The Grammy-winning singer and first American Idol winner recently returned to the NBC singing competition series as an adviser and is set to make her debut as a coach alongside Shelton and fellow returning stars Adam Levine and Alicia Keys in season 14 next year. Stefani had served as a coach on two past seasons. Clarkson has known Shelton for years, as her husband is his manager, and has met Stefani the past.
"You know, we've already [double dated] at her house, which is magnificent," Clarkson told E! News' Sibley Scoles this week at the 2017 Billboard Women in Music event. "And her family was there. It's a real testament of how she's such an awesome girl—it's 'cause of her family. Her family was super rad and down to earth. They're really, really cool."
Stefani, her three sons and other members of her family, including her brother and parents, have often hang out with Shelton.
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Clarkson said she "grew up listening to No Doubt, like everybody else" and that she was nervous at first to hang out with Stefani.
"And then she's like the coolest, most chill girl ever," she said, adding that they became friends right away.
She also continued to marvel at Stefani's Los Angeles home.
"Her house is insane. I walked in and I literally felt like the Pretty Woman scene when you don't belong in the store," she said. "I was like, 'I have a nice house,' but [with] hers, I was like, 'Damn!'"
In September, Clarkson said on the KTU 103.5 FM's Cubby and Carolina in the Morning radio show that she has taking part in group texts with Shelton and Stefani.
"Oh my god, Gwen is the nicest human ever. You can meet Gwen once and you're close to her!" she said. "I'm always on text chains with Gwen and Blake and somehow it always comes down to Blake and I being competitive in some sense because we both have songs and albums coming out now. I was like, 'Oh how was No. 7' because I was No. 6! I'm always like, 'Get used to it, because The Voice is happening soon!'"
"Gwen always starts this sweet, and positive text and Blake and I always take it somewhere else," she said. "Him and my husband have that boy humor going on."