Building attached to my gable end wall by neighbour

I have recently inherited my late mother's house and I am in the process of selling it. the house is detached but the next door neighbour has attached a flat roof side extension to my wall. They have never had permission to do this and when it wet to planning I objected on my mother's behalf as I stated that this should not be attached to the house wall as she has not given permission and there i no agreement to this.
Unfortunately my mother was in her 80s and I did not want to start a war with next door which could have caused her distress.
Now the buyer wants this flat roof off the wall and he extension down. I am waiting to hear from my solicitor but it has taken so long to get the house sold and we are so close does anyone have any suggestions before I am forced to move in, start a cattery and goth/ hippie commune out of spite.
TIA