Building next to neighbours extension....options?

Hello!

We are planning a single storey rear 3 x 5 m extension to our 1930 terraced house. Having got a few quotes from builders I'm a little confused by the advice on how we build next to our neighbours existing extension. They have the exact shape and size extension we'd like to build so we would essentially copying the structure and extending across the back of our house at the same height and depth. The neighbours extension appears to be built up to the boundary line so they own the wall entirely. See photo attached.

Here are the options I've been presented:

1) Build off the neighbours extension with a nib wall. This options saves both money, time and space but would involve the neighbour agreeing to use of their structure. I can also see potential issues with a) future sale and ownership of wall b) if they decide to rebuild their extension and knock the wall down c) issues of maintenance or subsidence.
2) Build flush to the wall and create a party wall. This option is most unclear, can we build flush and continue the fascia and roof without an air gap? Could we double the thickness of the wall and create a similar party wall agreement as our current internal walls? How would this affect foundations?
3) Leave a 200mm air gap and build an independent wall. This is the easiest option in terms of agreements with neighbour but would cost more, loose interior space and cost more. We are also concerned about damp, debris and pests from nearby river.

I'm interested in hearing thoughts on the options and any solves from previous projects.

Thanks in advance!
Chris

How have you got quotes, when there are no plans and no detailed design?

Btw, builders are not designers.

Thanks for the response Woody. The extension is pretty basic and I've been able to use neighbours plans from the council planning portal to gauge rough costs. Whilst builders aren't designers they do have hands on experience and have been all given me their point of view. I was hoping that someone on the forum could share constructive thoughts/ideas rather than stating the obvious!

It's obviously not obvious to you that a builder is going to recommend what is either easiest or more lucrative to him.

But if you want to be a cheap-skate and skimp, you will get the type of builders and extension that you seek.

Woody here is an example of what a value adding posts look like:

Ok , well I say definitely can not share the same foundation as your neighbour as there is no way of knowing if it was done correctly and could possibly move in time if not ?
2nd they have got the upper hand on you as their extension comes right out to the boundary , any form of attaching to their extension will not be approved by building control for several reasons, movement from one extension will damage the adjoining extension , sound travel, insulation etc etc
I think your only real and safe option is to come in 300mm from the boundary and create your own foundations

The back of my house projects beyond the back of my neighbour's house and they built a single storey extension with a new 300mm wide cavity wall on new concrete strip foundation abutting the existing foundation and leaving a gap of about 120mm between the new extension and the brickwork of my house. A stepped lead flashing over their monopitch tiled roof abutting the brickwork, and terracotta air bricks on end to close the gap between brick walls completed the ensemble.

Done this many times in my heydays.
Speak to the neighbour and let them understand that if build your extension as a continuation of theirs, there would be no risk of debris, mould, vermins etc in between the two.
It will look better and the builders will have no room for error as they would have to mirror exactly what's already there.
Yes, you can find out how deep and dense their fundations are as your external wall attached to theirs, will sit on it.
There's a rare, unknown tool called "spade" with which you can dig a hole next to the fundations and find out how deep they go.
Not many people can use one.
I have built many extensions like this and in west london, building control and planning dpt have never objected to it.