What legal concept applies when a significant investment is made in reliance on a granted permission to build a ditch?

The correct answer is that the legal concept that applies in this situation is an irrevocable license. An irrevocable license is a form of permission granted by a property owner to another to use the property in a certain way, which cannot be revoked after the licensee has significantly relied on it, typically through substantial investments or actions undertaken based on that permission.

In the context of building a ditch, if a property owner has granted permission to the builder to do so and the builder has made significant investments based on that permission, the builder may have an irrevocable license which protects them from having the permission withdrawn. This promotes fairness and prevents unjust enrichment to the property owner who would otherwise benefit from the builder's investments while revoking the permission.

An irrevocable license contrasts with a revocable license, which would allow the licensor to withdraw permission at any time, regardless of the licensee's reliance. An easement appurtenant typically involves a right to use another's land for a specific purpose, while a conditional easement would imply that the permission is contingent on certain conditions being met. However, in this scenario, the focus is on the significant investment and reliance on permission, which solidifies the concept of an irrevocable license.