Because it's "that house" and not "a house" A big house = Et stort hus That big house = Det store huset In your sentence, you are talking about a specific house, not just a house in general. Hope that makes sense

You are correct to think that (most, but not all) adjectives take the suffix -t in the neuter. The INDEFINITE neuter form: "A big house" --> "Et stort hus" However, when the adjective is placed before a noun in the DEFINITE form, the adjective will also be in the definite form, ending in -e. The definite neuter form, "The big house" --> "Det store huset" In your example, "Petter´s big house", you would also have to use the definite form, because you are not referring to just "a house", but to a specific house - Petter´s house. "Petter´s big house" --> "Petters store hus"