Do NOT build a handgun. You will be in violation of PC 12125, the "unsafe handgun" laws. This is totally aside from any AW issues or ATF Fed pistol vs rifle laws...
Unless it's a single-action revolver or (starting 2006) a single-shot pistol, you cannot build a handgun. For example, you can't build up from an 80% 1911 frame. There are exemptions for prototypes for certification, etc. but you'd have to be preapproved, etc etc.
For the record, this was confirmed to me by Cal DOJ Dir. Randy Rossi & Asst Dir. Tim Rieger at an NRA Members' Council Q&A dinner meeting last spring.
Bill Wiese
San Jose