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Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
Your neghbour is certainly not entitled to put scaffolding on your land or have it overhanging, without your consent, FULL STOP.
On a side issue, although I am not a Surveyor, I would certainly have thought that as he is building the extension on a party wall, he falls foul of the Party Wall Act 1996. This Act makes it a requirement that whenever a party wishes to build on a party wall or near to it, he must serve Notice on the adjoining owner (yourself), before any work commeces. The adjoining owner then can either consent to the work, or request that his own Surveyor liaises with the building party's Surveyor to agree the schedule of works to be done, and which would also include the erection of any scaffolding.
I hope this assists and answers your question.
You could indeed refuse access to your property for them to put up any scaffolding.
Hi Nick,Can I assist you any further?Kind RegardsAl