So the envelope operation uses:
envelope(direction, maxHeight, frontBaseHeight, frontAngle, backBaseHeight, backAngle,
rightBaseHeight, rightAngle, leftBaseHeight, leftAngle)
Within this, frontBaseHeight and frontAngle apply to the street.front edges if these are defined, otherwise they apply to object.front.
I would like them to apply to the northern edge of the parcel, which would be world.north.
Hope this helps make sense of it.