I signed a party wall agreement allowing my neighbour to carry out a loft conversion and put in 3 steels. What I did not know is that the wall in the loft is single skin.
The properties are terraced with a cavity wall to ground and first floor (which is a skin of bricks his side and a skin my side), however once you get into the roof space there is only a single skin of breeze blocks which was added years after the properties were built. When in my loft, you can see right into the cavity of the 1st and ground floor, so you could almost say the breeze blocks are his side of the cavity for the floors below.
My neighbour has put in the 3 steels and I can see all three of them in my loft space, one of them overlapping by some distance. Is this single skin of wall in the loft classed as the party wall whereby he should only have gone 50% into the breeze blocks from his side? Or is it the case that because the 1st and ground floor are double skin, his side of the party wall is classed as the full depth of the breeze blocks his side of the cavity for the floors below.
It appears that approximately half the thickness of the party wall was raised in the past by the owner of the adjoining property – the raised section is still a party wall even though it is thinner than the original party wall below. Presumably you could do the same on your side of the party wall if you wanted to convert your loft in the future.
Your neighbour does have the right to use the full thickness of the party wall to support his beams. It is considered good practice to only use half when the wall is thicker but he would not have had that option in this case as the beams require a bearing of at least 100mm.
Technically the beams can extend to the outside face of the original party wall although there is no structural benefit in doing that. If they extend beyond the thickness of the party wall below that is a trespass and you should request that they are cut back.