Among the most neglected aspects of basic grammar is how to make the plural possessive of a name ending in –s. It’s ignored in all editions of Strunk & White’s The Elements of Style, which announces on page one how to make a singular possessive—recommending Charles’s friend and Burns’s poem. It’s neglected in the AP Stylebook, which has a section “Singular Proper Names Ending in S”—recommending a different convention for singulars: Agnes’ book and Kansas’ schools.
In both books—and many others—the section on possessives simply doesn’t address the point of how to refer to the house of the Burns family.
Here’s the rule announced by all the authorities that do address the topic: pluralize first, then make possessive. Just as Garner becomes the Garners in the plural, and then the Garners’ house as a possessive, here’s what you do for John Burns and his family: Burns becomes the Burnses in the plural, and then the Burnses’ house as a possessive. It’s the only proper form.
You say there’s a happy hour today at the Powerses’ place? Great. See you there.
Garner’s Modern English Usage 712–13 (4th ed. 2016).
The Chicago Guide to Grammar, Usage, and Punctuation 31 (2016).
The Chicago Manual of Style 422 (17th ed. 2017).