Specifically, Swedish building codes don't apply to small constructions, hence the rise of the friggebod: https://en.wikipedia.org/wiki/Friggebod

> In Sweden, a friggebod is a small house which can be built without any planning permission on a land lot with a single-family or a duplex house. It is named after Birgit Friggebo, who was the Minister for Housing in 1979 when the new type of building was allowed. The word is a portmanteau of Friggebo and bod, the Swedish word for shed.

Yeah, the rules are similar in Finland (hardly surprising since the Nordic countries often try to harmonize laws), but there are specific requirements for buildings if one intends to live there permanently -- i.e. if you're building a shed in the yard then you might not require a permit, but you might need one if you build a small cottage and intend to live in it.

Regardless, it's usually a matter of documenting that the new building adheres to various standards.

Both a "Friggebod" and an "Attefallshus" requires the building to be built on "a land lot with a single-family or a duplex house", as you mention, which hardly applies for a house built in the middle of the forest. While I can't be sure, I don't think land lots with houses on them are merged with kilometers of owned forest. And even if it is: there are regulations on how and where you are allowed to fell trees.

I want to clarify that I'm not saying this building is illegal, which is what some people seem to choose to read: I simply said that a large step in this process often seems to be omitted.

> there are regulations on how and where you are allowed to fell trees.

If you own the land, and neither the land nor the tree (specimen or species) is protected, you should have no problem.

Since expanded to even larger dimensions:

https://sv.wikipedia.org/wiki/Attefallshus (link in Swedish)

Is that really relevant in this case though? The Wikipedia page you refer to specifically mentions that there has to be existing buildings on the property for those rules to apply.

No it's not really relevant, but then neither is the Friggebod, which was the subject of the parent of my comment.

These rules are mostly for built-up areas with neighbors. The rules used to be very draconian, but during the late 70s they were liberalized.

I believe in the US you can usually build small sheds (<100 sqft) in your backyard without a permit but IANAL.

Usually as an accessory structure, but things can get dicey if there isn’t a residential structure already on the property.