Net Operating Income (NOI) is the annual income a rental property generates after all operating expenses are deducted, but before mortgage debt service, income taxes, depreciation, or capital improvements are subtracted. Formula: NOI = Effective Gross Income − Operating Expenses. Effective Gross Income = Gross Potential Rent − Vacancy Loss. Operating Expenses include: property taxes, insurance, property management fees, maintenance and repairs, utilities (if landlord-paid), administrative costs, and capital reserves (for larger multi-family properties). NOI does NOT include mortgage principal or interest payments; this is what makes NOI a financing-independent measure of property performance, allowing fair comparison across properties with different capital structures. For a $22,800/year gross-rent Maryland SFR with $1,596 vacancy and $8,453 operating expenses: NOI = $22,800 − $1,596 − $8,453 = $12,751.

Maryland commercial and multi-family rental properties are valued using the income capitalization approach: Property Value = NOI ÷ Cap Rate. The cap rate is the market-derived rate of return that investors in a specific market and property type accept for a stabilized, unlevered investment. Maryland cap rates by market: Baltimore City 8%–11% for small multi-family; Montgomery County 5.5% — 7.5%; Howard County 6%–8%; PG County 7%–9%; Carroll County 8%–11%. An 8-unit Baltimore City apartment building with a $48,808 NOI at an 8% cap rate has an indicated value of $48,808 ÷ 0.08 = $610,100. If a renovation increases the NOI from $48,808 to $63,000, the indicated value increases to $63,000 ÷ 0.08 = $787,500 a $177,400 value increase from an NOI increase of $14,192/year.

Maryland rental property expense ratios (operating expenses ÷ effective gross income) vary by property type and scale: Single-family rentals: 35%–45% is typical. Well-managed SFRs with newer systems, no PM, and low vacancy can achieve 30%–38%. Higher expense ratios (45%+) indicate older properties with higher maintenance, or PM fees being charged. Small multi-family (2–8 units): 45%–55% is typical. The higher ratio vs. SFR reflects shared systems, PM costs on multiple units, and per-unit management inefficiency. Larger multi-family (8+ units): 50%–60% is typical for older Baltimore City stock. A property with an expense ratio significantly above these benchmarks may have management inefficiencies (over-market PM rate, deferred maintenance), utilities paid by the landlord that tenants should be paying, or tax assessment that can be appealed.

NOI specifically excludes: (1) Mortgage debt service (principal and interest payments) debt is a financing decision, not a property operating cost; (2) Income taxes on rental income; (3) Depreciation deductions (non-cash; does not affect operating performance); (4) Capital improvements (one-time costs like a new roof or HVAC system replacement these affect value but are not annual operating costs). These exclusions are what makes NOI a financing-independent and investor-neutral measure of property performance. Two investors financing the same property differently (one with 20% down, one with 35% down) have the same NOI but different cash flows. NOI belongs to the property; cash flow belongs to the investor.