At the age of twenty-five, Grant Cardone came out of a rehabilitation treatment center for a horrible drug problem. Five years later, he became a millionaire. Today, Cardone, 57, is an internationally renowned sales trainer. His primary company, Cardone Training Technologies, provides Fortune 500 companies, small businesses and entrepreneurs with an interactive sales training platform.

While he is mostly known for his success with helping companies increase sales, over the last two decades Cardone has almost single-handedly built a real estate empire. His investment vehicle, Cardone Acquisitions, has been involved in more than $425 million in real estate transactions, and the company currently holds $350 million in multifamily property throughout the United States.

Here is an overview of how Cardone built his multimillion-dollar real estate empire without raising external capital from anyone beyond his close family members, who own less than 2% of the Cardone Acquisitions portfolio.

Unlike the majority of individuals with successful property empires who successfully built their sizeable portfolios as a full-time career, Cardone's real estate holdings were slowly expanded as a side business. Cardone Acquisitions was not intended to be his primary business venture nor his main income . Instead it was created in order for him to have a stable place to preserve and grow the money he was making from his sales consulting company.

During a February 2015 interview with the BiggerPockets Podcast, Cardone said, Every time I get money, I go broke again because I shove it into this real estate thing [Cardone Acquisitions]. He went on to elaborate that I take these three companies that will probably be destroyed in my lifetime, that I've made a ton of money off of, and I take all that money and I park it over here so I am always broke running these three, or I am having to hustle every day to get new money and then I shove it in over here.

Though at the core he considers himself an entrepreneur and not a real estate investor, Cardone believed that real estate provided a wealth preservation and creation vehicle that his other business ventures could not offer. (For more, see Key Reasons To Invest In Real Estate.)

Since the age of fifteen, Cardone had been studying the ins and outs of real estate. During his childhood, Cardone and his father regularly visited different pieces of property as a family outing activity, and over time his interest in buying buildings developed. To this day, shopping real estate is still something he enjoys doing with his wife and children.

In 1981, at the age of 22, Cardone graduated from college with an accounting degree. Despite wanting to immediately acquire properties, he delayed doing so for a few years. This allowed him to grow the money that he would later use to make investments. Additionally, the delay gave Cardone the ability to soak up as much as he could on the subject of real estate.

In an October 2014 episode of his real estate show, Cardone shared that the knowledge he gained on property investing, such as "understanding different terms such as net operating income (NOI), what a pro forma is, and what a good market looks like" came not from reading books but actually "looking at different deals, and meeting agents." In fact, Cardone has never read a book on real estate investing: he replaced the knowledge that can be found in books with knowledge that can be attained by actually looking at listings in different markets. (For more, see How Much Money Do You Need to Invest in Real Estate?)

At 29, Cardone finally put his more than fourteen years of real estate studying into practice. He bought a single-family property in Houston, Texas that initially did well. However, after a few months, the tenants left, and Cardone's real estate income flow became dry. He often jokes about the experience saying, "My occupancy rate moved from 100% right down to 0%." He hated the fact that he had to lessen the focus on his main business in order to find new tenants. Afraid that this situation would recur, Cardone quickly sold the property, broke even and swore that he would never purchase single-family residential real estate as an investment ever again.

Cardone's second acquisition did not take place until five years after his horrible experience with single-family real estate. During that time, he continued to accumulate cash as well as increase his property investing knowledge base. His first multifamily deal was a thirty-eight unit complex in San Diego, California. Cardone acquired the property for $1.9 million and made a down payment of $350,000. Just over a month later, he acquired another complex. Cardone continued to purchase more complexes one at a time, and today his holdings are valued at $350 million.

His real estate holdings are based in Alabama, Arizona, California, Florida, Georgia, North Carolina, Tennessee and Texas. In 2012, Cardone Acquisitions made the largest private party acquisition of multifamily real estate in Florida. That year, he purchased five apartment communities, with 1016 apartment units, in the state for a total of $59 million. A large portion of acquisitions were financed with debt from the Federal National Mortgage Association (Fannie Mae).

In a March 2015 interview on the "Best Real Estate Investing Advice Ever" with Joe Fairless, Cardone disclosed that less than 2% of the Cardone Acquisitions portfolio is owned by external partners who are his close family members and friends. The majority of the acquisitions are funded with Cardone's personal cash as well as traditional bank financing.

Much more than a professional sales trainer, Grant Cardone is a real estate mogul who built a $350 million multifamily portfolio from scratch. Before making his first acquisition, Cardone spent several years consuming as much information as he could on the subject of real estate. At 29, he made his first acquisition which was a single-unit property that initially performed well but soon became a disaster when his occupancy rate turned to zero after the original tenants moved out. Years after that experience, he acquired a thirty-eight unit complex and never purchased another single-family property as an investment ever again.