So Cal garage conversion: Proper way to level floor
I would check Cali codes, but here in WA you can do all of those things you asked about without a permit. It would be considered a remodel and only certain things like some electrical work would require a permit. I personally would frame in the garage door area and use the self leveling product you can buy from the local home improvement store that mixes with water. Works well on concrete. I have used it to level and seal over my old basement floor that consisted of thin asbestos tiles over concrete. Have you talked to a local contractor? Ask friends/family for a trusted remodel contractor that will give you an honest assessment would be advice before doing anything.