Party Wall Query - Excavation

[SelfbuildExte]

Hi, we are getting close to digging footings for our single storey extension and am finding information on party walls confusing. All of the diagrams I have seen demonstrating the 3m rule are in relation to the adjoining property horizontally and not when the new wall is perpendicular to the party wall line? In fact, I can find no discussion or diagrams at all in relation to my query hence me wondering if I'm incorrect. Would I need issue a party wall notice to property A in the below diagram?

Many thanks in advance.

 

[stevie888]

You do need to serve a PWN as your hole will be within 3m of the neigbour's foundation, but it can all be very low key.

Talk to the neighbours if you haven't already: 9 times out of 10 it's a mere formality.

 

[SelfbuildExte]

Thanks, I've given them notice. They've been happy with everything to date and I'm sure this will be no different.