"I am in the white house!"
Translation:Ich bin im weißen Haus!
I made this mistake myself today and then realized why. We both overlooked that the form "im" is actually a combination of "in" (in) and "the" (dem). (The form "im" is always in dative case.) So with there actually being a definite article, the adjective "weiß" is then a weak dative form, so in this case, "weißen." Live and learn!
The preposition "in" requires either Akkusativ or Dativ. Here we don't have movement so it is Dativ. The adjective with neuter nouns in Dative finishes on -n when there is a definite article (in dem weißen Haus). Where there is no definite article the adjective ends on -m (weißem Haus).