Kitchen side-return extension - what height I am allowed to build the boundary wall up to

[Johnbuoy]

We live in an Edwardian house in London (Ealing) and we are planning to extend our single story kitchen into the side return up to the boundary with our neighbour.
I haven't discussed it with him yet but was wondering what height I am allowed to build the boundary wall up to without a party wall agreement?
I've had a couple of quotes and the first builder seemed to suggest we could only build up to 2 metres unless the neighbour allows us to go higher.
The second builder suggested that we'd be able to build to a height of 3.5 metres. (We'd actually only need to go to 3 metres to match the existing kitchen.)
Who is correct?

Surely if you are building it up even 1 cm you need to use a Section 2 party wall notice?

Ring your planning department. They are very helpful. It is my understanding that you have to get a party wall agreement for any building on or near the boundary.

That's not correct, there are certain requirements to issue a party wall agreement, eg you are building foundations lower than your neighbours and the works are within 3m. There are also many scenarios where you don't need to issue a party wall notice. Also party wall matters are not a planning issue so the local planning department won't care one bit.

You're better off speaking to your architect, or a party wall surveyor. Any decent party wall surveyor will give you 30 mins 'free' time so you should be able to ark yourself with the facts pretty easily.

It varies in London boroughs and whether your neighbour has extended. For instance, under planning permission in LBHF it’s 2metres unless your neighbour has also done a side return then it’s 3. There’s also the permitted development rule but this may not give you the length for a side return. Wandsworth has different rules. Upshot is, speak to your local planning officer who will offer short consultations for free and prepare for a party wall award - assuming you’re a terrace. As others have said, party wall notice is entirely separate from planning, as our building regs which you’ll also need to follow.