> No issue unless you fix it back up when you move out.
You can be completely prohibited from "damaging" things, including painting, in many jurisdictions. You also may have to match whatever grade of paint the landlord used. Some will use very expensive paints, or totally inappropriate paints, knowing it will lead to large deposit forfeiture, or the ability to collect excessive monies from the tenant.
> There must be laws in Germany, because I don't remember living in an apartment where the windows weren't top notch. That's also very necessary with temperatures sometimes going below -10°C.
In many apartments, especially illegal suites, you might not even get a window. The heating is your problem if the windows suck, and while in some jurisdictions there might be necessary temperature ranges the landlord must maintain, that's only if it's a "utilities included" apartment. Your heating bill could bankrupt you.
> Like all appliances, furniture, anything, it's "bring your own" in Germany (unless you have a common laundry area... not super common I think).
Often not the case, and is often a "feature". Again, it targets poor people, who then have a lower quality of life, and higher revolving costs. It's expensive to be poor.
> Same as before, as long as you fix up the holes when you move out, not an issue.
As before, you may be flatly prohibited from doing any damage to a wall.
> Don't know about solar panels. Like windows, never lived in an apartment without proper insulation in Germany (in the US on the other hand...).
Canada would like to have a word regarding insulation. Same with Australia, Japan, the US, etc.
You can't make structural changes to a rental, and that definitely includes solar panels. You can negotiate something with the landlord, but it doesn't stop them from evicting you after it's done. You'll need a lawyer.
> You can put in whatever floor you like as far as I know, and I've done so.
Nope. You might be able to negotiate it, but you'll need a lawyer.
---
When you rent, and you don't have a lot of money, it's fucking terrible. Low quality housing, low quality appliances, low quality life. It has higher revolving costs, and the absolute instability makes it untenable. Many of my family members bought and restored shitty in-bed campers as a safety net against eviction because it was constant and punishing.
This is advice for very high income, or already wealthy people.