There is no reason why the curtilage of one dwelling cannot overlap the curtilage of another dwelling, an inspector dealing with an appeal against the refusal of a certificate of lawfulness for a dormer window at a house in Sussex has maintained (DCS Number 400-019-492).
In the case before him the side wall of the dormer window would sit on a party wall. The council contended that it could therefore not be permitted development because it would not be wholly within the curtilage of the appeal property, and permitted development rights only apply to development within the curtilage of a dwellinghouse.
The inspector explained that adjoining curtilages can, as McAlpine v SSE  decided, overlap each other, and this was particularly relevant in a party wall situation where the result could be the collapse or partial collapse of both dwellings if the wall were removed. The party wall, he continued, was such an integral part of the two dwellings that there was no reason why their curtilages could not overlap because such small areas were involved.
Guidance on the definition of curtilage can be found at section 4.3444 of DCP Online.