Start with behavior, not the word clay
Some soils change volume as their moisture changes. Linear extensibility is a survey property used to describe shrink–swell potential. Clay percentage describes how much clay-sized material is present; it does not identify every mineral or tell you how a particular foundation is performing. Two soils with similar clay percentages can behave differently.
Read the class alongside the depth
Our checker groups reported linear-extensibility values into Low (under 3%), Moderate (3 to under 6%), High (6 to under 9%) and Very high (9% or above). It shows the highest available value in the top 100 cm for each mapped component. That is a transparent screening choice, not the official engineering interpretation for a building. The percentage is not the probability of damage or inches of movement.
A map unit is a mixture
A soil polygon may contain several components. Their percentages describe the estimated composition of the map unit, not a measured percentage of your yard. The largest component is a useful starting point, but it may not be the soil immediately beneath the building. An urban-land component or an unrated layer must stay unknown; it is not a low result.
Turn the result into better questions
Keep the map-unit name, component list and depth coverage with your notes. Ask whether there is an original geotechnical report, whether the site includes imported fill, and whether drainage or plumbing changes could explain observed conditions. For construction, foundation design or significant movement, have a qualified local professional investigate the actual site. Do not size a foundation or buy a repair solely from this map.
Sources and interpretation
USDA NRCS linear-extensibility definitions
Explanations and workflow are our own. This is educational screening, not a site investigation or engineering recommendation.