TT #275 · Parshas Tazria · Aliyah 7 · Passuk 51
English: The priest shall examine the lesion on the seventh day; if it has spread in the garment, warp or woof, or leather article, the lesion is malignant tzaraas; it is impure.
Torah Temimah cites Sifra (Toras Kohanim) for counting even a small amount of adjacent spread.
Sifra — Vayikra 13, Midrash Sefer
English: “The lesion has spread” includes any amount of spread immediately adjacent to the original lesion.
Textual derivation
For garment tzaraas, even a very small amount of new affected area immediately next to the original lesion counts as spread.