Which is...

"Which is/are" begins an adjective clause. If the clause is necessary to define the word it modifies and could not be removed without losing communication, then there is no comma before it. If the clause contains information which is nice to know but does not really define the word it modifies and is not really necessary to the meaning, there is a comma.

...The house which is next to mine is very large.
...The dog which is pictured here is a rescue dog

...Maria's house, which is next to mine, is very large.
...My dog, which is pictured here, is a rescue dog.
Happy punctuating! Margie