How to Stress-Test a Rental Property Deal
A base case is a guess. The useful output of an analysis is the list of things that would break it.
4 variables
Rent, vacancy, rate, exit. Every deal has a breaking point in each.
Why a base case is not enough
A single set of numbers produces a single verdict, and it always feels more certain than it is. Every input in it is an estimate with a range attached, and the range is what determines whether you can live with being wrong.
Sensitivity analysis answers a different and better question: how wrong can each assumption be before the answer changes? A deal that stays acceptable across the plausible range of every input is a deal. A deal that only works at the centre of every range is a coincidence.
The four break-even values worth computing
Every one of these is a single number, and each takes seconds once the model exists.
| Variable | Base case | Breaks at | Headroom |
|---|---|---|---|
| Monthly rent | $2,100 | $1,992 | 5.1% below asking |
| Vacancy | 5% | 8.95% | 3.95 points |
| Interest rate | 6.75% | 7.43% | 0.68 points |
| Exit cap rate (5-year IRR) | 6.25% | ≈ 7.2% | 0.95 points |
Read as a set, that table says something the base case never could: this deal’s tightest constraint is interest rate, with two-thirds of a point of room. Rent can fall 5%, vacancy can nearly double, but the loan is the thing that has to hold. If you were going to spend an extra hour on this deal, it belongs on locking the rate — not on a fourth opinion about rent.
Which variables actually matter
| Variable | A realistic swing | Effect on annual cash flow |
|---|---|---|
| Rent | ±10% | ±$1,940 |
| Interest rate | ±1 point | ∓$1,465 |
| CapEx assumption | 5% → 10% of rent | −$1,260 |
| Vacancy | ±3 points | ∓$756 |
| Property taxes | ±$800 | ∓$800 |
| Insurance | ±$500 | ∓$500 |
The third row is the uncomfortable one
The two-way table
One-way tests miss correlation. Rent and rates do not move independently — rising rates cool buying, which raises rental demand — so it is worth seeing them together. Annual cash flow at each combination:
| Monthly rent | Rate 6.00% | Rate 6.75% | Rate 7.50% |
|---|---|---|---|
| $1,890 | $114 | −$945 | −$2,039 |
| $1,995 | $1,084 | $25 | −$1,069 |
| $2,100 (base) | $2,054 | $995 | −$99 |
| $2,205 | $3,024 | $1,965 | $871 |
The shape is the point. Cash flow is positive across most of the top-left and negative across the bottom-right, and the base case sits close to the boundary — which is a much more honest description of the deal than “$995 a year”.
How to run it
- 1Build the base case honestly first. Sensitivity around an optimistic centre just moves the optimism around. Start from a real pro forma.
- 2Flex one variable at a time to find each break-even. Record the number, not the impression.
- 3Flex the two tightest together in a small grid. Two variables is enough; three-dimensional sensitivity is unreadable and nobody acts on it.
- 4Write down the tightest constraint in one sentence and put it in front of whoever is deciding. “This deal breaks above 7.43% on the rate” is the most useful sentence in the whole analysis.
- 5Decide what would make you walk before diligence, not during it. Setting the threshold in advance is the only defence against talking yourself past it.
Downside first, then base, then upside
The free calculator recomputes every metric live as you move an input, which is enough to find all four break-even values by hand. The full analyzer runs the grids for you and keeps both scenarios side by side, so the comparison survives past the moment you did it.
Frequently asked questions
What is a sensitivity analysis in real estate?
A test of how much each assumption can move before the conclusion changes. Rather than producing one set of numbers, it produces the break-even value for each key input — the rent, vacancy, interest rate or exit price at which the deal stops working.
Which variables should I stress-test on a rental property?
Rent, vacancy, interest rate and the exit assumption carry almost all the risk. It is also worth flexing the capital expenditure reserve, because on a typical deal a move from a 5% plug to a component-based figure costs more cash flow than a significant vacancy shock.
How much should I stress a rental property analysis?
Enough to cover a realistic range rather than a catastrophe: roughly 10% on rent, three points on vacancy, and a full point on the interest rate. The goal is to find where the deal breaks, so extend each variable until it does and record that value.
What is a two-way sensitivity table?
A grid that varies two inputs at once — commonly rent and interest rate — with the resulting cash flow or return in each cell. It shows where the zero line falls and how close the base case sits to it, which one-way tests cannot reveal.