Building onto neighbour's extension?

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We live in a semi detached house. Our neighbours already have a kitchen extension and we would like to extend ours too. This would mean building it onto their extension though- can we do that? I'm a bit clueless about all of this, sorry.

Depends if they have built on their side or astride the boundary.
If it's astride the boundary is a party wall and you can use it as your external wall. You may have to pay them to use the wall if it is, but they can't stop you.
If it's on their side you can still connect your roof or parapet or flashing into their wall as necessary.
An architect or builder or surveyor would be able to assist, and you could talk to the neighbours if you're on good terms

You can build onto their wall - our neighbours have an extension up to the boundary and we decided it would be easiest to do that; we were told they would need to consent.

Your other option is to leave a gap, which I believe is your only option if you want to build a bigger extension than they have. The gap can be super narrow or wide enough to access for maintenance purposes. The latter would obviously mean your extension was narrower than you might like.

In our case, we could either mirror next door’s extension and adjoin, or go further out and leave a gap. We didn’t want to lose the width, nor to leave a gap too narrow to access for maintenance purposes so we went for adjoining.

Thank you both for replying. We are on slightly dodgy terms but I don't think they would actively try and prevent us from improving our home. Let's hope not. :)

Just build up to the boundary without touching. Mine & my neighbours kitchens probably have 2 inches between the walls.

"slightly dodgy terms" and "let's hope not" means you are unsure. Why not ring the council and ask? Otherwise, if you might end up on the Daily Mail! If you do, please make sure you brush your hair first and tell us how much your house is worth! :D Jokes aside, if your neighbours are indeed dodgy they might take it upon themselves to make your life miserable so maybe this isn't the best place for accurate information.

Take advice on leaving a gap OP - can suffer from weather or other types of damage and be difficult to access. We agreed a shared party wall to avoid this!