The relevant legislation is below:
https://www.legislation.gov.uk/uksi/2015/596/schedule/2/part/1/crossheading/class-e-buildings-etc-incidental-to-the-enjoyment-of-a-dwellinghouse/made
It is not to be confused with Guidance.
It is the legislation that is enforceable in the courts of law, the guidance is not enforceable, and it is for the purpose of guidance of the public sector as to what to aim for nothing elese.
Therefore, if guidance is used in the court of law to demonstrate a point of law this easily dismissed as not part of the legislation, and therefore not enforceable but the courts, in essence what counts in the court of law is the law not guidance . If there is any inconsistency, the law is what is enforceable, not guidance.
It is important to emphasise that class E does not provide powers to impose any say on the internal layout and utilities of the outbuilding. It specifies only the permitted use of the outbuilding which can only be enforced using the standard procedures for use of a building. Therefore, if a bathroom or kitchen for cooking facility or any other amenities are installed, there can be still used within the scope of the part E as incidental to the use of the dwelling house. The legislation does not grant any special powers to the planning department to enforce the removal of any facilities onside the outbuilding. In general of course planning legislation is very clear as the lack of power of the planning department in connection to the, fixtures, utilities and internal layout of any buildings of any type in England and wales.