A former ad exec who sold his firm to Microsoft has spent the last 20 years restoring this mansion in Los Angeles - take a look inside

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 it.

"I've always been an autodidact. I always jump into learning what I'm working on," Fenton said to 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 historical properties, and got to work on the home, which is 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 plans to put the home back up for sale. The home has been on and off the market for several years, but it's expected to be relisted for just under $10 million.

Let's take a look inside Artemesia.