Absolutely. It is legal to build your own firearm for personal use in most US States so long as you can legally own a firearm. Always check local laws for your area. Rhode Island and New Jersey recently passed laws that may restrict the purchase or assembly of unserialized firearms.
80 percent lower receivers are parts manufactured just short of the legal definition of a firearm by the Bureau of Alcohol, Tobacco, and Firearms (ATF).
Some US States have additional regulations for firearms once they are completed, including California, Rhode Island, and New Jersey. Be sure to identify and comply with all local laws prior to your purchase.