Your first underwriting model does not need to be beautiful. It needs to be traceable.
A model nobody can audit is not advanced. It is a cockpit full of unlabeled switches, and the person who built it is asking you to enjoy the lighting.
In plain English, an underwriting model connects property evidence and deal assumptions to projected cash flow and value. Its job is to expose the path from input to output. If the path disappears, the model has failed before the deal does.
Build the control panels
Start with these tabs:
| Tab | Job |
|---|---|
| Inputs | Assumptions, sources, and dates |
| Rent roll | Unit-level rent and occupancy |
| T-12 | Historical income and expenses |
| Pro forma | Year-by-year projection |
| Debt | Loan terms and debt service |
| Capex | Renovation and reserve schedule |
| Exit | Sale value, debt payoff, sale costs |
| Sensitivities | Downside cases and key variables |
Keep each tab responsible for one job. When debt terms, renovation timing, and rent growth are hardcoded into five different sheets, one change becomes a scavenger hunt with a dollar sign.
Give every input a logbook entry
Use three columns for every major assumption:
| Assumption | Value | Source |
|---|---|---|
| Year-one vacancy | 7.0% | T-12 and current rent roll |
| Insurance | $92,000 | Broker quote dated 2026-06-18 |
| Renovation cost/unit | $9,500 | Contractor bid |
| Exit cap | 6.00% | Sales comp range |
Add the date because evidence ages. An insurance quote is not a family heirloom.
If the source is missing, mark it missing. A blank source cell is useful information; a confident guess is camouflage.
Wire the model in sequence
The core flow should be visible:
- Gross potential rent.
- Vacancy, concessions, bad debt, and other income.
- Effective gross income.
- Operating expenses.
- NOI.
- Debt service.
- Cash flow after debt and reserves.
- Exit value and net sale proceeds.
Returns come last because they depend on every connection above them. Moving the dashboard to the front does not shorten the circuit.
Run the formula check
In this hypothetical, if EGI is $1,050,000 and operating expenses are $505,000:
NOI = $1,050,000 - $505,000 = $545,000
If annual debt service is $430,000:
Cash flow before reserves = $545,000 - $430,000 = $115,000
If required reserves are $60,000:
Cash flow after reserves = $55,000
That is the number cash-on-cash should care about. Not the cash flow before a reserve expense the property still expects somebody to fund.
Pull every breaker
Audit the workbook before you trust it:
- Can every major input point to a source and date?
- Are formulas consistent across months and years?
- Are hardcoded numbers visibly marked?
- Do amortization, maturity, and any rate reset appear in debt service?
- Does exit value include sale costs and debt payoff?
- Can a downside case be run without rebuilding the workbook?
Then change one input at a time. Vacancy should affect collected revenue. Operating expenses should affect NOI. The interest rate should affect debt service. The exit cap should affect terminal value.
If a switch moves and no gauge responds, find the broken link before your money does.
Keep the first model small
Build the smallest version that can answer the decision. Save a clean copy, document the assumptions, and have another person trace one output back to the original rent roll, T-12, lender quote, or bid.
A compact model you can break on purpose beats a giant workbook you can only admire.
PR Steinfurth Equity provides educational information only. Nothing on this website is an offer to sell or a solicitation of an offer to buy any security, nor investment, legal, or tax advice. Any securities offering is made only to qualified investors through official offering documents. Real estate investments involve risk, including possible loss of principal. Past performance is not indicative of future results.