When to write Unit Tests : Is there any or list of terms to take right decision [closed]

Question

To write Unit Tests we have to write extra bits of code. For several projects we do Unit Tests and for several projects we do not.

As a Team Leader I face trouble to make the right decision.

Is there any or list of terms by which I can take the right decision whether we will do Unit tests or not?

EDIT: Please provide a comment while down voting the question. It will help me to improve my question.