Depending on the terms of your neighbors' lease with the landlord, this is probably a violation of the terms of their lease. Normally, a landlord will specify in the lease how many occupants are allowed to reside in the apartment. My guess is that the landlord has no idea there are that many people living there. Have you tried contacting your landlord to let them know of the situation? Most landlords would want to know what is going on in the apartment. The landlord should take appropriate steps to deal with the situation. However, if you do contact your landlord, or you have already, and they are not concerned about the number of residents living in such a small apartment, then your options are limited. If it is really bad, you might have other complaints you can lodge against them with the landlord (i.e. if the number of occupants is creating a noise issue, you could complain to the landlord about that). Also, if you think that the living situation is unsafe and/or unsanitary (for example, there are children living in the apartment and it's not a good situation for them with all of the people residing there), you could contact either the County Health Department where you live, or Child Protective Services. However, if it is mainly just a situation where you are irritated that there are so many people residing next door, but they are not really creating any real problems for you, I would see what the landlord does first, and if they aren't willing to do anything about it, decide whether you are willing to continue residing in an apartment building where they allow this sort of thing.