Plenty of homeowners in Scotland assume "planning permission" covers everything. It doesn't. There are two separate approvals, they're handled by different teams at the council, and mixing them up is one of the quickest ways to stall a project.
Planning permission vs building warrant
Planning permission is about what you're building and how it affects the area: size, appearance, impact on neighbours. A building warrant is about how it's built: structure, insulation, fire safety, drainage, ventilation. You might need one, both, or neither.
Many house extensions fall under permitted development, so they don't need planning permission. They often still need a building warrant. That catches people out more than anything else.