Neighbours wants to build extension onto mine

It may be best to get a party wall surveyor involved to help iron out the issues with this. You need to know where the boundary line is and if your extension is set back away from the boundary then you are essentially giving that space to your neighbour.

Your neighbour would have to pay for the party wall surveyor Not you

If you own the property I'd suggest looking at the deeds to the house, this will usually outline what your boundary is and show clear measurements. Your neighbour shouldn't be able to build onto the side of your extension. Doing so would usually require starter bars for the brickwork which are fixed in by screws or bolts after drilling using an SDS drill, this could cause problems to the structural integrity of your extension and even damage your internal walls. If you're unsure consult a party wall expert and create a party wall agreement.

As per a recent answer to your question, you need a party wall surveyor. Your neighbour is responsible for paying for this too. If you are not comfortable with using your neighbours chosen party wall surveyor, you can appoint your own at their cost. Sometimes it’s ok to give an inch to be amenable, but only agree to things through good advice from the surveyor and don’t be bullied into anything. If you have a friendly builder, ask him to look. If you’re near Margate, I would be happy to look.