"You have houses."
Translation:Du hast Häuser.
Yes, "Häuser" becomes "Häusern" in the dative plural. All nouns add an -(e)n in the dative plural. The only exception are nouns whose nominative plural already ends in -n (e.g. die Frauen) or ends in -s (e.g. die Autos).
Here, however, "houses" is the direct object. For almost all direct objects, you have to use the accusative case, not the dative. That's why no -n is added.
Just like Der changes to den when verb becomes as if you were telling somebody Ich mag der Ka(:)se Ich esse gerne den Kase / Den = Einen /Der/Das = Ein/ Die/Das Eine.
For example Hasutern becomes when you're talking about a past/future Ex, setence, and hauster is in the present plural setence.