Toni agrees with Collins Cobuild English Usage - Helping Learners with Real English - ISBN0 00 370258 8 Paperback.
Extract (allowable under fair use rule)
use an apostrophe in front of an 's' added to a noun or pronoun or after a plural noun
ending in 's', to show a relationship such as possession.
. . . my friend's house
. . . someone's house
. . . friends houses'