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South King County has long been a target for Puget Sound investors priced out of Seattle and the Eastside, and that interest hasn’t slowed down. But “cash flow” gets thrown around loosely in real estate conversations, so let’s walk through how to actually calculate it using real current numbers. (Note: I’m a real estate broker, not a financial advisor — this is educational information to help you run your own numbers, not personalized investment advice.)
Start With Real Rent Numbers
As of late summer 2026, average asking rents in the area run roughly $1,900-$2,000/month for a 2-bedroom unit and $2,350-$2,480/month for a 3-bedroom unit in Kent and Federal Way, with Auburn and Renton in a similar range depending on the neighborhood and property type. A single-family rental will often command a premium over an apartment-style comp, but so does its cost basis — the comparison that matters is rent relative to what you actually paid for that specific property, not city averages.
The Cash Flow Formula
Cash flow is what’s left after every expense, not just your mortgage payment. The full list: principal and interest, property taxes, insurance, any HOA dues, a vacancy reserve (even great tenants turn over), a maintenance reserve (a common rule of thumb is 1% of the property’s value per year), and property management if you’re not self-managing (typically 8-10% of collected rent). Subtract all of that from your expected monthly rent, and what’s left is your actual cash flow — not gross rent minus mortgage.
Financing an Investment Property Is Different
Lenders treat non-owner-occupied properties differently than primary residences: expect a larger down payment requirement (commonly 15-25%, versus as little as 0-5% on some owner-occupied loans), a somewhat higher interest rate, and stricter debt-to-income scrutiny, though many lenders will count a portion of the property’s projected rental income toward qualifying. Run your numbers with both the higher rate and the larger down payment before you fall in love with a property’s gross rent potential.
Appreciation vs. Cash Flow
South King County has generally offered a more attainable entry point than Seattle proper or the Eastside, which is exactly what makes the cash flow math work better here for a lot of investors — a lower purchase price against comparable rent tends to produce better day-one cash flow. That said, markets move at different speeds by city: Federal Way has shown the strongest price appreciation of the four core cities this year, while Renton’s median has cooled slightly. Whether you’re prioritizing cash flow today or long-term appreciation should shape which city and property type you target.
Want to Run the Numbers on a Specific Property?
If you’re evaluating a listing and want a second set of eyes on the real cash flow — not just the listing agent’s pro forma — email me at bmackcoldwell@gmail.com. I work with investor clients throughout South King County and I’m happy to run the math with you.
