This largely depends on where you live.
Some states or cities may have laws concerning it.
"In general" you can assemble a firearm that's a legal type for you own use.
The only rules are that it must be a legal configuration.
If you sell it it has to have a serial number, and you can't assemble firearms with the intent to sell them.
You need to check your local laws and the BATF website for information on what's legal.
An excellent site to learn more is the AR-15 web site. They have a builder site with lots of info.......