Costs of a real partition and subsequent development?

In which federal state is the property located?

We recently divided a property in NRW. The official subdivision survey alone cost us over 4100 euros. However, our property is only 241 m2 in size. The costs are always based on the smaller newly formed parcel.
In addition, there were 1000 euros for the subdivision permit and fees for the change in the land register.

A subdivision may result in necessary easements such as a right of way or rights to lines, which must be notarized and registered in the land register. This also costs money.

Of course, a public road leads up to the property boundary and somewhere there are also the respective supply lines.

That is called developed/fully developed

Nevertheless, these have to be continued on the property up to the actual location of the house.

That is called connection to the utilities… but I have already described that.

Would one bury own lines and pipes in a pipe manner and then connect them to the public network in the area of the street? (I guess the latter?)

No, you may dig yourself if applicable, not always, but connections are made by the utility providers. You have to apply for that and it concerns every house construction.

Therefore, the function of a project manager is unclear to me.

Look, we don’t know him. Probably the residents of the house, i.e. the owners, are somewhat uncertain and have commissioned someone. Of course, you may ask him!

Nothing is being torn down on the property.

And why are there demolition costs? Or does that concern the currently overgrown and later access path? And why don’t the owners pay for such things?
They should either sell everything neat and unused or correspondingly at a reduced price if you are supposed to pay for completing the access path (demolition).

Do you simply extend the supply lines from the already existing house to the new house, or would you bury your own pipes and lines in a trunk style and then connect them to the public network in the area of the street? (I guess the latter?)

The extension of existing pipes from one house to the next has not been done for years and is reversed again during construction work. The supplier builds its own lines from the new house to the connection point. This does not have to be the direct way to the street; it can also be, for example with telephone lines, the next distribution box. This ensures that each house has the full capacity available.

In which federal state is the property located?

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A subdivision may result in necessary easements such as a right of way or utility rights, which must be notarized and registered in the land register. That also costs money.

The property is located in Hamburg. No, it is a flag lot with its own access to the street.

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No, you might be allowed to dig yourself, not always, but the utility providers make the connections. You have to apply for that and it applies to every house construction.

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And why are there demolition costs listed? Or does that concern the currently overgrown and later access path? And why don’t the owners pay for something like that?
They should either sell everything neat and fallow or sell it at a discount if you are supposed to pay for the completion of the access path (demolition).

The question was not about doing the work yourself, but rather where the supply lines will physically run. Intuitively, I would say on the property itself, meaning under the access way "in the flag lot" up to the street. Alternatively, in my naive view, you could also lay them to the other house if that is shorter. But I guess technically that might not be possible, the other owner might not want their property dug up, and legally it might also be difficult if your own supply lines are on someone else’s land.

Yes, the future access path is heavily overgrown with bushes, but not wild; the property is maintained. In general, a lot of vegetation would have to be removed or at least relocated for the construction.

So I have learned that most of the above list are standard incidental construction costs and logically occur with every property, but that removing vegetation lowers the value because a normal building plot is usually sold fallow?

A public road leads up to the property boundary and somewhere there are also the corresponding utility lines. Nevertheless, these must be extended on the property to the actual location of the house. And it is exactly about those costs here. But that brings me to another question: Do you simply extend the utility lines of the already existing house to the new house, or would you bury your own lines and pipes in the style of a pipe stem and then connect them to the public network in the area of the road?

Thanks to the pipe stem, you do not need a right to lay your lines on the land of the front neighbor. Usually, it is the municipal utilities that have an advisory office for how the installation is carried out, where to submit applications and orders, and the like. They often handle water and wastewater themselves; for connection to the electrical grid, you must use an electrician registered with them as a concessionaire; and gas is increasingly optional according to one’s own choice; conversely, sometimes there is a mandatory connection to district heating networks (but usually only in new development areas). The downside of the "own way" is that it naturally also serves as the construction site access at the same time.

From my understanding, there are three major ways to build a house: I go to an architect, buy a prefabricated house, or go to a general contractor who builds the house. Until now, I assumed that no matter which option I choose, the questions about access roads, utility lines, construction site access, etc. would be planned and calculated exactly by that party.

A kind of "project manager" is also recommended for you, see above the coordination regarding the development through the construction site access. For some tasks (clearings, felling including hedges), coordination with the nature conservation authority may be required, and protective periods must possibly be observed.
I would never link an architect and a general contractor with "or," even though this costly mistake is a classic and practically a national sport. In "A House Construction Roadmap, also for You: The Phase Model of the HOAI!" I explain in detail which alternatives of smart approaches I recommend as clients. Take an architect even if the plot is not a triangle on a steep slope, and even if you do not want a designer house. His fee for performance phase 5 is best invested; it only looks expensive but pays for itself; and a proper tender also pays off. And no matter how sweet the word "fixed price" may sound, never commission a general contractor without a tender!
By the way, prefabricated houses are only as finished and solid houses are only as massive as a lemon butterfly folds lemons. A dog biscuit is a biscuit for the dog. An entire economic sector lives off the naive misassociations certain words generate in the ears of clients. "Turnkey" is just the tip of an iceberg called buzzword bingo. But somehow, I and other independent client advisors have to make a living as well.

By the way, it does no harm to deal with the plans for the now unbuilt semi-detached house. If there is already a preliminary building inquiry or even an approved building application for it, valuable clues can be derived from them regarding a buildable building body of the house, its location, and the position of supply and disposal lines.

If... so if you alone are to bear the costs of the division and the removal of the vegetation on the future driveway, then this should be reflected in the purchase price.