Remodeling costs depend on both the scope of work and the locale. If you've already committed to have plans produced, I suggest you wait until they are available for local professionals to price.

A patio provides insufficient foundation for framed walls and a roof. That will need demo first. You will need to build this all from scratch, with properly inspected footings, etc. Depending on site access, and permitting costs, and materials selections, 100K-200K in LA would be ballpark.

I would be surprised if you could get it done for less than $200/ft. Probably more like $250-$400/ft range which puts you $125K-$200K range. you will save some of this cost if you contract everything yourself, but make sure you check, then check again and check a third time then ask your friends to check you and your uncle, your neighbor... HAHA really though there are a million things to think about and not overlook. If you miss something and have to go back and fix it or add it, it will be very costly!