Here's a rather wishy-washy answer to the question. It does not take into account possible issues with the council.
Taken from: https://www.immoscout24.ch/de/c/d/im...he-lage?a=1565
You get to know each other, fall in love and at some point feel the desire to build a nest together - a nice feeling. If it weren't for the landlord, who wants to put obstacles in your way. He ruthlessly refers to the rental agreement, which states that the apartment is only intended for a single person. What to do?
The lease agreement must not prevent cohabitation
The number of persons stated in the rental agreement is a non-binding guideline. Therefore, as long as the apartment is not overcrowded, the landlord has no right to prohibit you from living together with your partner.
The situation is different if you want to take in a subtenant. In such a case, you would have to ask the landlord for permission. However, a life partner is not considered a subtenant, even if he or she pays part of the rent.
As with any rule, there are exceptions. If one partner of a married couple is from outside the EU/EFTA area, the migration office can make the residence permit conditional on the availability of a "needs-based" apartment. Under (certain) circumstances, the landlord's consent may also be required.
New contract: All parties must agree
If your partner moves in with you, the tenant cannot demand that a new lease be drawn up. However, you also have no right to do so. The agreement of all three parties is required for the conclusion of a joint tenancy agreement.
If the old contract remains in force, this means that only the registered tenant can assert his or her rights (except with a corresponding power of attorney). However, if you are married or living in a registered partnership, the partner not mentioned in the contract also has certain tenant rights.
Examples: If a notice of termination occurs, the unregistered partner can also contest it and request an extension of the tenancy. A notice of termination is only valid if each of the two partners has received a copy by separate mail. For this purpose, the landlord must use a special, officially approved form.
Joint contract: Yes or No?
A joint contract clearly has a certain symbolic power. However, in the event of a separation, such a contract can also become an obstacle, as it can only be terminated jointly. Accordingly, one partner cannot simply withdraw without the consent of the other.
You should also be careful if the landlord presses for a new, joint contract. Under certain circumstances, you may negotiate new and less favorable conditions.
Example: You signed the old lease at the beginning of 2013 - at a reference interest rate of 2.25 percent. If you replace it now (March, 2017) - at a reference interest rate of 1.75 percent - with a new one with unchanged rent, this means a hidden rent increase of over 10 percent.
Tip: Have your partner declared a co-tenant in an addendum to the existing lease instead of signing a new, joint lease.
Translated with www.DeepL.com/Translator (free version)