Difficult Plot and Monument - §34

I would not call it a problem property, but rather a lover's property.

After a night's sleep, you have me. Is there a possibility to change the title? I can't find an edit function. Otherwise, I would name it: "Lover's property with half-timbered monument and §34 development - first idea collection" I want to make people excited about the project.

I generally think the idea of building within an existing structure is great; you like the found property and you have a trustworthy architect with relevant references, so you are already riding high in this part of the forum, because the architect will also enjoy creating another reference project for herself and overcoming the existing challenges.
Such a project is therefore not comparable at all to the usual constructions of a single-family house on open land through a general contractor or a prefabricated house company.

P.S: I also really like split-level houses. That would probably make more sense with my elongated floor plan, then the stairwell would have to be in the middle.
What are the advantages and disadvantages?

Disadvantages can always be financial in nature or of a practical kind. Often, beautiful things are simply not practical but beautiful, and with such a property there will (hopefully) be a lot of beautiful and individual parts, which ultimately will not always be as practical as possible in a usual single-family house. For that, it can (and should) carry your very personal signature, and that applies equally to split-levels as to visible beams, etc. Such a house should especially receive elaborate planning/design on the inside; then it will be a gem.
You miss further participation here, but that is currently hardly possible since one is still in a vacuum until the first draft or at least a hand sketch within the framework of the possible building method is available.
You want to refine such a piece, then you will also be able to live well with and arrange yourselves with the fact that not all desired "standards" can be implemented. I would start by discussing basic ideas with the architect and have their feasibility checked by the authorities; then maybe start a new discussion or it will become more active on its own when it becomes concrete.
In this respect, I also believe that topics like car parking, terrace(s), storage rooms, access paths, etc. must only be implemented as a whole and maybe also in more unusual ways in order to create a beautiful overall structure.
I am already very curious and would be glad if someone brings their project here again with an open mind and as a return also receives the often important experiences of the participants.
It is not because of the name of your thread.

I basically think the idea of building within an existing structure is great; you like the found property and you have a trustworthy architect with the appropriate references, so you’re already riding high in this forum’s niche area, because the architect will also enjoy creating another reference project for herself and overcoming the existing problems. Such a project is therefore not at all comparable to the usual construction of a single-family house on open land through a general contractor or a prefab house company. ...

Thank you for your wonderful words! You are right, what are you supposed to do at the moment? My impatience stems from the uncertainty about the possible development. How are you supposed to take action when you have even less information than I do? The architect’s plan with the very rough exterior shell is already with the building authority and the lower monument protection authority. We have agreed to simply see what is possible and then to get the best out of the given conditions. I am also deliberately leaving the construction costs open because we just want to fulfill the dream of our life (family still takes a bit of priority). That does not mean that we have a huge space requirement; we just want something unique and suitable for us. All involved are very excited about the project; our architect is already enthusiastically imagining the front courtyard character through a wooden facade. For me, it’s always just the rational alarm bells ringing, wondering what the building authority will say. Yesterday, for example, I could not sleep because of the northern neighbor who built directly on the border. If now the building authority demands 2 * 3 meters from the border, our plot will be unbuildable. I hope such cases do not exist in reality...

Hehe, the mental cinema will have you build a series of different houses, which will then be prevented by the building authorities, neighbors, or some other disruptor. With such a dedicated architect, who also personally enjoys it, it will become a beautiful home, probably completely different from what you can imagine now. For some, that would mean stress, but I can understand and share your enthusiasm. If something doesn't go as planned, whether because of neighbors or building authorities, you shouldn't despair or be sad, because that in turn offers new paths you hadn't thought of (or had to think of) before. A friend of ours renovated a three-sided farmstead and got stuck somewhere again or had to give up original "dreams." But I experience him as very satisfied with his project and his awareness of creating something individual. People used to live differently there, so how could it be possible today to have a "modern" life there without any restrictions; that's just not possible. I always find it inspiring when people live differently than what is otherwise taken for granted. Such a building, half monument and half modern, is something great, and I would also like to use consciously chosen building materials in it. Looking forward to the first draft.

We are a family of four and have now found a nice hillside plot in NRW. [...] We are planning together with an architect whom we trust very much because she has really built and renovated some great houses within our circle of acquaintances.

Building within an existing structure is a whole different league of challenges, and within that, a half-timbered monument is a completely different matter than transforming a sixty-year-old house from oil heating to heat pump era. NRW is large; in Münstereifel there is and in Meschede Mrs. Kohnen ("Kittybob") also lives herself in a listed building. I gladly recommend both.

You miss further participation here, but that is currently barely possible,

... in my case, the overview is also missing: the "to be demolished" house in the opening post at the back looks like it is positioned with the eaves side facing forward in the picture, but in the cadastral extract / site plan it has a rather gable-facing format. I am also missing an aerial photo or cadastral extract with about 100 m image diagonal and photos of the situation of the garden floor described as level on the uphill side. I could imagine leaving the guest and workroom cottage standing solitarily, thus not necessarily wanting to connect it to a shared hallway area. That could also have the charm of considering it as an independent building and regarding energy renovation obligations separately and under different conditions. Shared connections / heat supply etc. are not necessarily a hindrance to that. But these are, as said, all aspects where expertise in "old" or "very old" houses is a completely different matter.

Building within existing structures is its own league of challenges, and a half-timbered monument within that is a whole different ballgame than beaming a sixty-year-old house from the oil heating era into the heat pump era. NRW is large, in Bad Münstereifel there is and in Meschede Mrs. Kohnen ("Kittybob") also lives herself in a listed building. I gladly recommend both.

Thanks for the tips. Bad Münstereifel is really not far away.

... in my case, the clarity is also missing: the "to be demolished" house at the back in the opening post looks eaves-aligned in the picture but has a rather gable-aligned format in the cadastral extract / site plan. I am also missing an aerial photo or cadastral extract with about 100 m image diagonal and photos of the situation of the garden floor, which according to the description is level on the mountain side. I could imagine leaving the guest and work room cottage standing solitarily, i.e., not necessarily wanting to connect it to a common hallway area. That could also have the charm of considering it as an independent building and being able to treat it separately regarding energy renovation obligations and under different conditions. Shared connections / heat supply etc. are not necessarily an obstacle to that. But these are, as I said, all aspects where expertise in "old" or "very old" houses is a different matter.

I am attaching an aerial photo. The demolition house stands eaves-aligned as in the photo. However, it was extended by an annex, which is why it appears gable-aligned in the cadastral extract.
Leaving the cottage standing solitarily would only be an emergency solution if the monument office requires it. I definitely want to integrate it into the new building because then it is used significantly more often. Also, a toilet would have to be installed there, otherwise the guests would not be pleased ;-)