I don't think I've encountered a builder who doesn't say "it'll be cheaper to get concrete done by 3rd party" (and you're talking quite a few thousand $$$ savings if you do).
Having the slab integrated into the house slab for the alfresco sounds nice and all, but we wanted exposed aggregate all around, and saved about $4000-$5000 by doing it after handover, which allowed us to get solar....