According to the wiki: "30 carpentry skill is needed to plan inner walls..." My character has 20 carpentry skill. I'm a free user so there's no way I can ever have 30 skill. But I was still able to plan indoor walls in my house. I planned and finished 4 walls already. Also according to the wiki: "Note: Only the owner of the house writ can add inner walls," I don't have the writ. My deed's mayor does. And like I said earlier, I can still plan it.