Floor plan of a new country house in a large garden according to §34 (with demolition)

Does anyone have a rough idea of how big the house needs to be to accommodate a closed kitchen and a guest room on the ground floor?

60-70 sqm per floor are more than enough for that 12 sqm kitchen, 10 sqm guest room, 25-30 sqm living/dining room, 4 sqm guest bathroom, 10 sqm heating/utility room, and then a few more sqm for the hallway. On the upper floor, three rooms plus a bathroom are possible. If the upper floor is a full story as planned, a nice studio is still possible in the attic with a standard gable roof. So overall, with 2 floors and 3 rooms, it is livable. You just have to want it...

Does anyone have a rough estimate of how big the house needs to be to accommodate a closed kitchen and a guest room on the ground floor? One of our core problems was that the house always seemed too small on the ground floor and then rather too big upstairs.

Occasional overnight guests per year

Honestly: occasional overnight guests could take a taxi home, sleep on the sofa, or use a sofa bed in an 8 sqm office.
And a closed kitchen in a double-row or L-shape can be achieved from about 10 sqm.
You are planning a kitchen over 16 sqm without added value. Quite the opposite: it is not ergonomic. The counter can't be used because it’s too tight against the wall, and the bottom right area is unnecessary space.
And then there's a pantry, which you don't necessarily need, but only want just to follow the mainstream trend of walking through the kitchen cupboard.
By the way, the closed kitchen with a counter is the most space-consuming of all kitchens.

Thanks again for the feedback. We had of course already considered the floor plan a bit, but admittedly it is somewhat wishful thinking.

The question for us now is: what next?

As mentioned, we have already been in contact with prefabricated house providers and had said that we would get back to them. Now you are probably right in saying that they will have little interest in seriously dealing with the floor plan for free.

Does anyone have a tip for a sensible approach? For example

[*]finding an architect and getting the floor plan done properly and then looking for a provider with a finished floor plan?
[*]going to the providers with the floor plan and getting another offer / hearing what they have to say?
[*]choosing a provider and entering the planning phase with them?
[*]... ???

If we have understood correctly, the prefabricated house providers only go into planning once the contract has been signed? We have also been in contact with two other providers who would charge separately for the planning phase, €2000 and €8000 respectively.

Does anyone have a tip for a sensible approach?

Take a look at the house-building schedule from (use the search function). Or under the keyword "Teigruhe" you will surely find various threads as well. He will probably also add his "two cents" right away...

As mentioned, we have already been in contact with prefabricated house providers and said that we would get back to them. Now you are probably correct in saying that they will have little interest in seriously dealing with the floor plan for free. Does anyone have a tip for a sensible approach? For example:

[*]find an architect and have the floor plan properly done and then look for a provider with a finished floor plan? [*]go to the providers with the floor plan and get another offer / hear what they say? [*]choose a provider and go into the planning phase with them? [*]... ???

If we understood correctly, prefabricated house providers only go into planning once the contract has already been signed? We also had contact with two other providers who would charge separately for the planning phase with €2000 or €8000.

Take a look at the house building schedule from @11ant (use the search function). Or you will surely find various threads under the keyword "Teigruhe". He will surely also add his "two cents" right away...

You can find my "A house building schedule, also for you: the phase model of the HOAI!" (because it is external) best together with the quotation marks. The Teigruhe is explained there and also here. Then 1. find or have someone find an architect (also stated there) and not a "finished floor plan," but work through "Module A," the "preliminary draft" is the interim result, with which you make the key decisions during the Teigruhe, so 2. go to a suitable selection of providers with the preliminary draft (a total of about half a dozen, two prefab brick builders, two timber builders, and a third from one of the two groups; providers from the first round can also be included in the selection); 3. continue with the free architect into performance phase 3 or the entire "Module B" with the result of the key decision phase. If you conduct the key decision offer request round according to my pattern, question 2 concerns similarly possible proven designs (catalog houses, customer houses). Separately billing the draftsmen of the general contractor only makes very limited sense. The problem is less with paying the so-called "acquisition phase drafts" but with the unclear probability perspective of closure.

I haven't read anything about a slope. Or overlooked it.