Not sure whether to start a clause with “that” or “which”? it’s all about the context.
If the clause is critical to the meaning of the sentence, it’s a “that” clause.
The buildings that have been vacant for years will be demolished. (Not the buildings that are occupied or the ones that have only been vacant for days or weeks.)
If the clause is merely describing the subject, and removing it would not change the meaning of the sentence, it’s a “which” clause and should be set off by commas.
The buildings, which have been vacant for years, will be demolished.
Take out the clause and the basic message of the sentence stays the same: The buildings will be demolished. The clause simply gives additional information about them.
Fun Fact: These two types of clauses are called “restrictive” and “nonrestrictive.” The AP Stylebook calls them “essential” and “nonessential” because it’s easier to remember.