A shed requires a building permit if it is over 10sqm.
Planning and building legislation has provisions relating to sheds on properties when they are not associated with dwellings. In a residential planning zone it is prohibited to have a shed without dwelling and in more rural settings Regulation 87 of the Building Regulations prohibits a shed with an appurtenant dwelling without the report and consent of the Municipal Building Surveyor. Consent may be given but would include strict provisions on the use of the building including the clause that the shed cannot be used for any form of accommodation including storage of a caravan.
Usually buildings like sheds won’t be approved for use as a home because they way they’re constructed may contravene residential building regulations.
If you are considering building a shed with a view to using it as a dwelling one day your first choice should be to construct the build that fully complies with Building Regulations. This could save you a lot of time and money to design footings/ frame/ living arrangements to appropriate building standards.