In other threads, we've wondered about the cottage's vacancy and whether John Dashwood grew up with the Dashwood sisters.

In Chapter 12, Marianne says she hardly knows John, though she lived with him for many years. So, they did grow up together at least part of the time.

In Chapter 19, when the Palmers come to the cottage, Charlotte says "Only think, mama, how it is improved since I was here last! I always thought it such a sweet place, ma'am!" (turning to Mrs. Dashwood,) "but you have made it so charming!"
So, someone has lived there before or Charlotte would not have seen the interior to compare it to the way Mrs. Dashwood has decorated it.