I had a leak this year on a new kitchen extension. My builder had given a 1yr guarantee for major issues such as this and were out within 2 days to look at it. As noproblem mentions, it was the lead flashing between the extension and the existing wall where the problem was. 1 of the eaves had come clogged with leaves and water was overflowing, down the exterior wall and inside the flashing.
They fixed it and have had no problems since.
And this bring Ireland, I'm sure you'll have plenty of rain before your 1st year is out to test the repair work