Outside Toilet in a Victorian Terrace - options?

[DollyParsnip]

We live in a standard Victorian Terrace which has an old outside toilet which, despite the loo being in place, is blocked up, unused and frankly needs tearing down work.

It is attached to the neighbours' outside toilet.

So. First up, would it be liable to a Party Wall Agreement if we were to extend it to the side (essentially elongating it) potentially doubling it's size? Secondly, if we did double it in size would we need any planning permissions (I'd says it's very roughly about 4 foot square at present) or would it be a permitted development?

The other option would be to extend it, but put a door in to the kitchen, either making it a utility area (although I'm not sure it'll be big enough) or a pantry-style cupboard.

Any opinions gratefully received!

My parents took the loo out and put the washing machine in there.

All walls left in place, some plumbing needed, but no PP I don't think.

This was 30 years ago mind.

I think it depends OP.
best to check out with planning dept anyway, nothing to lose.
You can extend a certain amount on a single story without planning permission but wouldn't know by how much or for what purpose.
you really need to check with them and not us, unless of course there's a specialist on here.
sounds like a good idea and either laundry room or pantry sound great.