This is a Guest Post from Paula from the personal finance and lifestyle blog AffordAnything.org.

My friend’s single mom has a problem: she hasn’t saved much for retirement. By “not much,” I mean she’s in her 60′s and has saved less than $30,000. She has no pension — her only source of retirement income is Social Security. Eek!

My friend, obviously, worries about how he’s going to help his mom retire. His grandfather left an inheritance in the form of property: a bare plot of land in the middle of nowhere. My friend knew that his mom could sell it for cash — but, realistically, that cash would run out quickly during retirement.

A few years ago, my friend hit upon a brilliant idea: why not sell the rural family land and use the proceeds to buy an income-generating rental house?

He came up with this idea in 2008 after the housing market collapsed, and it worked like a charm: he sold property that wasn’t generating any income, used the cash to buy a five-bedroom house close to the local university, and voila — his mom now has about $2,200 per month in rental income. After paying for property taxes, home insurance and maintenance, she usually pockets about $1,800 per month. It’s not an enormous amount, but with her modest lifestyle it’s enough for her to retire on, especially since her own house is fully paid off.

“What a fantastic idea,” I thought in 2008 when I learned about it. “I should do that, too, especially now that houses are so cheap.”

Two years later my boyfriend and I decided to buy a house. (We saved for a down payment the old-fashioned way: shuttling several hundred dollars from each paycheck into a money-market account). We agreed on one key principal: if we ever have to rent out our home, the rental income it generates should be higher than the mortgage, insurance, taxes and water bill combined.

We came to this conclusion, in part, because we’re not sure how committed we are to living in Atlanta. It’s a great city, but I’m 27: am I really living in the last city that I’ll ever call home? I feel too young for that.

The housing market is in the slumps, and no one knows how long it will stay there. This is great news for us as buyers. But we don’t want to be forced to be a seller. If we move to another city, we don’t want to become a “motivated seller” — we want the luxury of owning the house until we’re in a strong seller’s market.

With this in mind, we started keeping our eyes peeled for houses in the hip, urban core of the city. This is where young professionals live: the part of the city inhabited by 25-to-35-year-olds working as graphic designers or entry-level law associates. They’re stable and mature enough to be responsible renters with good credit, but not quite cashed-up enough to buy their own home.

Single-family homes in this trendy urban area are expensive. The neighborhood we want to live in is an area where you can walk to everything: restaurants, bars, concert venues, yoga studios, vintage shops. Everyone wants to live in this neighborhood, so single-family homes come at a high premium; the rent we could collect on one is much lower than the mortgage.

Duplex or triplex homes are much more reasonable: we can collect rent on each unit, which add up cover the mortgage. Plus the overhead is lower on a multi-unit home: for the cost of maintaining one yard, replacing one roof, and paying one property tax bill, we can own several units.

There’s another, more personal reason my boyfriend and I decided to buy a triplex: it offers flexibility. At this point in our lives, we love living with roommates; if we lived alone, just the two of us, we’d quickly get bored with each other. (It helps that our two roommates are, well, the perfect roommates. We’re like a little family.)

But it’s possible that in a few years, we might decide we’re tired of roommates and opt for a one-bedroom. And perhaps in a few more years, we might marry and decide we want kids, at which point we’ll need 2 or 3 bedrooms. Owning a triplex with a variety of unit sizes — like a 3-bedroom, a 2-bedroom and a 1-bedroom — will give us those options. We can live in whichever unit fits our immediate needs, and we can “upgrade” or “downsize” without having to incur the costs and hassle of buying and selling.

We decided on this vision before we started looking at places, so once we began the search, we knew what we were looking for and found our ideal triplex right away: a 99-year-old building walking distance to Atlanta’s central and most beautiful park. It featured an enormous three-bedroom unit, a very generous one-bedroom unit, and a modest one-bedroom unit.

While most couples examine the beauty of a home before they buy it, we examined the numbers: the rental history, the property taxes, the water bill. We threw all the numbers into a spreadsheet, factored a 10 percent presumed vacancy rate, and realized this triplex fit the bill.

There was one caveat: it’s in an egregious state of disrepair. Its foundation is sinking. Its floorboards are rotting. The siding is peeling off. The gutters are nonexistent. The home is the very definition of a fixer-upper. Have you ever seen that Tom Hanks movie The Money Pit, featuring a beautiful old home that’s completely falling to pieces? That is this house — except this house is worse, it even has foundational problems.

If my boyfriend weren’t a civil engineer who put himself through college rehabilitating old homes, I wouldn’t have touched it. And indeed, no one else wanted the building: it sat on the market for 16 months before my boyfriend and I inquired about it. Thanks to all that, we bought it far below market price.

We’ll have to pay tens of thousands to repair the house; we know that. But we’ll also be collecting rent that covers everything: mortgage, insurance, taxes and water. The two single-bedroom units are occupied. Starting this fall, we’ll move into the three-bedroom unit with our two roommates. The roommates plus other renters will cover the cost of the mortgage. We’ll chip in $1,000 per month towards repairs and upgrades. I envision a win-win for everyone: the renters get a place that’s being upgraded and beautified, while my boyfriend and I rest assured that repairs and upgrades are our only housing expense.

If you have invested in small multi-unit housing such as duplexes, triplexes, or 4-plexes, please share your experience in the comments!