Expand the monument-protected pavilion in the outdoor area

What do you mean by that?

Don't worry, "energetisch" here doesn't mean conspiracy theorists, but the Energy Saving Ordinance 2016 :-)

Sure. In case the permit is not granted, your architect’s task ends exactly at that point. It just has to be clear beforehand what you commission him for and how he will be remunerated for it.

Is there any legal obligation to carry out the measures agreed upon in the preliminary building inquiry/building application with an architect? For example, the window, which changes the exterior appearance, does it have to be installed under the supervision of an architect, or can a craftsman’s company also do this independently?

The architect also mentioned that it would make sense to submit a building application right away instead of a preliminary building inquiry, because for a listed building, a preliminary building inquiry is often not sufficient for a decision and then documents in the scope of the building application are required. In this context, he spoke about submitting plans (including of the interior). Now I wonder, since only the exterior view is protected and here "only" a new window is to be installed, why would building plans for the interior design also be needed for such a preliminary building inquiry or building application? Could this possibly be related to the planned conversion to residential space, so that such plans are required in this context?

Don't worry, "energetisch" here doesn't mean tin foil hats, but the Energy Saving Ordinance 2016 :)

Ah, okay. Yes, the whole thing would still need to be insulated, that's true. The current insulation is insufficient...

Now I wonder, though, whether - since only the exterior is protected and here "only" a new window is supposed to be installed

Only? If it is a listed building, that "only" is a very, very, very high hurdle. Most monument authorities do not cooperate with that.

Is there any legal obligation to carry out the measures agreed upon in the preliminary building inquiry/building application with an architect?

No. According to §42 (3) of the State Building Code BW, however, you will have to appoint a construction manager who must then coordinate all trades. If you have each trade carried out by a different company, the architect is naturally suitable for this.

The architect also mentioned that it would make sense to submit a building application instead of a preliminary building inquiry

I agree. A preliminary building inquiry is normally limited to the planning law permissibility. You can save the costly building documents for the building code verifications and their examination if the project is already inadmissible from an urban planning point of view. Since you are planning a non-privileged project in the outer area, IMHO you can only rely on §35 (4) 4. Building Code
the alteration or change of use of worthy of preservation buildings that shape the image of the cultural landscape, even if they are abandoned, if the project serves a purposeful use of the buildings and the preservation of the design value,
To prove this, detailed construction drawings and close coordination with the monument authority are already required. For a start, the oral, of course not legally binding, information from the building authority that the project is not fundamentally excluded would be sufficient for me.

Now I wonder, ... why such a preliminary building inquiry or building application would also require building plans for the interior design.

Because basically you are planning a complete residential house, very small but with all trades of a single-family house. Moreover, it is a listed building, which presumably makes a change of use possible in the first place here.

P.S.: IMHO the focus of your posts is too much on finances. Your project is more for the love than for making money. Keep in mind that an unmotivated (because underpaid) master bricklayer can ruin both.

Anyway. It’s not worth it energy-wise, is it?

What do you mean by that?

Don’t worry, with "energy-wise" we don’t mean conspiracy theorists here, but the Energy Saving Ordinance 2016 :)

Exactly, energy saving ordinance compliant. A listed pavilion... in my opinion, the whole effort isn’t worth it. Insulating walls, insulating the floor slab, insulating the roof. The thing also has windows. Then hot water, toilet wastewater, etc... it’s a lover’s project.
Just take a photo :)