The 10 year rule is a guideline set by the Planning Inspectorate, which states that planning permission should not be required for development that has been carried out continuously for 10 years or more.
This guideline is not set in stone, however, and there may be other factors that come into play when determining whether or not planning permission is required.
For example, if the development is located in an area of outstanding natural beauty, or if it would have a significant impact on the local environment, then planning permission may still be required.