I made this mistake on my first serious offer. The seller had a septic feasibility study from four years prior showing the land passed. I trusted it completely. My agent said it was fine, the price was right, and I was so emotionally attached to the property that I rationalized every shortcut.
After I closed, I commissioned my own test before breaking ground on a permanent structure. The sandy loam that passed four years ago had shifted enough — combined with a seasonal drainage change nobody disclosed — that my installer flagged serious concerns about long-term system viability in two of my three preferred building zones.
Soil hydrology is not static. Drought years, neighboring land clearing, even heavy grazing pressure nearby can alter drainage patterns measurably. A perc test is a snapshot, not a certificate.
I ended up making it work, but my building site is not where I wanted it, and I burned weeks renegotiating with my installer.
How old is "too old" for a seller's feasibility study in your region before you'd insist on a fresh one?