Joining my new 6m rear extension to neighbour's existing 3m rear extension

If the extension is built on your land then normally a party wall agreement should have been set up already and that means you don’t need to build a new wall along that wall. You just attach your roof to it and share the wall I have done this previously with extensions. A descent party wall surveyor will be able to sort this out and do the relevant searches. It will save you time and money using the already built wall

Hi Nasim if there is a partywall agreement previously for the build this has nothing to do with your build, you still need a partywall agreement with the neighbours consent, or at least to serve notice for the opportunity to agree or not. we have had the same situation where the neighbour consented to the build and we built a wooden partion with insulation , your architect can come up with the suitable methods if your neighbour consents , if not you will need to do a complete new extension and concrete foundations. And usually yes your extension will be the same height as the neighbours unless its a double , there is a maximum height for flat and pitched, if you go on to the planning portal you should find the heights there, usually unless otherwise agreed with the council your extension will match the finish of your house. hope this has been helpful