Digging to 3m deep near public sewer

Personally I say as you are 2 meters away from the sewer drainage there would be no need for any kind of build over agreement . If you made contact with water board in your area I think you wouldn’t need anything as long as during excavation of the footings being 2 m away and 3 m deep from sewer You just need to totally make sure that you excavate carefully safely without disturbing any of the ground towards the pipe to place your Footings for extension If you only need a footing 300mm depth by 600mm width Because the sewer is 3 m deep and 2 meters away from your new footing You wouldn’t need to involve water company at all. Really depends on how good the ground is and how deep you need to go, But being a single story Extention You should be absolutely fine . Your contractor would need to make those decisions once he does his test dig to ascertain The depth of footing on your home and how good the ground is .

At those depths I would recommend it to be piled rather than a conventional strip footing

Phone building control and they will have the right answer to your question.

As long as your 3m away from the pipe it does not concern the water board. If you are less than that then you will need to agree with them or contest it.