Boundary wall

[Gashead]

Basically my Grans neighbour wants to build an extension. On the plans in involves it ending 900mm off their side of the boundary wall. However, my Grans deeds, from when the house was built in the 80s state the boundary wall is her property. Weve heard they have brought in law or something regarding boundarys but as her deeds already state its hers, does this effect her? If it doesnt and the wall is soley her property, would this mean the extension needs to be moved over 100mm as on the plans its 900mm off their side of the wall, when it should be 1000mm of their side as thats where her property ends? I hope that makes sense.

Also, the wall itself has a few cracks in it, my Grans house is about 1.5m lower than her neighbours as its on a hill. If they go ahead and put in footings 1m down and only a meter away from the already cracked wall, will the wall be safe, and if they do it and it falls down who pays for it to rebuilt? Or should an engineer check it before work begins?

Any help would be appreciated as my gran is quite worried about it.