Expand the monument-protected pavilion in the outdoor area

So 20€/sqm is actually nothing extraordinarily unusual for Heidelberg, if you look around in the search portals, not even for 1-room apartments. And the pavilion is very nice, detached, plus there is a part of the garden that cannot be seen which can be used by the tenant alone, best location, etc. I still want to take your objections into account and initially calculate with 500€ rental income.

to partially cover the maintenance costs of the property.

It all goes towards the loan anyway...

Ground area 18m2, the attic is also to be converted (another approx. 12m2 area)

Uh, isn’t there a minimum square meter requirement for a rentable unit?
Anyway. Energetically it’s not worth it, right?

Since our property is located in one of the most popular parts of the city, on the edge of the forest, and also very central (1.5 km to the city center), I (and the local architects) expect a rent of 500-600 euros.

Since when does a student count as forest edge? The money has to come from somewhere...

So 20€/sqm is actually nothing unusual for Heidelberg

Quickly googled: normal student apartments (usually furnished): 25sqm room at about €350 cold rent and for apartments around 30sqm at about €450

You first have to compete with that – just because of the forest. Why should I go there if I can have it somewhere else (maybe more central)? But it cannot be judged due to lack of location information.

Oh yes, do you have to finance or pay from equity? Since you want to reduce the costs for the house/land, I assume it's a loan, otherwise you wouldn't be dealing with it. How does the situation look with the bank? Such a small loan ([subordinated in the land register?]) costs a lot of money and significantly lowers your return, even with €500 rent.

Thanks again for your assessments regarding the achievable rent. €500 is really realistic here, I am already renting out parts of the main house to students and have a pretty good impression of what people are willing to pay. I think it doesn’t make much sense now to justify the rent any further, because as nordanney already said, it is difficult for outsiders to assess without location information.

So if I go back to my original question, what do you think, is there a way to "scale down" the architect’s share of the project, or is it possible to carry out the change of use application and building preliminary request with an architect and exclude them afterwards once approved?

Oh yes, here are the answers to the questions:

Oh yes, do you have to finance or pay from equity? What is the situation with the bank? Such a small loan (subordinated in the land register?) costs a lot of money and strongly reduces your return, even with €500 rent.

I have €30k equity and would then have to take out another €45k as a loan. So far, no loans have been taken out on the house/land.

Anyway. It’s not worth it energetically, right?

What do you mean by that?

what do you think, is there a way to "cut down" the architect's share of the project or is it possible to carry out the change of use application and building enquiry with the architect and then exclude him after approval?

Sure. In the event that the approval is not granted, your architect's task also ends exactly at this point. It just has to be clear beforehand what you are commissioning him for and how he will then be compensated.