# Building a defensible valuation model with comps and cash flows
A broker tells you a mixed-use building on the corner is "worth about $12 million." A lender's appraiser says $10.4 million. Your investment committee wants a number they can defend, not a guess.
Here is the uncomfortable truth: three trained professionals can look at the same asset and produce numbers millions of dollars apart. The difference is not skill. It is method and assumptions.
In this lesson we value one property three ways, then watch two small assumption changes swing the answer by seven figures. That sensitivity is the whole point. A defensible model is not one that produces a precise number. It is one where you can show exactly which assumptions drive the result.
Let's use a single example throughout: a four-story mixed-use building.
Mixed-use means the building generates income from more than one property type (here, retail plus residential). Each income stream behaves differently, which matters when we forecast.
The sales comparable approach (or "comps") values a property by looking at what similar properties recently sold for, then adjusting for differences.
The standard metric is
Say three comparable buildings sold recently:
| Comp | Sale price | Sq ft | Price/sq ft |
|------|-----------|-------|-------------|
| A | $9.2M | 21,000 | $438 |
| B | $11.5M | 24,500 | $469 |
| C | $8.8M | 19,500 | $451 |
The three cluster around $450 per square foot. Our building is roughly 22,000 square feet, so a naive comp value is about $9.9 million.
But comps are never identical. You adjust for:
Comps are strongest when the market is active and similar assets trade often. They are weak for unusual properties (few true comparables) or in a frozen market with no recent transactions. For appraisal fundamentals, the Appraisal Institute is a solid free reference on standards.
Income properties are bought for their cash flow, so we can value them off income directly.
First, Net Operating Income (NOI): annual rental income minus operating expenses (property taxes, insurance, maintenance, management), before any mortgage payment or income tax.
Suppose our building produces:
Now the capitalization rate (cap rate): the ratio of NOI to property value. It is the unleveraged annual yield a buyer accepts.
$$\text{Value} = \frac{\text{NOI}}{\text{Cap Rate}}$$
If comparable buildings trade at a 6% cap rate:
$$\text{Value} = \frac{598,500}{0.06} = \$9,975,000$$
About $10 million, close to our comp number. That agreement is reassuring.
Here is the critical intuition: cap rate and value move in opposite directions. Lower cap rate means a higher price for the same income. Watch what happens if buyers demand a 6.5% cap rate instead:
$$\text{Value} = \frac{598,500}{0.065} = \$9,207,692$$
A half-point shift in the cap rate, driven by rising interest rates, just erased roughly $770,000. Cap rates track the broader rate environment, so this is not hypothetical.
Comps and cap rates are snapshots. Discounted cash flow (DCF) models the full holding period: project cash flow each year, project a sale at the end, then discount everything back to today's dollars.
We discount because a dollar received in year 10 is worth less than a dollar today. The discount rate reflects the return an investor requires given the risk.
The mechanics:
1. Project NOI for years 1 to 10, growing rent and expenses each year.
2. Estimate the terminal value: the sale price in year 10, usually NOI in year 11 divided by an exit cap rate.
3. Discount each year's cash flow and the terminal value to present value.
4. Sum them. That sum is your value estimate.
Here is a compact version in Python:
noi_year1 = 598500
rent_growth = 0.03
discount_rate = 0.08
exit_cap = 0.065
hold = 10
pv = 0
noi = noi_year1
for year in range(1, hold + 1):
if year > 1:
noi *= (1 + rent_growth)
pv += noi / (1 + discount_rate) ** year
# Terminal value: year-11 NOI capitalized, then discounted
terminal_noi = noi * (1 + rent_growth)
terminal_value = terminal_noi / exit_cap
pv += terminal_value / (1 + discount_rate) ** hold
print(round(pv))With 3% rent growth, an 8% discount rate, and a 6.5% exit cap, this returns roughly $10.4 million. Now all three methods sit in the same neighborhood, which is exactly what a defensible model should show: independent approaches converging.
🎬 [VIDEO: "Discounted Cash FlowDiscounted Cash FlowDiscounted Cash Flow (DCF) is a valuation method that estimates an asset's value by projecting future cash flows and discounting them to present value using a required rate of return.Voir la définition complète → (DCFDCFDiscounted Cash Flow (DCF) is a valuation method that estimates an asset's value by projecting future cash flows and discounting them to present value using a required rate of return.Voir la définition complète →) Model in Real Estate" — youtube.com — a walkthrough of building a property DCFDCFDiscounted Cash Flow (DCF) is a valuation method that estimates an asset's value by projecting future cash flows and discounting them to present value using a required rate of return.Voir la définition complète → from NOI to net present valuenet present valueNet Present Value is the sum of an investment's future cash flows discounted to today, minus the initial outlay. A positive NPV signals value creation.Voir la définition complète →]
A single-point value is a false comfort. The real work is testing how the number moves when assumptions move. Two inputs dominate residential and mixed-use models: vacancy and rent growth.
We assumed 5% vacancy. In a downturn, or if a new competing building opens nearby, vacancy could jump to 10%.
Rerun the income: gross rent stays $1,050,000, but a 10% vacancy allowance is $105,000. Effective gross income drops to $945,000. Hold expenses at $399,000 and NOI falls to $546,000.
Capitalized at 6.5%, that is about $8.4 million, down from $9.2 million. A five-point vacancy swing cost roughly $800,000.
In the DCFDCFDiscounted Cash Flow (DCF) is a valuation method that estimates an asset's value by projecting future cash flows and discounting them to present value using a required rate of return.Voir la définition complète →, rent growth compounds over ten years, so small changes matter enormously.
That is a swing of more than $3 million from a rent-growth assumption you cannot verify today. This is why underwriters treat aggressive rent-growth assumptions with deep suspicion.
Notice the ranking. Rent growth moved value more than vacancy, and the exit cap rate (a compounding, terminal assumption) rivals both. A defensible model presents a range, not a point, and names the two or three assumptions that drive that range.
Build a simple sensitivity table: rent growth across the top, exit cap rate down the side, value in each cell. Your committee should see the whole grid, not just the number in the middle.
Vérification des acquis
1. According to the lesson, what makes a valuation model "defensible"?
2. Why does the lesson argue that three trained professionals can produce valuations millions of dollars apart?
3. Why does the concept of "mixed-use" matter specifically when forecasting a building's value?
4. Select ALL correct answers about the sales comparable approach as described in the lesson.
Sélectionnez toutes les réponses correctes.
5. Select ALL correct answers about why a naive price-per-square-foot comp value can be misleading.
Sélectionnez toutes les réponses correctes.
You now have three estimates:
You do not average them mechanically. You weight them by reliability for this specific asset:
For our stabilized mixed-use building in a market with decent transaction volume, a reasonable conclusion is a value range of roughly $9.5M to $10.5M, with the caveat that a 100 basis point rise in cap rates or a stall in rent growth pushes it toward the low $9M range.
That sentence, with its range and its named drivers, is what "defensible" means. When your lender or committee pushes back, you point to the exact assumption and the exact dollar impact.