If you are building a new build property onto the end of a terrace and you have the full permission and co-operation of the owner of that property, is it normal to use their outside wall as your inside wall or do you need another seperate wall?

their terrace roof ending is apex (I think that's the term)

Thumbs up... makes a change to see a thread about two neighbours getting on well and doing a 'you scratch my back I'll scratch yours....'
Do you think in such a case, it would not only make sense, but almost definitely be approved with thumbs up all round, if you were to knock your garage down and build a smaller extension, leaving a gap between the houses, making them both detached? There's a house on the market that I am not looking at because of these garages attached, but that could be an option ...
That should be pretty straightforward for a builder shouldn't it? And, would a neighbour ever really say no to that?
Thumbs up... makes a change to see a thread about two neighbours getting on well and doing a 'you scratch my back I'll scratch yours....'
I think you mean they have a 'hip ended' roof, and you'll effectively convert it to 'gable ended' and then carry on building your own through.
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