The BATF says that they would like to see an identifying mark of some sort in case it gets "stolen", but that it's not required.You also need to be legally able to posses such a firearm as you are constructing and the firearm needs to be legal itself.
Keep in mind tho, that if you go the 80% route, that it's for your own personal use, you can't lend it out, or sell it or trade or anything else. You can will it to a direct descendant however.
It is a fair amount of work, I started my 10/22 mag from a block of 303 stainless, just about done now, picked up a factory mag barrel and stock for 40 bucks off ebay and one of my other 10/22's will probably donate a trigger group for the time being.
So while the ATF probably doesn't like people making their own firearms, they say it's legal if you follow their guidelines.