If the solution has not already been posted, the only other assumption I can imagine making is that the tether is linked to a railing that allows it to slide along the perimeter of the house. The the accessible portion of the yard in that case is four rectangles and a complete circle. I won't bother to draw it, but the area is clearly 80p + pi802 where p is the perimeter of the house.
I won't spoiler this, because don't see how the OP could be construed to describe that condition.
I don't see what else to try. If we're still missing something, a clue would be nice.