Wellington Council has voted unanimously to fund a $112.4 million strengthening and upgrade of the Wellington Town Hall.
There's also an 'undisclosed' contingency fund. Mayor Justin Lester said rumours that the contingency is $20 million are "just a guess", and he wouldn't confirm the figure.
But he did confirm there is a fixed contingency fund, and said any increase in that would need to go back to the council.
Mr Lester also rejected a suggestion by developer Ian Cassels that the council could retain the facade and build a new building behind it, for less money.
"Property developers have a very different view of the world. We have one of public governance and public good," Mr Lester said.
"Ian Cassels, when I asked him what he would do with the project, he said he'd put a 5-star hotel inside. That's of no public benefit whatsoever.
"This isn't about facadism, this is about the public wanting to retain the facility which is here, which is a public town hall."
Mr Lester said base-isolating will give the building seismic resilience for the next 100 years.
"As we've seen with the Christchurch Art Gallery, if we don't base-isolate we'll be back here in another 10, 15, 25 years' time, and have to do the job again," he said.
"It's a whole lot cheaper than building new. You cannot replicate this facility for the amount we're spending to strengthen it," Mr Lester said.
"We've had many Wellingtonians talk to us and say 'we love this building, it's a huge landmark building, and it's a civic and cultural heart, so we want you to get on with it and secure its future'," he said.
The Town Hall is built on reclaimed land. It's 60 metres down to bedrock for piling, and much of the engineering will be done below the water table.
It was most recently strengthened in 1992, but wasn't base-isolated.
On Wednesday, media were allowed into the auditorium, which has been closed since 2013 for strengthening after the Grade 1 listed building was deemed earthquake-prone.
The project is expected to take four years.