The revised tab for adding a rooftop terrace and ballroom to Denver’s Colorado Convention Center has ballooned to $233 million. That amount is more than twice the original estimate of $104 million. Denver voters in November 2015 approved an increase in hotel fees and taxes on car rentals to pay for the proposed expansion, which will create a 50,000-sq-ft outdoor terrace and an 80,000-sq-ft ballroom, to be built above an existing parking garage. Officials cite the complexity of the project and rising area construction costs for the large increase in the project’s final tab.