Advice please for Raising Garden

Hi everyone am new to this Forum but I am desperate for some advice please. My garden is on two levels and I'm looking to raise the lower level, Lots of hard work I know. The area is 12'x12' by 18" deep. I thought of filling it all with Topsoil but I have been told this will cost approx �£500. My other option is to part fill with hardcore and top this off with topsoil I am looking to turf the area once it is up to level but just what depth of topsoil will be ok? Any advice would be greatly appreciated.

 

Do you mean you want to raise it by 18 inches?