Class P site
A Class P site under AS 2870:2011 has problematic conditions (uncontrolled fill, soft soils, poor drainage). Standard deemed-to-comply slab designs cannot be used.
Ask Chalkline about this →A Class P site is the problematic site classification under AS 2870:2011 Residential Slabs and Footings. It covers sites with conditions that prevent use of the standard prescriptive slab and footing designs: uncontrolled fill, soft soils, poor drainage, highly variable ground conditions, or abnormal movement characteristics.
The key consequence is that the deemed-to-comply tables in AS 2870 do not apply. Full structural engineering design is required for the foundation system. In practice this means the structural engineer and geotechnical engineer work together to produce a site-specific design, which typically adds $15,000 to $40,000+ to the foundation cost compared with a standard stiffened raft on a Class M site.
Common triggers for Class P: pre-existing fill with no AS 3798 compliance documentation, demolition debris left in ground, sites with a history of flooding, or sites where the soil profile changes significantly between boreholes. The classification must be reassessed any time the site is cut or filled after the initial investigation.
Also known as: Class P, problematic site
Category: Site classification / earthworks
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Last updated: 2026-05-08. Verified: 2026-05-08. Quarterly review for currency.