Sharing side return with neighbour

[Moonlight46]

Hello we are planning to do a side return in a new property which is a classic London terrace. Our neighbour is also planning likewise and will likely do the build ahead of us. Can we share this party wall later down the line? Ie If they have already built the wall is it possible to ajoin? What are the pros and cons of doing so? And how do you go about setting a price with the neighbour? Would love to hear from anyone who has done this!

We tried to work together with a neighbour to share a wall for a rear extension. We didn't in the end.

Mainly because we couldn't do the build at the same time. You can't have one side of a (planned) internal wall exposed to the elements. It won't pass building regs unless it's built as an external, cavity wall.
Then where do you put a cavity wall? On the middle seems fair. But then the neighbour's extension is encroaching onto your property until you build your extension.
If you want to move that can cause problems with the sale.
If fully on the side of the person that builds first, then they get a smaller extension. In that case they'd be crazy to let you use their wall for your building

What happens if someone wants to change extensions in future like go further out, up, of remove it? It can be restrictive.

We wanted to go further out then out neighbour. That is really difficult with a shared wall because the extra bit isn't shared between extensions. As an external wall needs to be 350mm thick for building regs insulation. So how to design a strange step in the wall and roofing?

Something to be really wary of is overhanging eaves and guttering. A parapet wall with aluminium capping avoids overhang

If you can stay and finish at the same time then do shared plans, probably shared architect and builder. Then its possible.
But getting all the timing, design and contractors to align is hard.

Thank you! That is really helpful

Yes
you need a party wall award agreed with the neighbour.

Never share the costs of two builds by two owners using one builder at the same time
You have no idea how your true costs will aportion. The neighbours may require more work than yours, how do you prove that ??? They may change stuff on their side that delays yours etc. so so many problems. Speaking as an architect I’d avoid