Dhiren Barot, a Muslim convert, also admitted to targeting the World Bank, Citigroup and Prudential in Washington, New Jersey and New York.
"Explosions at these premises were clearly designed to kill as many people as possible", said Edmund Lawson, the prosecuting lawyer.
He also admitted planning strikes on unspecified British targets in a conspiracy called the "Gas Limos Project", which "involved parking three limos with gas cylinders with explosives and detonating them in underground carparks," Lawson said.
Amid tight security at Woolwich Crown Court in south London, Barot pleaded guilty to conspiracy to murder and prosecutors outlined the details of his confession.
Barot admitted another plan to detonate at least one "dirty bomb" contaminated with radiological material in Britain.
The prosecution said Barot claimed the bomb was not designed to kill but "rather to cause injury, fear, terror and chaos".
The plans did not seem to have reached an advanced stage and the prosecution said it accepted Barot's assertion that no funding, vehicles or bomb making equipment had been in place.
Barot was arrested by British police in August 2004 after a massive security alert in the United States.
The US Homeland Security advisory level was raised to "high", police with assault rifles were posted at possible targets, barricades were erected and traffic into Manhattan over bridges and tunnels was restricted.