If it was a fixed price one, the builder has to absorb the extra costs.
The problem is that a lot of builders will stop work if you don't let them pass on the extra cost to you, even with a fixed price contract. If they have no work left to do, they don't really have any leverage left.
Agreed. The op obviously requested a change from single block wall to cavity wall so the builder is right in looking for extra. I would query the amount though.
I hadn't clocked in the OPs pay that it was a change rather than just an increase in material costs.
Surely any change to the spec would have been agreed and the cost impact discussed. If so, it is only right you pay the builder for the extra work performed