Do all rooms need to be heated?

Insulate walls to the living area, install an airtight door so no moisture passes through, and design the storage room to be diffusion-open. The attic of immediately comes to mind.

We want to visit an existing house that is quite old and very large. Over 200 sqm of heated area, over 100 sqm of unheated area. Unheated rooms are, for example, a former storage, a former workshop, a vault room, etc. The thought of having to make the whole house heatable or having to heat initially unused rooms as well scares me.

You must adapt the property to the relevant thermal protection regulations after a deadline following the transfer of ownership. You can exempt rooms not intended for occupancy, but you must then also separate them from the "thermal envelope." Basically, I assume the burden of proof lies with the doubter if someone should not believe your personal need for so much storage space. Living and occupancy rooms behind that must not be "captured." Is your freight forwarding company / savings bank or whatever it was located in an area where you could also rent out storage space? (probably not in a WR or WA).

if the house is already older and has not developed mold so far, it will not do so in the future either. You just have to be careful not to make mistakes during the energy renovation. Whether renovations are required and checked during a change of ownership, you have to ask the chimney sweep. However, individual unheated rooms are not a general problem, so it is not necessary to install heating everywhere and to warm everything to at least 15°C. For example, you can also install a wood stove with an external chimney in the workshop that is only fired up when needed.