Who must be asked if one is allowed to build outdoors?

I would like to buy a plot of land in the [Außenbereich]. On it, I want to build a holiday cabin and sauna and such.

Who do I have to ask if I am allowed to do that? And do I have to present that person with an architect's building plan right away?

Who do I have to ask if I am allowed to do that?

Your building permit authority.

And do I have to immediately present a construction plan from an architect to that person?

No.

ok, thanks!