Legality of non-signatories of apartment contract living in that apartment

Question

Scenarios in consideration:

I have a few different scenarios in mind.

1. Landlord pays utilities. Non-signatory pays tenant for their stay in the apartment.
2. Landlord pays utilities. Non-signatory lives for free in the apartment.
3. Landlord pays utilities. Non-signatory doesn't pay money to live in the apartment, but is obliged (through oral agreement or contract) to pay in other ways, e.g. cleaning the apartment.
4. Landlord doesn't pay utilities. Non-signatory pays tenant for their stay in the apartment.
5. Landlord doesn't pay utilities. Non-signatory lives for free in the apartment.
6. Landlord doesn't pay utilities. Non-signatory doesn't pay money to live in the apartment, but is obliged (through oral agreement or contract) to pay in other ways, e.g. cleaning the apartment.

In all scenarios, the landlord has been unaware of the non-signatory's stay. Also, I'd like to consider another conditional to the "landlord doesn't pay utilities", dividing all those scenarios into two, some of which the landlord only pays for water bills. This is because it seems to me, from experience, that water bills are special (perhaps very cheap or subject to different laws), as I've never seen an apartment listed where the water bills aren't paid for.

---

Reflections on the scenarios' legality:

I assume 1. is obviously illegal. The landlord receives an increase in their expenses without their consent, and the tenant is making money off of the landlord's property, again, without the landlord's consent. In scenario 4., the latter part also applies. I think maybe both 1. and 4. would be considered theft?

Number 2. is problematic, as the landlord still receives an increase in their expenses. However, if the tenant decides to simply shower more and to live more luxuriously all of a sudden, the landlord's expenses would increase due to the tenant's decisions. Surely, the tenants decisions aren't illegal in this scenario, and the landlord's only option is to crank up the lease. So, why would then the scenario be different, if the decision causing the landlord's increased expenses, is the decision to allow a non-signatory to live in the apartment? Just like how the tenant choosing to live more luxuriously is their right, a right uninfringed by the fact that it leads to increased expenses for the landlord, surely allowing someone else to stay in the apartment for free is also within the tenant's rights? Maybe the difference is that one expense increase is caused by the intensity of use of the product of the product's legal leaser, whereas the other is caused by the addition of an illegal leaser?

Number 5. is still problematic for those same reasons. For example, if the landlord pays the water bills, their expenses will increase a little. Also, the wear and tear of the apartment will be worse, thus increasing the expenses. Not sure if this is impacted by the existence of a deposit. Then again, wearing that isn't covered by the deposit will still be worsened by more people living in the apartment.

If say, number 2 and/or 5 are legal, I could see that 3. and/or 6. would still be illegal. I guess it depends on what is legally considered payment or not.

---

What counts as staying at an apartment:

Is the difference between staying at an apartment and frequently visiting and sleeping at an apartment of a quantitative and/or qualitative kind? A tenant's romantic partner is likely to do either, and I expect differentiating between those two possibilities would be tricky.

Answer 1

In almost every residential lease that I've ever written or reviewed, this contingency is provided for expressly in a term of the lease with several terms, one on who may reside pursuant to the lease, and another on subletting and assignments. Categories #1 and #4 are subleases. The other categories are species of guest arrangements.

Those provisions, when they are present, typically prohibit occupancy by anyone other than the adults who are signed tenants and their children, and typically prohibit subleasing without landlord permission which shall not be unreasonably withheld (with the original tenant as a guarantor of the subtenant's obligation).

In most jurisdictions, the default rule of law is that you may sublet or have a guest who doesn't violate building code requirements unless the lease provide otherwise, but landlord-tenant law isn't entirely uniform from jurisdiction to jurisdiction. Many U.S. states, U.S. cities, and foreign countries have detailed statutory or ordinance rules on the subject.