Kai Andrew decided to buy his first house the year after he graduated from college. 

It was 2006. He was 21, saving what he was making in his engineering job, and plotting to move out of his parents' house in his hometown of Portland, Oregon.

Upon realizing that mortgage payments would be cheaper than monthly rent, he bought a Portland townhouse with a potentially risky interest-only mortgage. He recently told BiggerPockets' "Real Estate Rookie" podcast that his monthly mortgage payments at the time were about $1,100.

He didn't pay a cent himself, he told Insider.

Andrew rented out rooms in the house to his brother and friends, and he used their contributions to cover the mortgage payments and pocket "a couple hundred dollars a month."

That practice — buying a home and renting out spaces in it to create passive income — is known as house hacking.

The term "hacking" was coined by Maui millionaire Brandon Turner — a real-estate investing guru who has been preaching about pursuing financial freedom as a BiggerPockets executive since 2012. BiggerPockets is a real-estate media company that produces investing-advice podcasts and has over 100 million downloads on iTunes.

Turner previously told Insider that the company's how-to-invest content was so attractive for one simple reason: "I'm almost a picture of what's possible," he said. So are the podcast's guests.

Andrew, for example, told Insider he quit his day job by age 27 to focus on hacking full time. He said he earned six figures a year with an expanded strategy. He's not just renting out spaces in his residence: He also buys land, places shipping-container tiny homes on it, and rents those out to vacationers on Airbnb.

He owns eight properties — some of which he developed himself — that generate 12 different streams of income and have made him a millionaire.

"Equity in real estate is the fastest way to build real, generational wealth," Andrew said. Most Americans agree with him.

He recommended starting small. "When you're in your 20s, you don't need a lot of space." Buying a condo or townhouse that you can afford at that age can be a big step toward building that wealth. Even better if there is an additional room to rent out, like Andrew did.

"Once you have that equity, you can leverage it into a different property," Andrew said. "You're going to have big bubbles like right now, where you can take advantage," he said of the pandemic housing market. While home prices are sky high, mortgage rates are near record lows, and competition is starting to slow. It could be a good time to buy hackable properties.

To be sure, Andrew's advice isn't foolproof. There are a number of ways to successfully invest in real estate, but it's important to consider what strategies are best at a given time.

While the pandemic housing market is still hot, taking on an interest-only mortgage as Andrew did may prove too risky if home prices decline. If you've mortgaged short-term rentals, you need a steady stream of renters to pay your debts.

At this point, Andrew isn't necessarily worried about financing his investments. He's graduated to building shipping-container homes to be used as short-term rentals on more rural land. 

A prefabricated shipping-container home, he said, can be built with as little as $30,000 in cash. Even a flashier, more custom model is less expensive to build than a true single-family home and can largely be built in a couple months.

In 2018, Andrew bought seven acres 35 minutes outside of Portland for $90,000, and he put about $450,000 into solo-building four container homes there, as well as his own house.

The sum of those two investments comes close to the median home sale price in the Portland area right now. In October, that figure was $535,000, up 8.1% year over year, Redfin found. The median home sale price nationally isn't as high. It's about $376,000, up 13.6% year over year.

But then, all of the sudden, his handful of "brand-new, modern container homes that overlook a lake and valley" started hitting 100% occupancy rates at $175 a night, he said.

"We accidentally created a market — a destination," Andrew explained.

He said those rental units netted him hundreds of thousands of dollars a year, but that he was essentially primed to double his initial investment even without them: His improved property is now estimated to be worth $1.2 million, he said.