At what level can you start building a house?
I'm adding because on of my characters is a level 1 only who doesn't level up beyond 1 as a test how far I could go in this game.
I know Dawnguard for example normally start at level 10 but I could bypass that simply by walking to Fort Dawnguard myself without an invitation.
Perhaps such a thing is possible with Hearthfire as well?

One of the quests you need to do for a Jarl, I think Dawnstar's, is level locked at 22. If you don't do that quest you can't buy the land. The rest are fine I think.

Let me clarify, you need to have done a certain quest or quests for the Jarl of the hold you want to build in. To get on their good side y'know. One of those quests is only available if you're level 22 or higher.