The 'Since U Been Gone' hitmaker now stars as a coach on 'The Voice' US with the No Doubt frontwoman's boyfriend Blake Shelton, and she admitted to being in awe of the singer's Los Angeles digs.
Speaking to 'Entertainment Tonight', she joked: ''The first time I saw Gwen's house, it's insane!
''I was kind of like 'Pretty Woman' walking in the store, except I didn't look like Julia Roberts, and I was walking in the store and they were like, 'You don't belong here.' ''
Kelly was referring to a scene in the 1990 romantic comedy, in which Vivian Ward - played by Julia - is shot down by workers in a fancy store because of what she is wearing.
Thankfully, the reality for the former 'American Idol' winner is very different, and she and her husband Brandon Blackstock frequently enjoy double dates with Blake - whom Brandon manages - and Gwen.
She added: ''She's such a style icon as well as this amazing artist. I literally talked to my husband about her house for hours.
''I was like, 'I felt like I was a good interior decorator, and then I walked into Gwen's house.''
However, Blake - who joked he was surprised to be invited in himself - took a cheeky dig at Kelly's own pad.
He teased: ''[She] has the biggest house I've ever been in in my life. Kelly's house proves that there were more winnings from 'American Idol' than there are from 'The Voice.' ''
His quip comes after Kelly explained last year she snubbed the show that made her a star - which ended in 2016 but is being rebooted by Fox next year - because she didn't expect it to be back on screen after such a short break.
She said: ''Here's the thing, let's keep it real - I thought ['Idol'] was ending. Like a year ago, my kid's one, I was pregnant with my son. And they said, 'Oh, we'll bring it back'. And I thought in five or 10 years. But I didn't know they meant the next year.
''And so I had already been in talks with 'The Voice'. One time I wanted to do it and I was pregnant and the other time we were touring, but I've been a part of 'The Voice' team because my husband manages [coach] Blake Shelton. And I've been on 'Idol' a lot - I gave them 15 years! I was there every season!''
Celine Dion barely cracks a smile and yet she's the greatest guest yet.
Phildel has gone back a step, in terms of her musical compositions, and returned to a style more reminiscent of her debut album with her latest...
To promote the release of her latest album 'Designer', Aldous Harding has undertaken an extensive European tour. On Wednesday night Harding played to...