Greenwich magistrates fined Nzenwa £1500 and ordered him to pay £1000 towards costs.
Robin Vaughan, chief executive and registrar of the ARB, said: "ARB has a dual mandate – to safeguard the reputation of the profession and to protect the consumer. Both of these objectives are met when we are able to prosecute individuals who deceive the public by holding themselves out to be architects."
Nzenwa's deception was uncovered after his firm built an extension later found to be defective.