there will be no problem in getting an additional 20k towards the end of completion,he says the banks will not refuse as they have an interest too in having the house finished
I wouldn't be so sure of this especially as you say €150k is the maximum the bank would allow you to borrow. Some lenders included overruns when calculating how much of a mortgage they can lend as they expect self builds to overrun - ask the broker if your lender does this.
There have been instances where the banks have refused to release the last stage payment of a self build due to the value of the property being less than anticipated.
Although the bank has an interest in the property, this does not mean that they will lend for the overrun.
Depending on lender, you must qualify for 120% of mortgage amount at the outset as they expect overruns. You got a mortgage of 150k so you would have qualified for €180k based on this.
If it is one of these lenders then you should be ok.
Your broker sounds confident so if you have no reason not to, I would trust what he says.
The bank may ask your engineer how much it will cost to finish the house. Their priority will be to complete the property but not necessarily cater for a luxury finish ie granite work tops. As long as your just trying to complete the build to the original spec you should be ok.