Sensitivity Analysis: Stress Testing a Rental Deal Before You Buy

What Sensitivity Analysis Does

Sensitivity analysis asks a simple question: how much can my assumptions be wrong before this deal stops working? Instead of trusting a single set of inputs, you flex each one—rent, vacancy, interest rate, expenses—and watch how the cash flow and return respond.

Every underwriting model is built on estimates, and estimates are wrong. The point of a sensitivity analysis is not to predict the future perfectly but to find out which inputs the deal is most fragile to, so you know where a small miss becomes a big problem.

The Variables That Move the Deal

• Rent: test 5% and 10% below your projection.
• Vacancy: raise it from your base case to 8% or 10%.
• Interest rate: add 0.5% to 1.5% if the loan is not locked.
• Operating expenses: increase repairs, taxes, and insurance by 10% to 20%.
• Purchase price or rehab: model a cost overrun on renovation deals.

Running a One-Variable Test

Start by changing a single input while holding the rest at your base case. Drop rent 10% and see whether cash flow stays positive. Add 1% to the interest rate and check the DSCR. Isolating one variable at a time shows you exactly how sensitive the deal is to that specific assumption.

The variable that swings your outcome the most is the one that deserves the most scrutiny before closing. If a 1% rate move turns cash flow negative, financing risk is your dominant risk, and you should lock the rate or build in a larger cushion.

Building Best, Base, and Worst Cases

Beyond single-variable tests, build three full scenarios. The base case uses your realistic estimates. The upside stacks favorable assumptions. The downside combines several pessimistic ones at once—lower rent, higher vacancy, higher expenses, higher rate—to see the deal at its worst.

A deal that still survives the downside scenario, even at a thin margin, is far safer than one that only works in the base case. Real losses rarely come from one variable missing; they come from several missing together during the same rough year.

Turning the Analysis Into a Decision

Use the results to set your buy criteria and your reserves. If the downside case shows two months of negative cash flow, hold enough reserves to cover it. If a modest rent miss wipes out the return, negotiate a lower price or walk away.

Sensitivity analysis converts a single optimistic spreadsheet into a range of outcomes you can actually plan around. The best investors do not buy the deals that look great in one scenario—they buy the deals that still hold up when several assumptions go against them.

Frequently Asked Questions