InFill New Build - Party Wall - Wall Construction

[Zutnip]

Hi all,

I’m seeking some advice for an infill site that sits at the end of a row of terrace houses.

It’s quite a narrow site so I’m looking to maximize floor area. One variable that I’m unsure about is the wall construction against the existing gable end party wall.

 

The gable end sits astride the boundary (boundary is down the center line) and there is an existing lean to garage.

In the new build, would I need to build a stand-alone structural wall on foundations, with a cavity between that and the existing wall? Or would it be possible to attach to the existing wall and line it?

 

Any advice would be very helpful.

thanks in advance.