[Solved] Need an opinion on variation fees

(TL;DR at the bottom)
So.. long story short we are currently building a two storey with light weight upper floor, our builder's standard specification for main flooring usually comes with carpet.

During prestart we costed for tiles throughout, however due to a budget blowout (builder charged close to 200% of what tile supplier is charging) we took out the tiles from our contract and told them we'd do it through the supplier after handover.

That being said, our builder and prestart assistant were aware (verbally) that we are going to be using tiles on the second floor.

Fast forward to about 5 months, they've completed the electrical wiring and aircon ducting throughout the house and about to proceed to plastering / closing the ceiling.
Then after a site meeting for the balustrades, the client liaison asked what we're putting on the second floor as flooring.
When we said tiles, they then said they'll need to do additional work to provide better support as the upper floor was calculated to support carpet weight.

At that time in an email they gave approximately $3k as the additional cost of the remedial work to stiffen/strengthen the upper floor (complete with the work details), which we then agreed on.

Fast track about 1 month after that agreement, the remedial work is all done and they came back with $7k cost in the variation.
$3k for the actual remedial work, $4k for taking down the wires, aircon ducts & plumbing and putting them back in after the remedial work on the upper floor.


Are we (the client) actually liable for that additional $4k cost that we've never heard of before?



TL;DR
Builder said a remedial work will cost us 3k which we agreed on, once the work is done variation was raised as 7k due to additional work.