What Does a Class P Site Mean for a Home Building Project?
Owners, builders and project clients
Class P means the site has a problem or condition that cannot be adequately described by the ordinary normal-site reactivity classification alone. It does not mean the house has failed, and it does not automatically mean piles or underpinning are required. The useful question is: why was this particular site classified P?
P has more than one possible reason
Possible reasons include inadequate bearing strength, potentially excessive settlement, certain fill conditions, landslip, mine subsidence, collapsing or erodible soils, abnormal moisture and other movement influences outside the normal reactive-soil assumptions.
Those situations need different information. A site affected by deep fill poses a different investigation question from a site affected by existing tree-related drying or a possible slope problem. Two reports carrying the same letter may lead to quite different footing proposals.
The reason belongs in the report
A Class P classification report should identify the reason and recommend further investigation as needed for adequate design data. The basis needs to be recorded with further geotechnical recommendations.
If a quotation or drawing says only Class P, ask for the underlying report. Find out whether the reason has been investigated, which assumptions the designer adopted and whether any recommended work is still outstanding. P alone will not usually give enough information to prepare an appropriate footing design.
A classification investigation is not always a complete design investigation
A classification investigation may not provide enough information for Class P footing design. A completed soil test does not confirm that every relevant ground question has been resolved.
Ask what the next investigation would establish and how the result could change the design. That makes it easier to distinguish necessary information from repeated testing that has no stated purpose. It cannot guarantee fewer tests or a lower project price; it gives the team a clearer basis for deciding.
P does not select the remedy for an existing crack
Standard deemed-to-comply footing designs discussed here exclude Class P sites. A site-specific engineering response must address the actual condition, not substitute a generic heavier slab or deeper pile. A letter on a soil report is also not a retrospective explanation of damage in an existing building.
Keep the classification report, design response, identified site works and any unresolved assumptions together. Where there is current distress, provide the movement history separately. Urgent change or apparent instability needs prompt assessment regardless of the site's letter.