Sources close to the couple tell us ... Kanye's a huge fan of Axel Vervoordt's work, so he sought him out and convinced him to help design their mansion. We're told Kim gave Kanye the reins ... and he collaborated with Axel on nearly every aspect of the house.

As for the design ... it's what's come to be expected with Kanye -- VERY minimal ... much like their recently sold Bel-Air mansion. The partnership makes sense too -- Vervoordt's known for minimalist interiors "infused with a serene sense of history and timelessness."

Our sources say Kim and Kanye's new home features other signature styles too ... like tonal colors and every piece of furniture being custom made.

As we reported ... all the remodeling and attention to detail's paid off, because the home is valued at $60 million -- $20 mil more than what they put into it.