Underwriting a multifamily deal in Orange County requires more than plugging numbers into a spreadsheet. You need to know which assumptions kill deals, how Prop 13 reassessment changes your tax line, and why OC’s compressed cap rates demand a different lens than most markets. This guide walks through every line of the model ? the way we actually use it before writing an offer.
Most investors approach multifamily underwriting backwards. They find a property they like, pull the seller’s pro forma off the OM, and start tweaking numbers until they get an IRR that feels acceptable. That’s not underwriting — it’s rationalization.
Real underwriting starts from first principles. You build your own income and expense model from market data, not from numbers a listing broker assembled to make the deal look good. In Orange County, where Class B and C multifamily vacancy sits around 2.8% and cap rates for coastal product compress to 3.8–4.5%, the margin for error in your assumptions is thin. A 5% error in your expense ratio or a half-point optimism in your rent growth projection can swing a deal from marginally positive to deeply negative cash flow.
Here’s the exact process we use at NextGen Properties before writing an offer on any multifamily asset in Orange County.
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Step 1: Build Your Own Gross Scheduled Rent Figure
Start here, not with the seller’s stated rent roll. Request the current rent roll as part of due diligence, but verify every unit against your own market data before accepting it as your baseline.
For each unit type, pull comparable active rentals and recent lease signings within a one-mile radius. In OC, you’re looking at rent comps from CoStar, Apartments.com, and the actual rental history the seller must disclose. If the property has below-market rents — common in older OC assets with long-term tenants — model both current rents and market rents separately. That gap is your value-add thesis.
Gross Scheduled Rent (GSR) is the theoretical maximum if every unit rents at market and no unit sits vacant. It’s your starting line, not your finish line.
Step 2: Apply a Realistic Vacancy and Credit Loss Factor
Sellers often use 3–5% vacancy in their pro formas. That’s actually reasonable for stabilized Class B and C product in OC right now — the market vacancy is genuinely that tight. But vacancy in underwriting isn’t just physical vacancy. It includes:
- Physical vacancy — units that are vacant between tenants
- Credit loss — rent that is owed but not collected (evictions, partial payments, write-offs)
- Concessions — free rent or other incentives to attract tenants, which reduces effective revenue
For stabilized OC workforce housing, we typically model 5% total economic vacancy (including credit loss). For value-add assets mid-renovation, we model 10–15% during the lease-up period. Never model vacancy below what the market actually produces unless you have specific operational data that supports it.
Effective Gross Income (EGI) = GSR − Vacancy & Credit Loss
Step 3: Build Operating Expenses from the Ground Up
This is where most amateur underwriters get destroyed. Seller pro formas notoriously understate operating expenses — particularly in California, where property taxes, insurance, and regulatory compliance costs have all moved sharply higher in recent years.
| Expense Category | Typical % of EGI (OC) | Notes |
|---|---|---|
| Property Taxes | 15–22% | Prop 13 reassessment at acquisition — model the new assessed value, not the seller’s |
| Insurance | 3–6% | Up 20–40% in Southern California in 2025–26 due to wildfire exposure |
| Property Management | 6–10% | 8–10% is typical for full-service management in OC |
| Repairs & Maintenance | 4–8% | Higher for older stock; budget for deferred maintenance in value-add |
| Utilities (common area) | 2–5% | Water/sewer/trash especially; RUBS can shift this to tenants |
| Landscaping | 1–3% | Higher in communities with significant common area |
| Marketing & Leasing | 1–2% | Listing fees, photography, and leasing commissions |
| Administrative & Legal | 1–2% | Compliance costs — California eviction law requires legal counsel |
| Reserves | 3–5% | CapEx reserves; lenders typically require at least $250/unit/year |
Total expenses in OC typically run 40–55% of EGI for stabilized residential multifamily. If a seller’s pro forma shows expenses below 35%, they’re either omitting lines or using their actual (pre-reassessment) tax bill. Rebuild the expense model yourself.
One OC-specific line that frequently gets underestimated: property taxes post-Proposition 13 reassessment. When a property sells in California, it is reassessed at the purchase price. A building held for 20 years may carry a $400,000 assessed value on the seller’s tax bill but a $3,000,000 purchase price — meaning your annual property tax will be roughly 7x what the seller pays. This alone can swing NOI by $40,000–$60,000 on a mid-size acquisition and is the single most common underwriting error we see from buyers new to California.
Step 4: Calculate Net Operating Income
NOI = EGI − Total Operating Expenses
NOI does not include mortgage payments, depreciation, income taxes, or capital expenditures. It is a financing-agnostic measure of what the property produces on its own — and it’s the number that drives cap rate and valuation analysis.
Step 5: Cap Rate Analysis
Cap Rate = NOI ÷ Purchase Price
| Submarket / Asset Class | Cap Rate Range (Q1 2026) |
|---|---|
| Coastal OC (Newport Beach, Laguna Beach, Huntington Beach) — Class A/B | 3.5–4.2% |
| Costa Mesa / Irvine — Class B | 4.0–4.8% |
| Anaheim / Fullerton / Garden Grove — Class B/C value-add | 4.5–5.5% |
| Santa Ana / Orange — Class C workforce housing | 5.0–6.0% |
See our guide on cap rate vs. cash-on-cash return for a deeper breakdown of when to use each metric.
Step 6: Model Debt Service and DSCR
Once you have NOI, layer in your financing assumptions. Agency (Fannie/Freddie) multifamily rates are running approximately 5.5–6.5% as of Q1 2026 for 5–10 year fixed at 65–75% LTV.
Debt Service Coverage Ratio (DSCR) = NOI ÷ Annual Debt Service
Lenders in OC typically require a minimum DSCR of 1.20x–1.25x at underwriting. If your DSCR is below 1.20x, either the deal doesn’t pencil at this price, you need more equity, or you need a bridge loan with a value-add business plan.
Step 7: Cash-on-Cash Return
Annual Pre-Tax Cash Flow = NOI − Annual Debt Service
Cash-on-Cash Return = Annual Pre-Tax Cash Flow ÷ Total Cash Invested
In OC’s current environment — compressed cap rates and elevated financing costs — cash-on-cash returns on stabilized coastal acquisitions are frequently negative. A property at a 4.2% cap rate financed at 6.5% on a 70% LTV loan produces negative cash flow in year one. That’s not a reason to automatically pass — but it requires honesty about what you’re buying. See our full analysis of negative leverage in OC real estate for the detailed math.
OC-Specific Underwriting Factors You Can’t Ignore
AB 1482 Rent Control: Properties with a certificate of occupancy issued more than 15 years ago (and not single-family or condos) are subject to California’s statewide rent control law — this is a rolling exemption, so in 2026 buildings built before 2011 are covered. Annual increases are capped at 5% plus CPI (maximum 10%). See our full guide to AB 1482 and what it means for OC landlords.
Insurance Costs: Wildfire exposure and rising reinsurance costs have pushed insurance premiums 20–40% higher across Southern California in the past 18 months. If you’re using the seller’s 2023 insurance figure, you’re likely understating this line by $5,000–$15,000 annually on a mid-size asset.
Common Underwriting Mistakes That Kill Returns
- Using the seller’s property tax line. Reassess at your purchase price. Non-negotiable in California.
- Accepting the seller’s vacancy rate without validating against market data.
- Forgetting closing costs in your equity calculation (typically 1.5–3% in California).
- Modeling rent growth above the AB 1482 cap on covered properties.
- Ignoring capital expenditure reserves in operating expenses.
- Using a single-year NOI snapshot instead of a 5–10 year projection model.
If you want a second set of eyes on a deal you’re evaluating, our acquisition team reviews OC opportunities regularly and is happy to share how we’d model it.
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