Understand comps, true monthly costs, and one indicator that can help you rank options
Most rental property analyses fall apart for the same reason: they compare asking rent to purchase price and stop there. Real underwriting is more layered — it accounts for what a unit actually rents for (not what’s listed), every real monthly cost, the difference between short-term and long-term strategies, a single comparable indicator that lets you rank deals against each other, and a worst-case scenario that tells you whether the deal still works if things don’t go as planned. This article walks through that full process, with a specific look at what changes the math in Miami.
Introduction: The Spreadsheet Isn’t the Problem — The Inputs Are
Almost every investor who calls me has a spreadsheet. Purchase price, estimated rent, a rough mortgage payment, and a number at the bottom that looks promising.
The spreadsheet isn’t usually the problem. The inputs are.
Rent estimates pulled from asking prices instead of what units actually lease for. Cost columns that leave out vacancy, maintenance, and the capital improvements that come up every few years. No consistent way to compare a Pinecrest single-family home against a Cutler Bay condo, a Coral Gables duplex, or a short-term rental in Brickell. And almost never a worst-case scenario built in before the offer goes out.
None of this requires complicated math. It requires better inputs and a consistent method — which is what the rest of this article walks through.
Research Rent Comps, Not Asking Prices
The single biggest input error in most rental underwriting is using the asking rent on a competing listing as if it were the achieved rent.
An asking rent is a seller’s or landlord’s opening position. It tells you what someone hopes to get, not what the market actually paid. The number that matters is what comparable units in the same building, same street, or same immediate area have actually leased for in the last few months — not what’s currently listed and unproven.
Miami-Dade’s rental market illustrates why this distinction matters right now. According to MIAMI REALTORS®, Miami-Dade asking rents rose 1.5% year-over-year to $2,660 as of May 2026 — outpacing the national rent growth rate of 0.2% — with stabilized occupancy at a healthy 95.4%, above the national rate of 94.1%. Meanwhile, Zillow’s own rental market data shows the average rent across all bedroom counts and property types in Miami running higher, near $3,100 per month, and actually down slightly year-over-year — a sign of a market that’s cooling from its post-pandemic highs, not accelerating.
Those two figures aren’t contradictory — they’re measuring different things (median asking rent for a specific tracked segment versus a broader average across all unit types). But that’s exactly the point: rental numbers vary significantly depending on the data source, the property type, and the specific pocket of Miami-Dade you’re in. A generic “”Miami rent”” figure from a national report is a starting point, not an underwriting input. Pull real comps for the specific submarket before you trust any number.
Account for All the Costs Involved — Not Just the Mortgage
The second most common error is building a cost column that only includes the mortgage, taxes, and insurance.
A realistic monthly cost picture also includes:
- Vacancy. No property rents 12 months a year, every year, without interruption. Build in a vacancy allowance — even a conservative one — rather than assuming continuous occupancy.
- Maintenance and repairs. Ongoing upkeep, not just emergencies. Older properties and properties with pools, elevators, or extensive landscaping carry meaningfully higher ongoing costs.
- Capital improvements. Roofs, air conditioning systems, water heaters, and appliances all have a finite life. A property that looks fine today may need a five-figure system replacement in year three or four — that cost should be reserved for, not treated as a surprise.
- HOA or condo association dues, where applicable, and any pending or likely special assessments.
- Property management, if you won’t be self-managing — typically a percentage of collected rent.
- Insurance, which in Florida is one of the largest and most volatile line items in the entire cost stack, and one that can shift meaningfully based on flood zone designation, roof age, and construction type (more on this below).
Skipping any one of these doesn’t make the cost disappear — it just means the deal’s true return is lower than the spreadsheet suggests, and you find out after closing instead of before.
Short-Term Rental vs. Long-Term Residential Rent
Miami offers a real choice most markets don’t: a viable short-term rental (STR) strategy alongside traditional long-term leasing. But the two are genuinely different businesses, not just different pricing models.
Short-term rentals can produce higher gross revenue per month, but they also carry higher operating intensity — cleaning and turnover costs between every stay, more frequent furnishing and maintenance, dynamic pricing management, and materially higher management costs if you’re not doing it yourself. They’re also the more heavily regulated option: short-term rental rules in Miami-Dade vary sharply by municipality and even by zoning district within a single city, with some areas permitting STRs with registration, others restricting them to specific zones, and others prohibiting them in most residential areas outright. Rules and enforcement in this space also change more frequently than most other real estate regulations. Before underwriting an STR strategy for any specific address, verify the current municipal and, if applicable, condo association rules for that exact property — not for “”Miami”” generally.
Long-term residential rent is simpler to underwrite, carries lower turnover and management intensity, and is far less exposed to regulatory risk. It typically produces a steadier, more predictable monthly number, even if the headline revenue ceiling is lower than a well-run STR in a strong location.
Neither strategy is inherently better. The right choice depends on the specific property, the specific municipality’s rules, and how much operational involvement you actually want. What matters is underwriting the strategy you’ll realistically execute — not the one with the most attractive spreadsheet assumptions.
CAP Rate Is a Good Indicator to Compare Options in Miami — Here’s How It Works
Once you have real rent comps and a complete, honest cost picture, you need one consistent number to compare deals against each other. That’s what the capitalization rate — cap rate — is for.
The formula is simple:
Cap Rate = Net Operating Income (NOI) ÷ Purchase Price
Net Operating Income is the property’s annual income after operating expenses (vacancy, maintenance, management, insurance, taxes, HOA) — but before mortgage payments. That last part matters: cap rate is intentionally calculated as if you paid all cash, which is exactly what makes it a fair way to compare a financed purchase against a cash purchase, or one neighborhood against another.
Why it’s useful for comparing Miami options specifically: a $850,000 property in Cutler Cay and a $2.0 million property in Pinecrest are not directly comparable on price or on gross rent alone. But their cap rates can be compared directly, because the ratio strips out the difference in price and isolates how efficiently each property converts its price into income.
According to CBRE’s cap rate research, institutional multifamily cap rates nationally have generally sat in the mid-4% to upper-5% range through 2026, varying by asset class and market. That figure is a useful frame of reference, but it describes large institutional multifamily buildings — not a single-family rental in Pinecrest or a duplex in Cutler Cay. Individual residential rental properties should be compared against each other within the same market and property type, not against a national institutional benchmark. Calculate the cap rate for every property you’re seriously evaluating, using your own honest cost inputs, and use it to rank your options against each other — that’s where the number earns its value.
Build a Worst-Case Scenario
Every deal looks reasonable under its base-case assumptions. The real test is what happens when a few of those assumptions don’t hold.
Before making an offer, run the numbers again with deliberately conservative inputs: rent 10-15% below your comp-based estimate, an extra month or two of vacancy per year, a maintenance reserve on the higher end of your range. If the deal still produces a return you’re comfortable with under that scenario, you have real margin for error. If it only works under the optimistic case, you don’t have a deal — you have a bet that requires everything to go right. That distinction is the difference between an investor who sleeps well through a slow rental season and one who’s forced into a reactive decision the first time a unit sits vacant for an extra month.
Important Considerations That Make the Difference in Miami
A few factors matter more in Miami-Dade than in most markets, and they change the math even when the rent comps and cap rate look identical on paper.
Location relative to major demand anchors. Proximity to the University of Miami, Baptist Hospital, the Brickell financial district, and the US-1 corridor consistently supports rental demand, because each of these anchors a stable population of tenants — students and staff, healthcare employees, finance and professional-services workers, and commuters who value transit access along US-1. A property that’s a short, verified distance from one or more of these anchors tends to lease faster and with less seasonal fluctuation than a comparable property with no nearby demand driver. Always verify actual distance and drive time before including this in your analysis — don’t assume proximity based on the neighborhood name alone.
Flood zone designation. FEMA flood zone maps directly affect insurance requirements and cost, and in some cases affect financing terms as well. A property in a higher-risk flood zone can carry meaningfully higher insurance premiums than a comparable property just a few blocks away in a lower-risk zone — and Florida homeowners insurance is already one of the more expensive and volatile cost lines nationally. Check the FEMA flood zone designation for the specific parcel, not just the general area, before finalizing your cost assumptions. This applies across Miami — flood zone designations can vary block by block even within a single neighborhood.
Exit Strategy: Can You Sell This If You Need To?
The final underwriting question is one many investors skip until they actually need the answer: if circumstances changed and you needed to sell, how easy would it be?
A property with broad appeal — to other investors and to owner-occupant buyers alike — gives you more flexibility than a property that only makes sense as a rental. Consider whether the property, layout, and location would appeal to a local family looking to live there, not only to another investor running the same numbers you just ran. Properties located in areas with a narrower buyer pool, may be harder to exit if your plans change or the market shifts.
This doesn’t mean every rental needs to be a property you’d personally want to live in. It means understanding, before you buy, who else would want to buy it from you later — and how large that pool of future buyers actually is.
What This Means for You, Investor
Buying a rental property in Miami isn’t complicated because the math is hard. It’s may become complicated because most of the numbers people start with — asking rents, rough cost estimates, a single optimistic scenario — aren’t the real numbers.
The investors who consistently do well are the ones who replace assumptions with verified comps, build a complete and honest cost picture, choose a rental strategy that matches what they can actually execute, use cap rate to compare options on equal footing, and stress-test the deal before committing to it. Doing that consistently —anywhere in Miami-Dade — is what separates a good-looking spreadsheet from a property that performs the way you expected it to, year after year.
Sources cited: MIAMI REALTORS® (South Florida Rental Market Report, May 2026); Zillow (Miami rental market data); CBRE (cap rate research, 2026); FEMA (flood zone designations).
The Author
Marina Orfali DeRobertis is a seasoned Realtor with Berkshire Hathaway HomeServices EWM Realty and a real estate investor. She brings extensive expertise in South Florida’s real estate landscape and is committed to creating a seamless real estate experience for her clients.
She built a highly accomplished career in business development, with more than 20 years of experience delivering results in sales, multi-million-dollar negotiations, business management, growth strategies, and acquisitions for multinational corporations and real estate investors.
Born in Brazil and based in South Florida, Marina is fluent in English, Portuguese, and Spanish and brings a unique international perspective to Miami real estate.
Whether you’re buying, selling, or investing in Pinecrest, Coral Gables, The Falls, Cutler Cay, or other South Florida communities, Marina combines local market knowledge with sophisticated negotiation and business expertise to help her clients make informed real estate decisions.