That would be "The garage is by the house." Mellett makes the two buildings neatly aligned, -nál makes it sound here that the garage is just somewhere in the house's vicinity.
Why isn't it " The house is next to the garage"? I thought words like mellett are combined by the word before. So "the house is by.." - ... a ház mellett van
mellett goes with the thing before in the way that preposition go with the thing after in English.
So a ház mellett is "by the house", and ... a ház mellett van is "... is by the house".
mellett is right after a ház, just as "by" is right in front of "the house".