The starting point is always the decision served on the party. It shows authority, parties, operative part, conditions, reasons and appeal information. Without this text the position cannot be assessed reliably.
It also matters to whom the decision was served and when. The time limit generally runs from service on the entitled party. With several participants, service defects or unclear party status, the chronology must be reconstructed carefully.
Keep the envelope, proof of service and all annexes. In building procedures, details such as plan version, condition or neighbour submission often decide whether a complaint has prospects.