Houses 17 and 18 are not just attached, it appears they overlap.
The question was how to alter mine retrospectively?
The hard way of doing it would be to cut a section out of the side of the garage on number 18 (the house on the right) so that the next door house sits 'inside' it a little.
But that would be quite an involved process, so I thought of an easier way.
I've added an extra strip of thick styrene to the side of the garage and bonded on a tiny section of slates on the top to create the same effect.
It does mean the left hand wall is a wee bit thicker than it was supposed to be but I hardly think anyone's going to notice.
And, yes, it was most definitely easier.