My new neighbour is building something under his house.. it is illegal building work - what to do?

They're new neighbours. I've introduced myself but they never talk and they run away when we are outside. (LOL). They're building like a granny flat under their house or something without building permits. If it is a flat it will impact on my amenity by reason of the increased density, noise and... show more They're new neighbours. I've introduced myself but they never talk and they run away when we are outside. (LOL).

They're building like a granny flat under their house or something without building permits. If it is a flat it will impact on my amenity by reason of the increased density, noise and parking requirements.

Should I dob them in to the authorities and let them handle it - putting my name on the public record? What would you do?
Update: I'd love to help them, by making them get approval. I can't help them build illegally..... I'm bound by a code of professional conduct as an Architect. It's a simple little addition involving lots of digging. There is a risk to the house they adjoin. There is an environmental risk in the way the... show more I'd love to help them, by making them get approval. I can't help them build illegally..... I'm bound by a code of professional conduct as an Architect. It's a simple little addition involving lots of digging. There is a risk to the house they adjoin. There is an environmental risk in the way the work is being performed. There is a risk that they will be unable to sell their property under our current laws... and there is the risk to future owners that the work is faulty if undetected. Bricks are being delivered for some underpinning. There is a risk to themselves and to the neighbour whose house shares a common wall.
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