It sounds very complicated and complicated is not good when looking for a loan these days.
What is the value of the house / site now? The total loan would be €330k, assuming that an 80% LTV is assigned to an investment property loan, the current site / house would need to be valued at over €410k. That is unless you can find a bank that will lend based on your expected future sale price.......which is not likely. It is advisable to get one or more local estate agents or valuers to look at the property and give an idea of the current value and the value after the work is carried out.
If it still makes sense to proceed, you should retain the excellent tracker rate that is in place main loan. As you have the funds/equity to guarantee the loan, you could take out the loan in your own names to fund the renovation. There would be other implications to this such as taxation but it would have the advantage of you controlling the spend on the renovation and protecting your 'investment'