I agree with Woofters. He can not "attach" structurally to anything of yours without your express permission.
The pergola is out of order, as is the outbuilding containing the pizza oven. They may well be allowable under "Permitted Development", but should be "self-contained". The back wall of the oven must be unattached to your garage.
While he may be allowed to do these without Planning Permission, subject to what I said above, he would need "Building Regulations" approval if the outbuilding is within 1 metre of your boundary.
Also, if the pergola and attached oven-house is over 30 square metres, then he would need Building Regs even if he was 1 metre from your boundary.
Speak to "Building Control" at your local Planning Dept.