Why It Matters
The base case scenario answers the question: "If nothing unusual happens, how will this deal perform?" Investors use it as the primary benchmark for deciding whether a property is worth buying. If the base case still generates acceptable returns, the deal has a reasonable margin of safety. If the base case barely breaks even, a single surprise — a vacancy, a repair bill, a rate adjustment — can tip it negative.
At a Glance
- Middle projection between best and worst case
- Uses realistic (not optimistic) rent and occupancy assumptions
- Drives the core go/no-go decision on a deal
- Should assume some vacancy, some expense overruns, and no unusual windfalls
- Paired with sensitivity analysis to stress-test key variables
How It Works
Every investment property carries uncertainty. Rents may come in higher or lower than expected. Vacancy may stretch longer during a slow leasing season. Repairs may cost more than the initial estimate. To account for this uncertainty without being paralyzed by it, investors build three projections: a best case, a worst case, and a base case.
The base case scenario is built on the most probable assumptions — not the most flattering ones. For rental income, that means using current market rents, not aspirational rents that require a full renovation and a perfect lease-up. For vacancy, it means using local market averages or slightly above, not zero. For expenses, it means including maintenance reserves, property management fees, and insurance — not just mortgage and taxes.
A well-constructed base case walks through every major input:
Revenue: Use current in-place rents or verified market comparables. Apply a realistic vacancy factor — typically 5–8% for stable single-family rentals, higher for value-add multifamily or short-term rentals.
Expenses: Include all operating costs — property taxes, insurance, property management, maintenance, utilities (if owner-paid), HOA fees, and a capital expenditure reserve. Skipping any of these makes the base case a best case in disguise.
Financing: Use the actual loan terms — the real interest rate, real amortization schedule, and real points paid at closing. If the deal depends on a rate that hasn't been locked, model the current rate, not a hoped-for future rate.
Exit: If the strategy involves a sale or refinance, apply realistic appreciation assumptions based on historical market data — not peak-cycle extrapolation.
Once the base case is assembled, investors run sensitivity analysis by adjusting one variable at a time: What happens to cash flow if vacancy rises from 5% to 10%? What if the refinance rate comes in 50 basis points higher than modeled? This stress-testing reveals which variables matter most and how much cushion the deal truly has.
Real-World Example
Darnell is analyzing a duplex in a midwestern market. The seller is asking $285,000, and the listing advertises potential rents of $1,400 per unit per month.
For his base case, Darnell doesn't use $1,400. He pulls comps and finds that similar units in the same zip code are actually leasing for $1,275. He plugs in $1,275 per unit and applies a 7% vacancy factor, giving him effective gross income of $28,458 per year.
He runs a full cash flow analysis, including a careful expense analysis that accounts for property taxes ($3,200/year), insurance ($1,400/year), property management at 9% of collected rents, a $100/month maintenance reserve per unit, and an estimated $1,800/year capital expenditure reserve. Total operating expenses: roughly $13,200/year.
He also checks the revenue analysis against the listing's pro forma and finds the seller omitted management fees entirely. With those included, the net operating income drops noticeably.
Darnell also reviews a rehab analysis for the light cosmetic work the property needs — about $12,000 — and factors the holding costs into his total acquisition basis. Finally, he models the deal using his actual financing analysis at 7.25% on a 30-year loan with 25% down.
The base case produces a cash-on-cash return of 5.2%. Not spectacular, but positive and stable. His worst case — with rents at $1,150 and a 12% vacancy — still doesn't generate a loss. He makes the offer.
Pros & Cons
- Forces realistic thinking before money is committed
- Provides a clear benchmark for measuring actual performance after purchase
- Helps identify which deal assumptions carry the most risk
- Makes it easier to compare multiple deals on an apples-to-apples basis
- Supports lender conversations and partnership discussions with credible numbers
- Only as accurate as the underlying market data — garbage assumptions produce a garbage base case
- Investors sometimes unconsciously build a best case and call it a base case
- Does not account for black swan events that fall outside normal variance
- Requires discipline to update as market conditions shift
Watch Out
The most common mistake is anchoring the base case to the seller's pro forma. Sellers — and their brokers — present numbers that make the property look as attractive as possible. Vacancy is often shown at zero or near zero. Management fees are omitted. Maintenance is underestimated. Taking those numbers at face value and calling it a base case means you've built a best case projection and paid a base case price.
Also watch for base cases that use future rents rather than current rents. If the property needs a full renovation before it can command higher rents, those higher rents belong in a best case tied to a specific rehab timeline and budget — not in today's base case. Model the property as it stands, then model the upside separately.
The Takeaway
The base case scenario is the honest center of your deal analysis. It's the projection built on what the market actually supports — not what the listing promises and not what could happen if everything breaks your way. If a deal works in the base case, it has real legs. If it only works in the best case, you're not investing — you're speculating.