Section 1903 requires that foundation design be based on soil conditions determined through investigation. The design professional must verify that the recommended foundation type and depth is suitable for the building loads and soil properties. Boring and sampling must be conducted at adequate depth to ensure all soil layers affecting foundation performance are identified. Soil strength increases with depth in most locations, so deeper borings may reveal higher bearing capacity than shallow soil layers. Testing of soil samples in the laboratory provides bearing capacity estimates, settlement predictions, and other properties informing foundation design. The foundation design report must clearly state the allowable bearing pressure, which is the maximum downward stress the foundation can exert on soil. Bearing pressure is calculated as the sum of dead load plus maximum expected live load, divided by the foundation area. Total and differential settlement estimates guide foundation design decisions. Shallow foundations are economical for most buildings, while deep foundations (piles or drilled shafts) are required when bearing capacity is low or settlement must be minimized. Underpinning of existing foundations may be required when new adjacent construction affects bearing capacity. Seismic site class is determined from soil properties, influencing seismic design parameters.