Building work - cost escalation

ArbitraryRandom

If the contract states that it would be using the existing house supply and this is now found to not be possible, then that's a material change which would 'frustrate' the contract (the work simply can't be completed as specified in the contract). 

So the choices are to make a new agreement (amend the contract) or pay for the work completed to date and consider the contract terminated.

i.e. you negotiate with the current builder or find an alternative contractor to finish the job - I'd bet money the quote for completion will be higher than renegotiating with this guy, but if you don't trust your current contractor then...