score:5
If you want to play it safe, your best bet is to ask the hotel. Maybe she can stay for free, maybe you need to pay a bit extra, maybe you need to switch to a double room for those few days -- it will depend on the hotel.
You can also just try to bring her in, and many larger hotels wouldn't even notice, but the longer the stay and the smaller the hotel, the higher the odds of being caught, and the potential penalties are high.
Upvote:4
I would suggest simply emailing the hotel and saying that the booking actually should be for two people, and that you trust they can update the booking.
See what they say.
If it's a guest house there might be an extra charge (at least to cover breakfast), for many hotels the price is the same for one person or two.
Upvote:5
Definitely contact the hotel and explain your situation. Particularly in Germany. Rules are expected to followed in Germany and it keeps things running efficiently and smoothly. You can always email them anonymously if you want to test the water and find out if it's possible to add her to the room.
Besides, most hotels are booked based on double occupancy and you are likely paying a single-person supplement anyhow. Does your room contain a double bed? Won't be a problem to add a guest. Is it a single bed ( doubtful )? If it is, I'm guessing it's a very small hotel... but many hotels can bring in a cot upon request.
That being said, many hotels in Europe are based on number of people, not number of beds.