Purchase price determination - Acquisition of land from private individuals

You should say goodbye to the standard land value. Here it starts with double that.

we have been looking for a for building a single-family house for quite some time.
ideally, we want to stay in our municipality

That is always my advice. Focus on your favorite municipality (or neighboring towns in the same school district), don’t create concentric circles around it. If hunting success fails or is unsatisfactory, change the tactic, but not the focus.

there are many building gaps here in private hands, for which I have created a list with address, size, standard land value, and price according to BRW.

You are providing the interested reader with a good example of a tactic that you will soon change because of its inefficiency.

I am aware that most people see their parcels as capital investments etc. The market here is tight (Middle Hesse), so the price must "speak" or be attractive enough.
Can you roughly estimate from your experience how to make a proper offer? What is important in the call to avoid being dismissed right away?

You first speak of a standard value – which (if at all) can only be used in balanced markets. In a seller’s market, you will always pay more, and in a buyer’s market correspondingly less. Then you speak of a tight market – so you certainly have to offer more than this “average price for statistics fans.” I gladly give coaching for real estate hunting success, but not in the public consultation. You will always be dismissed if you contact as an “unsolicited worm.”

as an example: 5-year-old new development area, 700 sqm, BRW €330/m2, value according to standard land value €231k (at that time €240/m2 was paid to the municipality)

That speaks for an extremely sleepy municipal government if such a case can even occur. Standard-value-close transfer without construction obligation, I can’t believe it. And it is certainly not a plot accessible to a beginner. Furthermore, I share the doubts that you will receive the contact details of the owners. Everyone keeps such a carte blanche in their own safe.

A municipality that leaves the creation of building gap registers to the interested parties themselves is obviously not motivated to resolve building gaps. In this case, I would exceptionally prefer the neighboring towns – specifically one that acts more up-to-date there.

Consider what you could offer the owner of your future building plot besides money. Then you can at least soften such potential sellers who are open (and just don’t know it yet).

You won’t get into circles that want to remain closed even with larger suitcases of money.

Tips on how to get properties I have given repeatedly here in many dozens of threads - use the forum search.

We also bought our plot privately without any building obligation (the plot was already in their possession before development) and ended up, after negotiations, at about 1.6 times the official BRW.

If THIS is the plot for you, then opening your wallet wide and digging deep is the only argument on your part.

Good luck with your search, we also approached every potential seller during walks every Sunday for 6 years to ask about the situation. In the village, everyone knows each other, but I can't even imagine that in "foreign" places. For the landowners who inherited and no longer live in the village themselves, the only answers were "We are keeping the plot for our children or grandchildren" or "we don’t need the money." We would still be going door to door today.

Good luck with the search, we also approached all potential sellers every Sunday for 6 years while walking, asking about the status of things. In the village, people just know each other, in "foreign" places I don't even want to imagine it.

That's actually how it only works internally in villages, and even there according to "right of way" rules that only medicine men and rainmakers can understand with a lot of experience, and where "strangers" (meaning not even related by marriage) come last, and "complete strangers" (meaning also from outside) not at all.

With the landowners who inherited and no longer live in the village themselves, the only answer was "We are keeping the property for our children or grandchildren" or "we don't need the money." We would still be walking today from to front door.

This is not limited to those mentioned.

We also bought our plot privately without a building obligation (the plot was in the owner’s possession before development) and ended up after negotiations at about 1.6 times the official BRW.

There is no building obligation with private sales, rather the opposite: "I only sell you the property if you don't build over the view during my lifetime – practically only for your children’s house."

If this is THE plot for you, then opening your wallet wide and digging deep is the only argument you have.

There isn’t enough money to buy something that someone doesn’t want to give up. And if someone just wants to drive up the price, then usually without the slightest sense of shame.

Every Sunday while walking, approached all potential sellers

That's how it goes! That's how it can work. Direct approach, small talk, the first impression counts.
Stalking through offices does not go over well. Yes, some older people from the land registry office still know the relaxed and spontaneous way without data protection, you can be lucky. However, it is not the case that the plots are undeveloped because the landowners have forgotten this property or own too many of them and have lost track. Most leave it fallow, exactly for the reasons that says. Anyone considering selling their property contacts an agent and does not wait.
But: of course, you can also look for a needle in a haystack. Sympathy, possibly through the future neighbor, contributes to success.