Insulate cold roof attic for office

What do you mean by "partially"? Separating a room with drywall and insulating it?
Or a bit of insulation everywhere?

And by itself, it won't get warm despite the insulation; it just keeps the heat longer or better inside the envelope.
Is there a heating system or an option for one (other than electric) available?

Insulating the attic as a DIY project is basically not a problem, only the (in my opinion nicely done) cold roof structure is problematic to retrofit :-( due to the missing vapor barrier.

Can you share the details of the roof structure?

That's exactly how I imagined it. We have underfloor heating downstairs and ceiling heating/cooling via a horizontal collector... So I think there's no option, unfortunately, except to connect an electric heater, or just let the heat from below rise through the attic hatch?! But that wouldn't really be a proper solution either. However, the window wouldn’t be a big problem in my opinion; I marked it in the attachment, it can actually be installed there quite easily. Photos of the attic structure in the attachment, instead of the EPS, the OSB boards have already been laid.

Excuse me, you left out the insulation under the OSB?

You should also check whether the attic is suitable for heavier loads. Our [GU] explicitly points out everywhere that there is an extra charge if you want a reinforced attic. So a few boxes are okay, but a desk and office equipment also weigh a bit in certain spots....

How come you left out the insulation under the OSB?

on the insulation boards*, not instead, at least I think that's what I remember... an independent expert of mine was on site anyway, and I think he would have vetoed that, he was ultra precise. Thanks, I will ask about the load matter

except for unfortunately connecting an electric heater

Do you have a sufficiently sized electric cable going up? Usually, only a single cable for a lamp and at most one socket goes up.

Additionally, you need internet in the office, right? There won't be any network cables lying in the attic, will there?

How is the access supposed to be made? If the attic stairs are open, they will probably be in the way somehow?

could contribute here. He can build everything himself and wants to help.