Building on family-owned property - green space with ground monument

it is a heritage site

If it should be a heritage site, then there should be something archaeological there. Is it not known what could be there? The term definitely does not come from the swamp.
My research (why the streets are always obscured)
The whole village seems to be a listed building. No, seriously: just research your village with all the historical stuff that happened there (brickworks and so on, much of it is in Wikipedia): it is all a local recreation area, so close to the lake. The property includes some holiday apartments and houses.

In my opinion, there is a development plan because of the newly built houses that have appeared nearby in recent years

Is there one or isn’t there? Ultimately, it does not have to affect your green area, what is on the other side of the street. Development plans are very often limited by roads. Left and right of a main road are usually considered separately.

Since this adjustment, we have been mowing the property regularly; this was a recommendation from the former building board of the municipality so that the property does not become overgrown.

Does that mean in your scenario that it is your land? How long has it been in the family’s possession?

The plan is to take a roughly 1000 sqm plot from the total area and build a city villa of about 140 sqm,

The plan could work if, for example, the upper area at the street is developed and you align yourselves with your piece.

Only an inquiry at the municipality will probably bring a result

I would have done that long ago. There are enough arguments (provided the frog does not still live there).
Possibly your family was a part of the history there, so you can also use that argument.
However, I estimate it will take a few years.

Up until 7 years ago, the plot was still designated as a nature reserve

Then there is 100% no development plan, unless one has been drawn up in the last seven years. And then you could definitely check it online.

So go to the building authority and find out if there is any chance.
New buildings in such locations (outer areas) almost always have something to do with old farmsteads. Building completely new is almost always impossible, as urban sprawl is to be prevented.

If it should be an archaeological monument, then there would have to be something archaeological there. Is it not known what could be there? The term certainly does not come from the swamp. My research (why is the street always obscured) The whole village seems to be a listed building. No, seriously: research your village with all the historical things, what happened there (brickworks and so on, much is in Wikipedia): it is all a recreational area, so close to the lake. The property includes some holiday apartments and houses. Is there one or is there none? Ultimately, your green space does not have to be affected by what is opposite the street. Development plans are very often limited by streets. Left and right of a main street are usually considered separately. Does that mean in your scenario that it is your land? How long has it been in the family? The plan could work if.. e.g. the upper area on the street is built up and you align yourselves with your piece. I would have done that long ago. There are enough arguments (unless the frog still lives there) Perhaps your family was a part of the history there so you can also use that as an argument. However, I estimate it will take several years.

Hello, The village had a brickworks and various gypsum quarries in the surrounding villages, the SS had caused trouble there, It is not known to me or my grandfather that something is supposed to be there (the brickworks is quite a way from the property) About ~50 years ago, I think, fruit trees were planted there, that is all we know. The land use plan in the municipality is from 2017 how often is something like that updated? As said, a development plan for the area has been applied for but I do not have it and yes, the property has been in the family ownership for many decades Because we have many plots, which are mostly used agriculturally, and I am the first grandchild who wants to build, now the thing is gaining momentum. Hence, visits to the office are now gradually pending. Maybe I'll pull out the old family tree, maybe the history of my family will be enough : D Thanks for the answers

The land use plan in the municipality is from 2017, how often is something like that updated?

When needed. It can take up to 30 years.

As I said, a development plan for the area has been applied for

Who applied for it and why? There is actually a formal development plan procedure for that. You can’t just “apply” for one. If it exists, you can view it in BB online.

Where exactly were new houses built? Can you mark them on your sketch? It would probably be crucial if there were new buildings on your side of the street.

If needed. It can take up to 30 years. Who applied for it and why? There is actually a formal development plan procedure for that. You cannot “apply” for it. If it exists, you can view it online in BB.

Hello, I think the site was called Bebauungsplan-online, we accessed it via the parcel numbers there And then once again informally requested access through the municipality after consultation with the municipal administration at the office, that was this Tuesday.