There are many disagreements regarding interpretations of parables. I will present the principles of interpretation of parables used by the majority of conservative evangelical theologians since I prefer their approach as the most matching the normal interpretation of the Bible.
The following material is based on the books:
1. George Eldon Ladd A Theology of the New Testament
2. Carl B. Gibbs Principles of Biblical Interpretation
3. Henry A. Virkler Hermeneutics: Principles and Process of Biblical Interpretation
4. Understanding the Bible (published by ICI)
The main principles of interpreting parables:
1. In parables, the truth is recounted in earthly actions or events that were well known to most people.
2. Parables always contain a spiritual lesson.
3. There is always an analogy between the spiritual lesson and its earthly illustration.
4. Both the spiritual lesson and its illustration should be correctly interpreted.
5. Parables are not allegories. In an allegory, every detail is controlled by the author and the author can give a symbolic meaning to every detail. A parable is a story from daily living. The author does not invent this story and does not invent all the details. The details are often not important. The parable has one main idea and contains one truth, not many truths. So, in every parable we should seek for this main truth.
6. Interpreting parables, it is important to consider the prerequisites, the event, and the application. Prerequisites are the circumstances when the parable was told. Sometimes parables are preceded by the problems or event that Jesus was talking about in His parable. Parables are often followed by the way they should be applied. Sometimes Jesus explained His parables. So, try to find the explanation given by Jesus.
7. Parables in the Gospels are about Christ and the Kingdom of God.
8. Interpreting parables, it is important to consider the place and time of their origin.
9. Often biblical culture (symbols, daily life, and so on) gives a key to understanding parables.
10. Compare the teaching you found in the parable with the full context of the Holy Scripture. No doctrine should be based on a parable. Doctrines should be taken from clear biblical texts, and parables should serve only as illustrations of them.
The last principle is also important because parables are often used for twisting the Bible that takes place in abusive churches and cults. Their leaders use their own interpretations of parables in order to make the Bible speak what they want.