Invoice for water connection despite payment through property price?

As you present it, it can only be about the point "Vorauszahlungen". That simply means you have made a down payment, more may still come. It does not say that you have paid.

This is how it works with us:
When purchasing land, the municipality immediately demands a fixed amount of about €2000 for sewer and water, followed by an annual so-called availability fee of about €400, which is then credited back upon construction for 5 years retroactively to the actual connection fee of €2500.
So anyone who builds within 5 years only pays a remaining amount of €500.
The municipality wants to prevent people from buying land and never starting construction. Anyone who builds only after 10 years or never pays about €400 annually and never gets it back, except through rising land prices, which is unrealistic in our small town.

I also suspect that the word advance payment will backfire on you. Because it is nothing more than a flat-rate partial invoice for a service that can only be specifically calculated later.

For us, the development of the land and thus the connection to the municipal water and sewage network were included in the purchase of the property. During construction, the pipes were extended up to the multi-utility entry point, which you have to pay for yourself.
For electricity, for example, this is not the case, which is why a few meters of cable can cost several hundred to a thousand euros...
What we had to pay additionally ourselves was about 800 for the water connection (6m + faucet) and about 2000 for electricity.

As you present it, it can only be about the point "prepayments." That only means you have made a down payment; there may still be more to come. It does not say you have paid.

I also suspect the word prepayment will backfire on you. Because it is nothing other than a flat partial invoice for a service that can only be calculated concretely later.

That is clear to me – a notice from the municipality will also come when the construction measure is completed. The price is defined very precisely: about €1 per sqm of land area and €7 per sqm of living floor area.

However, in ADDITION to that, we have received this invoice.

With us it works like this:
When buying a plot, the municipality immediately demands a fixed amount of around €2,000 for sewer and water, then an annual so-called provision fee of about €400 comes, which you then get credited against the actual connection fee of €2,500 for five years retrospectively when building.
So anyone who builds within five years only pays a remaining amount of €500.
The municipality wants to prevent people from buying plots and never starting to build. Anyone who only builds in 10 years or never continues to pay around €400 annually and never gets it back, except through rising land prices, which is unrealistic in our small town.

Interesting approach! For us, it’s the other way around: If you don’t build within five years, there is a hefty penalty, and the municipality can reclaim the land...

Making does not automatically mean paying.

I feel confirmed in my suspicion. When buying it was a construction cost subsidy and now the connection of the plot. Only the lack of transparency of the price of the connection is annoying.

The word construction cost subsidy does not appear anywhere here. The statute’s definition very precisely says that the line runs up to the transfer point INSIDE the building.

The contribution rates are:
“The contribution amounts to a) €1.00 per sqm of land area excluding VAT, b) €1.00 per sqm of floor area excluding VAT” according to the statute.

We received an invoice from a civil engineering company for a total of nearly 20 meters of connection line, of which everything beyond 10 meters is charged. But we find no indication of such a regulation anywhere in the municipal statute, the purchase contract, or anywhere else.

If the amount is not exorbitant, I would probably pay for the sake of communal peace. After all, you want to live there. Maybe you just openly address the matter and ask the building authority if they miscalculated or what the issue is?

Something similar has also crossed my mind. I have already spoken to the municipality in person twice; two neighbors have also done so. The only answer is always "That's just how we do it." No exact basis for this procedure can be named or shown in writing.
They then showed me a cost acceptance form for an "overlong water house connection" that all builders supposedly had to sign. Neither my neighbor nor I knew this form, nor did we sign it.
I built with a local construction company – I showed the form to the site manager, who said he had never seen it in all his years there.

I have also spoken several times with the managing director of the civil engineering company. He was clearly frustrated by the situation and told me “since some builders overdid the connection length,” the municipality has now decided to charge everything beyond 10 meters to the owner.
But this can hardly be correct in terms of procedure.

I have also asked the civil engineering company several times for proof of order. They said they were commissioned by the municipality.

To me, honestly, this looks like a shady beer tent agreement, which in this manner borders on an attempt at fraud.

Regarding the amount itself, it is not just a few euros but about €1,800 – that is a family vacation. I have ZERO problem paying an invoice if it is justified. It simply does not make sense to me that within four weeks I receive no written information (or any information at all) about the legal basis of this invoice. I finally demanded the municipality to respond in writing. Then I received a phone call offering to gift me some meters of connection length. That smells to me like "hush money."

My tip: They don’t know either. The "we’ve always done it this way" has become so entrenched that no one remembers what would be legally correct – so far there have apparently been no inquiries.

1,800 loops are unfortunately a bit too much to just keep quiet. That’s why I also believe in the hush money.

Imagine 50 other builders find out about your case and try to hold the authorities accountable afterward... they would be scared.

This is my personal opinion, which of course could also be wrong.