Room division "with little effort"

How high is the financial effort to install a drywall partition instead of the gallery in the attic in order to create a room closed off from the rest of the house?

In DIY under €1000 by far. Drywall construction is really cheap.
With a craftsman, low four-figure.

Is it possible to install a second radiator "with little effort"?

What does "little effort" mean to you? Are you okay with surface-mounted heating pipes? Then probably yes. Otherwise, chipping out, laying pipes, plastering, wallpapering, and painting. You have to do part of that anyway for the drywall walls. Still manageable.
But also think of further chipping work, e.g., for your own lamps, switches, sockets, network, etc. Also consider soundproofing and flooring.

Yes, warmth in the attic would be important, depending on how it is insulated. You write the year of construction is 2017, so one can hope that it is already well insulated. We insulated in 2017, with insulation between the rafters as well as under the rafters, a total of 21 cm of insulation. Good roof windows and on 2 of the 3 roof windows still exterior blinds and on the 3rd an interior blind (east side). I strongly recommend exterior blinds in the attic because without them the best insulation is useless. We can sleep quite well at our place, even if it can be a challenge during heat waves in the summer. However, with such continuous heat eventually nothing helps anymore. We have windows on 3 sides and then create a cross breeze. We will retrofit an exterior blind on the 3rd window when we have some cash left^^.

Hello, dear advisors,

thank you again for your advice and your time!
The mentioned house is not going to happen after all.
I am not very sad about it, because another option has come up, which I am very hopeful about. A mid-terrace house from 2005, which has 6 rooms plus a finished basement room. As it is always the case, there is another catch here, which, however, has nothing to do with the construction itself, but is of a legal nature. The house will be handed over no earlier than autumn next year, because the owners (probably expensively) are building a new house and the bank, it seems, is forcing them to sell their house now (?) in order to grant the loan. Tricky to create legal certainty for both parties here.
I will get back to you with the outcome of this matter...
Best regards!

Hello, dear advisors, Thank you again for your advice and your time! The mentioned house is no longer an option. I'm not very sad about it because another opportunity has arisen, which I am very hopeful about. A mid-terrace house from 2005, which has 6 rooms plus a converted basement room. As is often the case, there is another catch here, but it has nothing to do with the construction itself; rather, it is of a legal nature. The house will be handed over at the earliest in autumn next year because the owners (probably expensively) are building anew and the bank, it seems, is forcing them to sell their house now (?) in order to grant the loan. Tricky to create legal certainty for both parties here. I will get back to you with the outcome of this matter... Best regards!

It's actually not that tricky. My brother just did the same thing. Bought the house, paid the purchase price, and beforehand agreed in writing (I even think in the notary contract) on the corresponding rent that must be paid until the time of move-out. Additionally, a date is agreed upon after which the sellers must move out, regardless of whether their new build is finished or not. You just have to wait a little longer to move into the property, but with the sometimes limited selection on the real estate market, this is not uncommon. Get advice on the options so you know all possibilities. Maybe this is even an advantage for you, because some people shy away from it (just like you are now). But actually, it’s not a big deal...

It's actually not that tricky. My brother just did it too.

Several forum members have already practiced this here, I can't quite remember who all ... (?)

Ok, thank you very much for the hints! I am not afraid either, I would do it. But I am a bit worried about the rental agreement because tenants are rightly very well protected legally. I #möchte# and cannot simply terminate it if, for example, the developer goes bankrupt and they do not get a new home at all. Or if the fixed moving-out date is delayed by more than a year due to construction defects in the new building or delivery bottlenecks. Etc. Pp. I actually did not want to become a landlord. But I will ask the forum about it again at the appropriate time if we get the “Zuschlag.” But thank you for the encouragement, I am very interested in that house!