In many forms of housing, a “shared wall” will exist between two separate properties. Examples can include a wall that both you and a neighbor share in an apartment or a duplex. This wall essentially divides both properties, and the colloquial term is a “party wall”.
When living with a party wall, it is important to understand what you can legally do with it, as it is technically shared property.
Getting neighbor permission
Home Guide discusses the importance of maintaining and even soundproofing a party wall. One of the most common complaints of those who have party walls is the noisy neighbors on the other side. When you share a wall with another home, you can often hear noises from the other side. This is especially annoying if the sound maintains persistently over hours or starts late at night and goes into the early morning.
However, since party walls belong half to you and half to your neighbors, you cannot always take action on the wall without their approval. For example, installing blown-in insulation to the stud cavities will affect both sides of the wall, so you must gain approval.
Methods to dampen sound
You have other methods of dealing with pre-built walls that could use some noise reduction, too. Outside of the above method, you could also have sound-reduction drywall added to the outside of the wall itself. You can also dampen the sound by fireproofing the wall, such as caulking outlets on a party wall with fire-rated caulk.
Fortunately, many buildings constructed in recent years will have offset stud construction or double-wall construction, both of which automatically dampen or even eliminate sound transfer without you having to do anything extra.