A former ad exec who sold his firm to Microsoft spent the last 20 years restoring a mansion in Los Angeles -and now he's selling it for $10 million

When Leonard Fenton first bought his home - a 13,000-square-foot architectural masterpiece called "Artemesia" - he had no idea just how much work he would end up putting into it. He was in his 20s, and though he had previously restored homes while funding an earlier music career, he had never before worked on a project of this size.

Still, he knew a valuable opportunity when he saw one.

"I've always been an autodidact. I always jump into learning what I'm working on," Fenton recently told Business Insider.

At the time of the purchase, Fenton was heading up an advertising firm, Automotive Dealers' Marketing, that he would later sell to Microsoft.

He called up a few architects who specialized in preservation, consulted the National Trust's guidelines for historic properties, and got to work on the home, considered to be the largest ever built in the Craftsman style.

"The people and sources I consulted often didn't have the answer, but they taught me how to research and get the right answers," Fenton said. "I didn't just want a neoclassical house. I wanted a piece of art."

Nearly 25 years later - most of which he spent working on the home part-time, though he has been working on the restoration efforts full-time for the last six years - he's putting the home back up for sale. It has been on and off the market for several years, but is now listed for $9.995 million with Sally Forster Jones of John Aaroe Group.

"I have enjoyed living in this beautiful castle and being the custodian of this cultural landmark immensely. In fact, I literally grew up as an adult living in that house," Fenton said. "Now that everything is done and the challenges completed, there's really nothing more for me to do. I'm ready to pass on this wonderful property to a new family who will protect and cherish it for the next twenty or thirty years as much as I have."

Let's take a look inside Artemesia.