This is an easy-to-read page.
If you have a single-dwelling house or a two-dwelling house, you can build a so-called "Attefallshus" of maximum 25 m2.
You do not need a building permit, but you must submit a notification to the building committee and receive clearance before you start building.
You can read about
New building, rebuilding or extension
Applying for a building permit
Building without a building permit
Boverket's rules for how you can build
CE marking of building products