Close with a different selling price than discussed with the broker?

but rather to warn to remain realistic despite all tax avoidance, otherwise one might have trouble afterwards.

Is there a discreet property value to installation value ratio? 330,000 to 20,000 for example, rather discreet?

Womit denn täuschen? Im Notarvertrag würden 330,000€ für das Haus & 20,000€ für Möbel/ Gardinen festgehalten.

That's correct, the tax office will have to be able to read that themselves.

We had to specify to the notary in quite some detail which items were taken over for how much (as of 2022). This was attached to the contract as an annex. When the condominium was purchased in 2001, a simple note stating that items worth … were taken over was sufficient.

T€ 350 = 88% T€ 350 = 78% Where do the 10% come from? Does the house now cost T€ 400 and the fittings T€ 50 in your example?

No, the house costs T€ 450 and the accessories T€ 50 = purchase price T€ 500. But as a bank, you separate market value/sales value and lending value. These are different quantities, which are also defined differently. For single-family houses or condos, it is common to calculate the lending value with a 10% deduction from the market value (purchase price). It gets really interesting with income-producing properties, such as commercial real estate (from multi-family houses or offices or hotels, etc.). There, the purchase price = market value can easily be twice as high as the lending value. So with a 50% equity investment, you still have to pay financing costs for 100%.

Is there a discreet property value to installed value ratio? 330,000 to 20,000 e.g. rather discreet?

It also depends on what is included in the 20,000€. For example, if only a kitchen is specified, I would personally find a value of 20,000€ for a used kitchen strange in relation to the house price. But I am not the tax office. No idea when they approve and when they become suspicious, it probably also depends on the individual persons in charge.

a value of €20,000 for a used kitchen in the house price is strange. But I am not the tax office.

There are depreciation rules (AfA) for that.