Admission means "allowing to enter" or "permitted entrance" and applies to acceptance that carries certain rights and privileges. Admittance means "physical entry" without reference to rights or membership privileges."He sought admission to the party." "Admittance is by invitation only." "It was easy to secure admission to the public library, but she soon found several doors marked 'Admittance to staff members only. '"
Dictionary of problem words and expressions. Harry Shaw. 1975.