Using neighbours existing extension to enclose my proposed one

Good evening Combo, I am going to make a large assumption in that your property is in England. If it is, then my first place to guide you would be here: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/393927/Party_Wall_etc__Act_1996_-_Explanatory_Booklet.pdf This is an explanatory document outlining your responsibilities under the Party Wall Act . If you have a read through it and establish which parts you believe cover your proposals, it will point you in the direction of template letters and notices which you can issue to your neighbours. Please exercise some patience and caution around party wall matters, the advice in the document is to discuss all of your proposals with your neighbours, show them plans, make them fully aware of what you wish to do before you consider issuing your notice. I say be cautious because if your neighbour does not respond positively within fourteen days of you issuing your notice then a dispute will have been deemed to have arisen. This can involve some significant delays in your works and worse still get costly. I hope that this hasn't all sounded too negative and if you get on with your neighbours then I am sure all will be just fine, but please do talk things through with them and make sure that they are ready to sign the notice before issuing it. Good luck with your project gclarkitecture