The Four Numbers to Run Before You Get Excited About a House Hack
5 min read
The most expensive mistake in house hacking isn't buying a bad house. It's buying a fine house on numbers you never actually ran.
It happens because the math feels obvious. Mortgage is X, rent is Y, Y is close to X, therefore this works. Then you close and find out about everything that wasn't in the mortgage calculator.
Here are the four numbers I run before I let myself get excited. All four take about twenty minutes.
1. What it actually costs to own
Not your mortgage payment. The whole thing. Principal and interest, plus:
- Property taxes — these vary across the metro, and they change when a property changes hands. What the current owner pays isn't necessarily what you'll pay.
- Insurance — and once you're renting out part of the property, the coverage you need may not be what a standard quote assumes. Ask specifically.
- Mortgage insurance, if your down payment triggers it.
- HOA, if there is one.
- Utilities you're covering. Renting furnished or by the room? You're probably paying these.
- A maintenance reserve. The line everyone skips, and the one that gets people. Things break, and they break more often when people are living in your house. Set aside a percentage of rent every month and treat it as spent.
The number you land on will be higher than the mortgage calculator told you. That's the point.
2. Realistic rental income
The key word is realistic, and there are two ways people get this wrong.
They use the best-case rent — the highest number they saw one listing ask for, on a nicer version of the space, in a better part of the metro. Use what comparable spaces are actually renting for, and when unsure, use the lower end.
They assume full occupancy forever. Rooms sit empty between residents. Build a vacancy allowance in rather than treating every month as full.
Vacancy math differs by model, and it's worth understanding why. If you rent one basement unit and it's empty, you've lost all that income. If you rent four rooms and one is empty, you've lost a quarter. Renting by the room spreads risk across more residents — a real structural advantage that partly offsets the extra work.
3. Your effective housing cost
Number one minus number two. This is the number that matters.
Analyze
The Four Numbers Worksheet
Run these on any property in twenty minutes. Fillable worksheet for cost to own, realistic rent, effective housing cost, and the move-out math.
Your alternative isn't zero — it's whatever you'd otherwise pay to live somewhere comparable. So the real question isn't "does this cash flow," it's "will I pay less to live here than I'd pay in rent, while building equity?"
That framing changes what counts as a good deal. A house hack that leaves you paying something every month can still be strong if it's meaningfully less than market rent on a property you own. People talk themselves out of good deals because the number isn't zero — but zero was never the benchmark.
4. What it looks like when you move out
The number most people never run, and the one that separates a house hack from an investment.
Eventually you'll want to leave. When you do, you'll rent out the space you were living in — and the question is whether the property covers itself at that point.
If it does, you own a rental that pays for itself while you go do something else. That's the foundation of everything after.
If it doesn't, you have a decision to make later that you'd rather understand now. Maybe you sell. Maybe you keep it and accept a monthly cost. Neither is a disaster — but finding out at the moment you want to move is much worse than knowing going in.
A house hack that only works while you live there is a smaller opportunity than one that works after you leave. Both can be worth doing. Just know which one you're buying.
Run them in order
Each number can end the analysis. If one is much higher than you assumed, stop. If two is much lower than you hoped, stop. If three doesn't beat your alternative, stop. If four doesn't work and you're not okay with that, stop.
Most properties fail at one of the first two — which is why doing this in twenty minutes is so much better than doing it after an inspection.
One habit worth building: run these on properties you have no intention of buying. Pull a listing a week and do all four. After ten or fifteen you'll start seeing the answer before you finish, and you'll stop wasting time on properties that were never going to work.
The honest part
None of this makes a deal good. Numbers tell you whether something works — not whether you want it. A property can pencil out perfectly and still be a house you'd hate living in.
Run the numbers to eliminate what doesn't work. Then use your judgment on what's left.
Atlanta REALTOR®, investor, and serial house hacker.
REALTOR®, Keller Williams Metro Atlanta