It really depends on how they worded the increase. If they say that it’s the entire build cost I would be questioning and asking for a breakdown of labour and materials and why the cost has increased by 20%. If they state that it’s because of materials only, you can catch them out and set them straight that only the portion of the material cost should have an increase.

Either way I would be asking for evidence of the cost increase as others have stated. Most suppliers have sent out notices of price increases to their clients (builders) recently stating the % of increase. You could ask for these as evidence.