The Caravan Act is best explained in this link.
https://www.planningportal.co.uk/info/200130/common_projects/43/outbuildings/7
Your project may benefit from referencing the Caravan Act as guidance as well as the permitted development guidelines. We have seen customers use the Caravan Act to build slightly larger buidings than would be allowed under permitted developemt so it may be worth a read and then consulting your planning professional.
