Party walls in terraced houses

[SALL2009]

We have an alleyway between ours and the neighbour’s house. In the alleyway there are two arches at second floor level. The arches align with the two chimney breasts in the house.

I am curious about the layout of walls upstairs.

Would there be a void between the two properties (above the alleyway) and the two arches are supporting the chimney flues for both property?
Is our bedroom wall a party wall? Or does that wall belongs to us and the neighbours have there own wall and there is a void between the two walls?

 

[joinerjohn]

Take measurements (upstairs)from the party wall with your neighbour on the other side and the wall where the alleyway is. Do this from front to back of the property and repeat downstairs and compare these measurements. Should show whether the bedrooms upstairs are over the alleyway or just in line with the downstairs alleway wall.