John's or John's house

They both have the potential of being correct.

In the first sentence, you are stopping off at John's house. In the second sentence, you might be stopping off at John's house or a place of business named John's.

In British relaxed speech, you can use a person's name in the possessive to denote their residence, e.g I have been round at Jim's; we are all meeting at Bob's. Also 'yours' and 'mine'. Probably Oz/NZ too. Maybe SA.

And the US, as Sturgis has noted.