Using the Law of Cosines.
As with many math problems, there are a number of ways this problem can be worked. Perhaps the most straightforward is to use the Law of Cosines to compute the side (c) opposite the given angle (C), given the sides (a, b) adjacent to that angle.
c² = a² + b² -2ab·cos(C)
Then the desired width of the house is ...
c = √(c²)