I was taught that the s is omitted after plural nouns, not after nouns ending in s.

Julia Evans's house, but the Evans' house.

FWIW, I'd accept either one. Some names don't take an extra s by convention, like Jesus, but otherwise it seems in practice that names that feel like plurals don't get it. I'd write "Jess's" but "Jenkins'".

(I consider myself a follower of Lynne Truss's, and have deliberately not considered a grammar reference for this answer, as I'm sure this is within the interested reader's ability and shan't spoil these readers' fun.)