Can builder hit me with a huge bill at end of project

Hi looking for information we are two weeks off completion of a 150 k renovation and builder has just said there is 40 k owed and we exploded to the builder our budget was 150 k and we didn’t have money for extras.so we upgraded our stairs so I knew we would owe for that .But he saying we owe extra for electrical work for tilers for insulation for front door extra for sliding door also he told us if we didn’t do double leaf extensions it would save us about 5 k so we decided on single block and he now says they cost the same .To say I’m shocked is an understatement as can’t sleep or eat to be honest I don’t have that money .Also he was onsite three times throughout the build and never ever said we owed him anything till now

What does your contract say?

That's a heavy over spend.was there a QS involved ?

A chancer

Marseypan said:

also he told us if we didn’t do double leaf extensions it would save us about 5 k so we decided on single block

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You understand you will likely spend a lot more than 5k in the coming years heating that space. How is that wall complying with the current building regs?

I hope you have someone independent assessing and signing-off on compliance of the build works.