Building side return next to a neighbour - is a new cavity wall the norm?

[Snapdragonsoup]

Where a neighbour's side return wall sits on their side of the boundary (i.e. only built up to the 'line of junction' they don't have to let us use it do they as I don't think it counts as a party wall?? If they don't want to share it do we have to build a new cavity wall alongside their cavity wall or can we build a thinner wall because our wall is going to be protected by their pre-existing wall? Would building control allow this and do party wall surveyors allow this/not try to prevent it? Also, are we allowed to put a flashing onto their wall to create a weathertight seal between the two walls? I am asking these questions on the basis that there is no agreement with the neighbour on this and neighbour not wanting to share their wall so we build a new wall alongside theirs.

Build a new cavity wall next to theirs. The gap does need to be sealed at roof and walls.

How far do you need to excavate the ground to build your wall as that will determine whether you need a party wall agreement. Building control will only look at your wall so you need to go by the regs.

You need to meet the u value of .18, so you’re going to need a cavity wall with a 150mm cavity. Otherwise you’ll be insulating it inside and losing even more space.

@BriarHare, doesn't internal insulation take up less space than a cavity wall with 150mm gap though? Does building control ignore another cavity wall built alongside our wall in calculating u-values then (i.e are the calculations based on the assumption that our new wall Is open to the elements with no wall alongside it?

Also, does anyone know if the wall needs to be build as weatherproof (like an external wall) or can it be sealed at the top by putting a flashing into the neighbour's wall so that it doesn't need to be weatherproof (e.g. can be made from blockwork)? Do the neighbours have to agree to it if their wall isn't a party wall or can a party wall surveyor impose such a requirement on them in a party wall award?

Also, if we aren't allowed to put a flashing into the neighbour's wall are there other (less intrusive ways to seal a gap between wall (e.g. using a sealant)? Or doesn't that work?

Bumbing this to see if anyone can help me further on this? Thanks!

I think you need to ask an architectural technician (both cheaper and more practically minded than architects on the whole !) who understands both the regulations, and building physics ie will know what the risks of condensation, water penetration etc are, as well as what building control will require. They probably have standard details that they already know work, and are legal.

"Sealant" doesn't sound a very durable way of keeping the rain out! It is important that your addition does not inadvertently start directing rainwater into your neighbours' brickwork for example 😬

If you both build to the line of junction then it is normal to seal the gap. It sounds like you don't get on with your neighbour. Even so, ask your party wall surveyor to get this agreed with your neighbour. It is in both of your interests.

You will most likely need a PW agreement when you excavate for foundations any way won't you? Might as well get it all done properly and get the surveyors for both parties to do it legally. You might not get on with them, but to me that means you need watertight legal agreements dealing with this.

Thanks for the suggestions, everyone!

I am doing this because the AO was adamant that she did but want me to have an extension that did not mirror hers. You will need a PWA but will not be paying the AO a fee for enclosing if they have built on the OW or within their boundary. There are PW surveyors to avoid - they will charge you in 15 mib blocks and delay matters. Nit sure if you can object to the person your neighbour from hell proposes as her PW surveyor but check him/her out and ask for the fee structure upfront.