When forming the possessive of a plural noun that already ends in “s,” you should add an apostrophe after “s” and not add an extra “s.” For example, the plural noun houses becomes the possessive noun houses’.
This is different from forming the possessive of a singular noun (e.g., “house”) or an irregular plural noun that doesn’t end in “s” (e.g. “men”). In those cases, you add an apostrophe followed by an “s”: house’s; men’s.