Filling and compacting the ground for the house

Yes, because that's just how it is ... I even called again to check. But hey, all the better. Otherwise, I wouldn't have come across the current company, where I get the material for free and have it installed for 2000-2500€.

Congratulations that everything has turned out well for you. But a yes just because it is that way does not exist and may unsettle readers and other home builders who then confront their construction companies with this and who then feel a bit fooled, which puts strain on the relationship from the start.

Imagine you order a phone from Amazon for 300 euros, and the invoice then states 1000 because 300 is only for delivery... No one would probably come up with that idea.

If there is an invoice for exterior or garden installations, as far as I know, labor costs, I think about 1000.-, can be deducted from taxes

You can claim household-related services and get 20% back. In the case of garden work, however, only after moving in. And always make sure to have an invoice with a bank transfer, otherwise it will not be recognized.

Can I only claim 20% or do I generally get it back?

You can deduct a maximum of 20% but 4000 euros directly from the tax liability.

As far as I can see, Z0 material may be quite contaminated with lead, chromium, arsenic, and other substances. Would I really want to have this dumped on my property?

NO.

Because I would hardly want to live on something like a hazardous waste landfill. And if things go really badly, it could suddenly be discovered that this material is contaminating the groundwater. And then?

This is meant just to be a food for thought.