I rented my house out when I lived in Germany.
I found my local tax office very helpfull, just go and speak to them.
Basicaly you need to fill in a tax return if you make money from a property ie renting it out. There is no list of things you can claim for. But the tax office did tell me that any money paid out by me to enable me to rent the property can be put down as an expence and off set against the tax due on the profit made. That includes thing such as improvements ie curtains, etc I dont think a new conservatory would count though. If you rent it out through an agency. If I remember correctly you can claim that back as well
Also check with your mortgage provider as it can effect the payments. But I would recommend your first stop is the tax office. They even helped me fill in the form one year.
Floppy