Can you really never have anyone over?

It is a real rule and it is generally enforced. Plan around it rather than hoping.

Why it exists: rooms are single occupancy and priced that way, the building has fire capacity limits, and — the practical reason — a guest means an extra person using a shared bathroom and kitchen that other residents are paying for. Operators are consistent about it for that reason.

What it means in practice:

- A friend visiting your room for an hour: usually nobody notices or cares.
- Someone staying overnight: not allowed at most buildings. Many have a CCTV camera at the entrance and a keycard log.
- A partner staying a week: this is squarely against the rules everywhere, and it is the situation that gets people asked to leave.

For your specific case in October, the options that actually work:

1. Your partner books a guesthouse or hostel nearby. Hongdae, Sinchon and Myeongdong have plenty at 35,000 to 60,000 won a night, and a week is 250,000 to 400,000 won. This is what most students do and it is the simplest answer.
2. An Airbnb or a short-stay for the week, which for two people is often similar money and more comfortable.
3. Ask the operator directly. A small number of buildings allow a guest for a fee, sometimes 10,000 to 20,000 won a night. It costs nothing to ask, and asking in advance is very different from being discovered.
4. If the visit is important and you are early in your stay, consider that a share house or one-room does not have this restriction at all. If you expect visitors regularly, that is a reason to choose differently.

What not to do: sneak them in for a week. The consequence is being asked to leave, usually with your notice period, and you then have to find somewhere new while hosting someone. It is a bad trade against 300,000 won of guesthouse.