Why underwriting is the whole game
Every bad real estate investment shares one root cause: someone paid too much because they didn't run the numbers before they fell in love with the deal. Underwriting is the process of running those numbers — systematically, before you make an offer — so the price you pay reflects what the property can actually produce, not what you hope it will produce.
In Florida specifically, underwriting is more important than in most states because two line items — insurance and property taxes — are high, volatile, and frequently underestimated. A deal that looks like a 6% cap rate with the wrong insurance number is actually a 4.5% deal. You need the real figures.
This guide walks through the full underwriting process in five steps, with a worked example at the end using a real property type in a real Florida market.
Underwriting is not about finding reasons to buy a deal. It's about finding out what the deal is actually worth before you offer. If the numbers don't work at the asking price, that's a signal to walk away or negotiate — not a problem to rationalize around.
The five-step underwriting framework
Establish gross potential rent (GPR)
Gross potential rent is what the property would generate at 100% occupancy at current market rents. This is your starting point — not the seller's proforma, not the current rent on a below-market lease, not what Zillow's rent estimate says.
Pull 3–5 active rental comps within 0.5 miles that match your property on beds, baths, and general condition. Use the median, not the high end. If the comps cluster between $1,700 and $1,900 for a 3BR, use $1,800.
For a Jacksonville 3BR at $1,850/mo: GPR = $1,850 × 12 = $22,200/yr
Data sources worth trusting: Rentometer (paid), Zillow rental comps (free, use conservatively), direct MLS rental history, and a local PM who will give you their actual vacancy experience in that ZIP code.
Calculate effective gross income (EGI)
EGI adjusts GPR for vacancy and credit loss. No property runs at 100% occupancy. Even strong Florida markets experience turnover, and every turn means at least a few weeks of vacancy plus leasing costs.
Use a 5% vacancy rate for stable, in-demand markets (Jacksonville, Lakeland, Pasco). Use 7%–8% for markets with higher tenant turnover or seasonal demand fluctuation. Don't use less than 5% regardless of how hot the market feels — markets change.
Jacksonville example: $22,200 × (1 − 0.05) = $21,090/yr EGI
Calculate net operating income (NOI)
NOI is EGI minus all operating expenses. This is the most critical number in the analysis and the one most often gotten wrong. Operating expenses in Florida have several line items that require Florida-specific assumptions.
Florida operating expense defaults for a standard SFR (use these as starting points, then replace with actual quotes):
| Expense | Inland Counties | Coastal / High-Wind |
|---|---|---|
| Property taxes | 1.0%–1.2% of purchase price/yr | 1.0%–1.3% of purchase price/yr |
| Insurance | $2,800–$4,000/yr | $4,000–$9,000+/yr |
| Property management | 8%–10% of gross rents | 8%–10% of gross rents |
| Maintenance & CapEx | $150–$250/mo (<20yr home) | $200–$350/mo (<20yr home) |
| Landscaping / misc | $50–$100/mo | $50–$100/mo |
| Vacancy (already in EGI) | — | — |
Jacksonville example (inland, $285,000 purchase):
- Property taxes: $285,000 × 1.1% = $3,135/yr
- Insurance: $3,200/yr (actual quote range)
- Property management (10%): $2,220/yr
- Maintenance/CapEx: $200/mo = $2,400/yr
- Misc: $75/mo = $900/yr
- Total expenses: $11,855/yr
NOI = $21,090 − $11,855 = $9,235/yr
Calculate cap rate
Cap rate measures the property's return as if you paid all cash — no debt. It's a property-level metric that lets you compare deals across markets and price points without the noise of financing structure.
Jacksonville example: $9,235 ÷ $285,000 = 3.2% cap rate
Wait — the county table in our Florida investing guide shows Jacksonville at 6.1%. Why is this example only 3.2%?
Because county averages are built from deals that were sourced well — below-market acquisitions, properties with value-add upside, or recent rent growth that hasn't been priced into the asking price yet. A standard MLS deal at full ask in Jacksonville won't automatically produce a 6.1% cap rate. The 6.1% is what's achievable in that market with correct sourcing — not what every listing delivers on day one.
Use cap rate to compare deals to each other and to market benchmarks. A deal at 3.2% in a 6.1% market is overpriced relative to what the market can produce — that's useful information.
Calculate cash-on-cash return (COC)
COC is what actually matters to a leveraged investor. It measures your annual cash flow against your actual out-of-pocket capital investment — down payment plus closing costs plus any upfront repairs or reserves.
Annual Cash Flow = NOI − Annual Debt Service
Total Cash Invested = Down Payment + Closing Costs + Upfront Repairs
Jacksonville example with 20% down ($57,000), 7.1% rate on $228,000 30-year fixed:
- Monthly P&I: ~$1,532
- Annual debt service: $18,384
- Annual cash flow: $9,235 − $18,384 = −$9,149
- Total cash in: $57,000 + $5,500 closing costs = $62,500
- COC: −14.6%
This deal, at full ask with standard financing, doesn't cash flow. That's an honest result, not a reason to stop investing in Jacksonville — it's a signal that this specific deal requires either a lower purchase price, more equity, a value-add plan, or a different strategy.
Get the Florida Deal Analyzer spreadsheet
All five steps above, pre-built into a spreadsheet with Florida expense defaults. Drop in an address, get a full underwrite in under 5 minutes.
A full worked example: Lakeland value-add deal
Here's what a deal that actually works looks like when you underwrite it properly. This is a hypothetical 3BR/2BA SFR in Lakeland (Polk County) acquired below market with a value-add component.
3BR/2BA · Lakeland, FL · Built 2004 · Acquired off-market
Still negative cash flow — but now you see the full picture. The investor acquired $22,000 in equity at closing through the below-market purchase and renovation, which represents a 27.7% return on invested capital on day one. The monthly carry cost is approximately $489/mo. For a patient investor using a HELOC or cash reserves as the down payment source, this is a legitimate play.
The math changes materially if the investor is using a HELOC draw as the down payment. Model that layer separately — the HELOC carry cost needs to be factored into the total cash flow picture. Use the HELOC calculator on the homepage to model the combined return on the full capital stack.
Stress-testing the deal
A deal that pencils at your base case isn't necessarily a good deal. Run it through at least two stress scenarios before you make an offer:
Scenario 1: 10% rent decline
What happens to your NOI and COC if rents drop 10% — either because you can't hit your projected rent or the market softens? In the Lakeland example, a $190/mo rent reduction cuts NOI by roughly $2,100/yr and pushes annual cash flow to approximately −$7,970. Still survivable if you have reserves, but you need to know this ahead of time.
Scenario 2: Insurance spike
Florida insurance costs have been rising. What if your insurance premium increases by $1,500/yr at renewal? That's $1,500 directly out of NOI. Model it. If the deal collapses under that scenario, you need either a larger cushion or a different deal.
Scenario 3: Extended vacancy
What if it takes 60 days to place a tenant after acquisition or at first turnover? That's two months of gross rent ($1,900 × 2 = $3,800) absorbed as a loss plus the mortgage continuing. Add this to your cash reserve requirement — not your underwriting assumptions.
If a deal only works in your best-case scenario, it doesn't work. The stress test is not a pessimism exercise — it's how you find out what size cash reserve you actually need to hold this property through a rough stretch without being forced to sell at the wrong time.
The one metric that lenders use: DSCR
If you're financing with a DSCR loan (common for investors with multiple properties or non-W2 income), the lender will underwrite the property using DSCR — Debt Service Coverage Ratio.
PITIA = Principal + Interest + Taxes + Insurance + HOA
Most DSCR lenders want a minimum ratio of 1.0–1.1, meaning monthly rent must equal or exceed total monthly housing cost. At $1,900/mo rent and a PITIA of roughly $1,870/mo (P&I $1,362 + taxes $234 + insurance $258), the DSCR is approximately 1.02 — just above the minimum threshold. Lenders at 1.0 DSCR typically price the rate higher than those requiring 1.2+, so it's worth shopping multiple lenders on any deal near the threshold.
How to find deals that actually underwrite
Standard MLS listings at full asking price rarely produce strong underwriting results in today's Florida market. The deals that work come from: off-market sourcing (direct mail, driving for dollars, wholesaler relationships), distressed or deferred-maintenance properties where you're adding value post-acquisition, properties that have been sitting on market for 45+ days and have pricing room, and portfolios where a motivated seller is discounting for speed of sale.
For county-by-county context on where the best sourcing opportunities are right now, see the Florida county rankings. For the broader market context — financing options, current rate environment, and strategy overview — see the Florida investing guide.
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