Dont just wack up a brick wall on the existing concrete slab. it will fall down and kill you. I presume the concrete was part of a carport or just a hard stand. it will have no reinforcing or very little and more importantly it will most likely have no footings. therefore it wont support the bricks, one day it will fail and kill you or worse damage the car!

Always check with the local council, as stated its quick and easy to do and will give you advise that wont find you having to pull whatever you built down. Building regs are there for a resson, to protect not only you but your neighbours and future owners of the property. You build it wrong and it kills someone even in the future you go to jail.
Ask first, get the right advice and go from there.

Justin P