What significance do standard land values have?

Sure, but a simple numerical example to explain the background of my question (purely hypothetical assumption):

Total budget: 600,000 euros (100,000 for the land and 500,000 for the building)
Equity capital: 300,000 euros (50% loan-to-value, right?)
Loan: 300,000 euros

Now find a plot of land for 85,000 euros. Incidental costs simplified to 15,000 euros (notary, land register, taxes, commission)
You take the 100,000 euros from the equity capital. For the later financing you would have:

Total budget: 500,000 euros (for the building)
Land: 85,000 euros
Equity capital: 200,000 euros
Loan: 300,000 euros

Both variants are identical, aren't they?!
Equity capital commitment in both examples EUR 300k - in the first example €300k cash / in the second example also, here cash is divided into an already purchased plot of land and incidental purchase costs.
Loan-to-value ratio in both variants is identical (in the first variant, the incidental costs share would still have to be deducted for the loan-to-value ratio, since this share does not affect the value of the property).

Both versions are identical, aren't they?!

Only if you take the already paid additional costs into account in version 2.

Only if you take into account the ancillary costs already paid in option 2.

This does not matter at all for financing, whichever option you choose. In the lending assessment, ancillary costs play no role in either option – the financing amount is also identical, as are the loan-to-value ratios!

Total budget: 600,000 euros (100,000 for the plot and 500,000 for the building project)
Equity: 300,000 euros (50% loan-to-value ratio, right?)

From this, the loan-to-value ratio cannot be determined. Hypothetical example:
Plot 100,000 euros
Ancillary purchase costs 7,000 euros
Construction costs house 350,000 euros
Various other individual costs 50,000 euros

Total budget: 507,000 euros.

Equity: 200,000 euros.
Debt capital 307,000 euros.

Loan-to-value ratio roughly:
Debt capital / (Plot + House) = 68%.

As already said: It does not matter whether you bring in the money in cash and then pay for the plot, or whether you bring in the plot itself (as long as the numbers remain unchanged. Tricky: Cash is worth exactly as much as the amount states. There can be differing opinions on the plot value).

Note: Typically, the bank has a different idea of the "total budget" than the builder. Because the bank completely ignores many costs that the builder has to bear ("these are supposed to be paid out of equity anyway"). That means, of the equity actually available to the builder (what the builder sees in their account), only a part is available for determining the loan-to-value ratio. The rest evaporates elsewhere.

There is no room for interpretation regarding how the bank values the property, since the bank is bound by the BelWertV. It is relatively clearly regulated there, for owner-occupied residential properties using the cost approach, what the bank has to do.

Thank you for the explanation! For the bank there is a standard procedure. For the buyer it looks different and therefore there can be different opinions (nowadays usually to the detriment of the buyer, who has to pay more than the bank considers it worth). That is what I wanted to point out.

Simply sqm x BRW is therefore not...

Exactly!

That means, conversely, that one is generally worse off with property (paid for with equity) than if one can still show the planned investment in cash.