/ˈhaʊsɪz/

Is it not all native speakers that pronounce it like that? For me I find that it's actually really uncomfortable to pronounce it as an 's', I have to think about it in order to say it.

It really depends on the person, Mikkel. Both are correct and acceptable for either plural or possessive. I use two z sounds. In my opinion the first s and z at the end is hyper-correction. I would expect to hear a non-native speaker use s then ending z. To clarify, the grammar does not change the pronunciation.

That's fascinating. Yes, I pronounce ”houses” as /ˈhaʊzɪz/ and ”house's” as ˈhaʊsɪz/.

Either pronunciation is acceptable, unless you are using "houses" as a verb. A skyscraper ˈhaʊzɪz offices; it cannot "houses" offices. Barracks ˈhaʊz soldiers, they do not ˈhaʊs soldiers. The middle sound has to be a 'z'. Hope that helps!