What underwriting actually is
Underwriting is a structured estimate of what a property might earn, what it will cost to run, and how much cash you must invest. It does not predict the future. Its job is to show you which assumptions the deal depends on, so you can test them before you buy.
A good underwriting model is simple enough to explain in a few minutes and conservative enough that a single bad assumption does not sink the investment.
Step 1: Estimate gross revenue
Revenue for a short-term rental is built from three inputs: average daily rate (ADR), occupancy, and the number of available nights.
Formula: Annual gross revenue = ADR x occupancy x 365 (plus any cleaning fees you keep, if you model them separately).
Source ADR and occupancy from comparable listings with the same bedroom count, similar amenities, and a similar location. Our guide on reading AirDNA data covers how to pull comps responsibly.
Example (illustrative only): ADR of $300, occupancy of 55 percent. Revenue = $300 x 0.55 x 365 = $60,225 per year.
Step 2: List every operating expense
Investors most often go wrong by leaving costs out. Build the list before you look at the result.
- Platform fees and payment processing
- Cleaning and laundry (often passed to guests as a fee, but model the true cost)
- Property management, if you are not self-managing
- Utilities, internet, streaming subscriptions
- Supplies and consumables
- Property tax and insurance (short-term rental coverage, not a standard homeowner policy)
- Occupancy and lodging taxes, where applicable
- Software: pricing tools, channel manager, smart-home subscriptions
- Repairs and a maintenance reserve
- HOA or resort fees
Example (illustrative only): Suppose total operating expenses, including management and a reserve, come to 45 percent of revenue. On $60,225 that is roughly $27,100.
Step 3: Calculate net operating income
Formula: NOI = gross revenue minus operating expenses (before debt payments).
Example (illustrative only): $60,225 minus $27,100 = $33,125 NOI.
Step 4: Subtract debt service to find cash flow
Debt service is your annual mortgage payments (principal and interest). Use a real rate quote for your loan type. See financing options compared for how loan types differ.
Example (illustrative only): Annual debt service of $24,000. Cash flow = $33,125 minus $24,000 = $9,125.
Step 5: Measure returns
Two common measures:
- Cap rate = NOI divided by purchase price. It ignores financing, so it lets you compare properties.
- Cash-on-cash return = annual pre-tax cash flow divided by total cash invested. It reflects your actual financing and upfront costs.
Example (illustrative only): Purchase price $500,000. Cap rate = $33,125 / $500,000 = 6.6 percent. If total cash invested (down payment, closing costs, furnishing, reserves) is $170,000, cash-on-cash = $9,125 / $170,000 = 5.4 percent.
Our post on cash-on-cash return goes deeper on this metric.
Step 6: Count total cash invested honestly
The denominator matters as much as the numerator. Include:
- Down payment
- Closing costs and lender fees
- Renovation and repairs
- Furniture, décor, linens, kitchen supplies, and smart-home gear
- Permit and licensing costs
- Startup operating reserve (several months of fixed costs)
Step 7: Stress test
Re-run the model with pessimistic inputs. A useful set of tests:
- Occupancy 10 percentage points lower
- ADR 10 percent lower
- Expenses 10 percent higher
- Interest rate one point higher, if the loan is adjustable
- A three month ramp-up before the first strong bookings
Example (illustrative only): With occupancy at 45 percent instead of 55, revenue falls to about $49,275. If expenses stay near $27,100 (many costs are fixed), NOI drops to roughly $22,175, which is below the $24,000 debt service. The deal would lose money in that scenario. That is the kind of finding underwriting exists to surface.
Common underwriting mistakes
- Using averages from the whole market. A four bedroom with a hot tub does not perform like a market-average listing.
- Ignoring regulation. A deal that is not legal to operate has no revenue. Check the permits and zoning checklist first.
- Forgetting the ramp-up. New listings rarely hit stabilized performance immediately.
- No capital reserve. Roofs, HVAC systems, and appliances fail eventually.
- Treating projections as promises. Comps describe the past; they do not guarantee your results.
A one-page underwriting checklist
- Confirm the property can legally operate as a short-term rental.
- Pull at least five true comps and note ADR, occupancy, and seasonality.
- Build a full expense list, with management priced even if you self-manage.
- Compute NOI, cash flow, cap rate, and cash-on-cash.
- Run the stress tests above.
- Decide in advance the minimum return and the worst-case loss you will accept.
Some investors hand this analysis to a done-for-you service such as BnB Accelerator, which operates this site; the framework above applies whether you do it yourself or review someone else's numbers.
Choosing a conservative revenue assumption
Comps describe what others earned, not what you will earn. A practical habit is to set your base case at or below the median of your true comps, then treat top-quartile results as upside. If the deal only works at the top quartile, the margin for error is thin. Write down which comps you used, why they resemble your property, and what would make yours weaker or stronger.
Questions to ask before you trust a model
- Would this deal still make sense if a manager ran it, and I paid the full fee?
- What is the cash needed if the first three months earn half of what I projected?
- Which single assumption, if wrong, hurts the most?
- Is there a legal fallback use if short-term rental rules change?
Save each version of your model with the date and inputs. Comparing your first estimate with actual results after a year is one of the best ways to improve future underwriting.
Educational only: this guide is general education, not financial, legal, or investment advice, and all numbers are illustrative.