Builder claiming variation for his own problem

Hi. I'm building with a project builder - standard house but with changes to the design (mainly made the house bigger). We signed off on the architectural plans 12months ago and recently signed off on the structural, hydraulic and updated architectural plans. Now they have started building they found the steel columns don't actually fit in the wall like shown on the architectural drawings so they want to box out around them (no charge).

The columns are to the area added to the house so was an extra which we signed off the cost. The designed columns are standard steel UC columns but we weren't happy with changing the design of the house to accommodate them so we spoke to the engineer and he said custom steel columns would work (ie 3 flat plate pieces welded together -so a custom UC column). The builder wants to charge a variation of almost $4k to change the columns (about $45,000/t).

Can they charge me for this? Or should they wear this cost to provide the architectural design as per the contract (with no boxing out)?
The builder wants to charge a variation of almost $4k to change the columns (about $45,000/t).

So we are talking 100kg of steel? Doesn't sound right.......:confused:

How long will it take them to remove the existing?
How much to 'custom' fabricate + materials?
How long will it take to re-install?

These questions will for the basis on how they arrived at the almost $4k figure, but when you start talking 'replace' and 'custom' the costs will escalate.

They are responsible to design and build it. It's their engineer, etc. is not built -about to do it now so there is no abortive work. Is about 80kg of steel
I signed the contract for them to design and construct the house based on the agreed architectural plan and specification. It's not a latent condition and they want to change the design for convenience as they didn't coordinate the consultants properly.