Start with the jurisdiction
England, Wales, Scotland and Northern Ireland do not share one set of householder permitted-development limits. A dimension that is allowed in one nation should not be copied into a project elsewhere.
Previous extensions count
Permitted-development rules often refer to the original house, not simply the building as it looks today. An older extension built by a previous owner can reduce what remains available.
Restrictions can remove normal rights
Listed status, conservation areas, planning conditions and Article 4 directions can change what can be done without a full application.
Planning and Building Regulations are separate
An extension may not need planning permission and still require Building Regulations approval or, in Scotland, a building warrant. Do not treat one approval as evidence that the other is unnecessary.