"I was over the moon, very stoked,'' he told the Bay of Plenty Times.
"This is massive. It's the top award in New Zealand, it doesn't get any better than this. It's a real honour and I'm very proud of what we have achieved."
Over a 14-month period, Mr Riordan and his team of three transformed a 1970s clinker-brick, two-storey home in Matua into a modern, elegant four-bedroom classic with 180-degree panoramic views of Tauranga Harbour.
The work was "a complete renovation'', with everything stripped back to its bones, Mr Riordan said.
"The frame and the roof were the only things left intact. The footprint of the house didn't change much but we remodelled the whole inside space."
Judges described the renovation as "exceptional'' and were particularly impressed with the way builders incorporated the old and the new, particularly relaying the original roof tiles and cobblestones and the "exquisite restoration of fireplaces and handrails''.
"This is not only the Renovation of the Year, but a renovation for many years to come,'' judges said.
Watching the house transform from a lived-in family home into a place of beauty was special for the owners, who wished to remain anonymous.
The owner said her father built the home in the 1970s and even though she and her husband wanted to renovate, they also wanted to keep it similar, while enhancing its character
The owner's mother still lives in the home, in her own space down one end of the house.
The family worked closely with Riordan Construction and architect Fraser Cameron. "We are so delighted and blown away with the award. It's so exciting and they so deserve this award," the owner said. "The whole process was easy, nothing was ever too hard for Paul and the whole team was a joy to work with."
The family have been living in the renovated house for about a year.
Mr Riordan put his success down to the team, including wife Helen, who is the company's business manager.
"When you surround yourself with good people you get good results. I would like to thank everyone involved for helping us pull this off," he said.
Meanwhile, a cliff-top Remuera home won the House of the Year supreme award, the House of the Year over $2 million category and the GIB Living award.
Winners at the national awards were all top winners in 12 regional competitions held throughout New Zealand during July and August.
