I have a 1 year old rosemary. I read in several (non scientific) articles that I have to put it to 10°C in winter, so it can hibernate (I guess). (Outside it would freeze I think, we sometimes get -15°C during the winter.) Now my problem is that I don't have a room with 10°C. So what will happen with the rosemary if I put it into my room in which the temperature is about 20°C?
Nothing, it'll be fine - it will, though, need as much sunlight and bright daylight as you can possibly give it, so a southfacing (or, second choice, east facing) windowsill would be perfect, provided there's no heat source directly beneath the window. If you can, move it into a terracotta pot with a good sized drainage hole, preferably using a sharp potting medium which drains freely - this will help to keep the plant from being too damp.Being mediterranean plants, they don't appreciate heavy soils and don't like being too wet at the root for any length of time. Watering should be done with care, only watering when the top of the potting medium feels a bit dry to the touch, but not so dry its shrunken from the sides of the pot, and when you water, water well, but don't let the plant stand in anything that catches the water, such as a tray or outer decorative pot - be sure to empty it after 30 minutes, and again 30 minutes later if necessary. Keep it trimmed and tidy by snipping it when it starts to look as if its getting too large or out of shape, assuming you don't use it in cooking frequently.
Here's a link for you to read.
It might help (or not) for you to know that I've got the same potted rosemary plant I've had for 3 years indoors in winter - but I do put it outside in early spring/late winter, when its still cold, (but not so cold the pot will freeze), after hardening off, when it recovers from its time inside - the UK isn't really sunny enough during winter, even on a south facing windowsill. Rosemary is pretty hardy, but I don't want to risk the smallish pot its in freezing, and I use it all winter for cooking.
Scienctific aspects aside, just some practical advice:
If I were you, I'd take the plant inside, but leave it outside als long as possible. Slight frost shouldn't harm it. Older plants might even survive outside, especially in larger containers. I've had the same plant outside for three years, originally because I had no space indoors & was too lazy to throw it out right away. Lowest temp. last winter was about -15 during the night, but the pot was just next to the house. If it's covered with snow, this will also protect from hard frost. I found that the hardiness depends somewhat on the variety. Some will do fine, others won't. So If you aren't too attached to it and willing to buy a new one, you could try leaving it outside on a sheltered spot. Cold greenhouse, perhaps?
Indoors, opt for the lowest temp. and the maximum of light possible. If in doubt, I'd rather choose the place with more light than lowest temperature. The plant probably won't hibernate anyway, so light is essential. For cool places, think outside the box: The plant doesn't have to go in your livingroom. A cool bedroom or storage room, the toilet, the stairways in larger houses, even an attic, as long as there's a decent window (and you don't forget to water it occasionally) make good wintering spots. On warmer winter days, you could even put it outside again for a couple of hours or even a few days during mild weather.
If your rosemary gets too warm (and possibly too dark) inside during winter, it will grow thin, lanky shots that can be pruned. The shots then can be used right away, the flavour will be less strong than during summer, though. Be gentle with watering, but never let dry out fully. Don't fertilize; also, be careful with fertilizing during summer - rosemary needs soil low on nutritients.