No, it's not making a rule about what's sin. R.V.G. Tasker writes, "Tracing back to 4.4, this is another example of how worldliness can cause people to neglect God and arrange their lives as though he did not exist and as if they alone were masters of their destiny." That's what it's about. He is not rebuking responsible planning, or saying that if you don't recite the mantra ("if it's in God's will) you have sinned. He is instead rebuking presumptuous confidence.

Garry Friesen (Decision Making the Will of God, pp. 203, 209-210): "Here James is portraying a group of successful, confident, self-made merchants. They are hovering over a map discussing plans. But James condemns arrogant, self-confident planning which does not recognize God or His sovereignty. Their self-confidence is born of pride, and is evil (16)."

Again, James is not saying that you have to say this ritual whenever you plan. That's not the point. What is sin is pride and confidence in oneself that ignores the place of God in one's life.

I have a question in regards to James 4:13-17. My interpretation of this piece of scripture is that it’s a sin to say our plans without saying if it’s in Gods will. Is this the right way to understand this scripture or is it saying something else, am I missing some context behind why James is saying this?