made at a constant depth from the natural ground surface. Flat sites allow easy access and generally are simpler to excavate than sloping sites.
The base of the excavation is always horizontal and parallel to the natural ground surface.
Obtaining horizontal excavation bases on sloping sites presents problems. If the slope of the ground exceeds 1:10 fall (a 1 in 10 or 10% fall) it is unacceptable to excavate parallel to the natural ground surface.
Sloping footings can slide down an allotment due to gravity. For this reason the base of strip footings should be excavated horizontally, where possible.