A small rental portfolio is usually a concentrated bet — often one asset type, one neighborhood, sometimes one tenant per building. Concentration isn't automatically wrong (it's how expertise compounds), but you should know exactly which single events could break your portfolio, and diversify against those first.
The concentrations that actually hurt
Tenant concentration: with three units, one nonpayment is 33% of revenue — the strongest argument that units four through eight de-risk more than they add work. Employer/economy concentration: a town dominated by one plant or industry moves all your rents and values together. Property-type concentration: all student rentals or all Class C shares one demand curve and one regulatory exposure. System-age concentration: three buildings of the same vintage replace their roofs and furnaces in the same expensive decade. Financing concentration: several loans adjusting or ballooning in the same year is a liquidity event on a schedule.
Diversify the cheap ways first
Full geographic diversification (multiple metros) costs you your local knowledge edge and adds management overhead — it's the last resort, not the first move. Cheaper risk reducers, in rough order: more (and smaller) tenants per dollar of exposure; different micro-locations within your metro (different school zones, employers, price points); a mix of unit sizes and price tiers (working-class 2BRs and mid-market SFRs draw on different renter pools); staggered loan maturities and reset dates; and staggered capital-expenditure cycles when you can choose what to buy.
Worked example
An investor's four properties: all 1960s buildings within a mile, all Class C 1BRs, all financed 2021–2022 with two loans resetting the same year. Stress event — the area's hospital campus (largest employer) downsizes: occupancy drops portfolio-wide simultaneously. Her next two purchases deliberately differ: a 3BR single-family in a school-driven suburb and a small mixed-tenant fourplex across the metro, one on a 15-year fixed. Same city, same expertise — but the portfolio no longer fails all at once.
When this rule of thumb breaks
Diversification dilutes focus, and focus is the small investor's real edge: deep knowledge of one submarket routinely beats shallow presence in three. Under ~5 properties, tenant quality and reserve depth reduce risk more than any diversification scheme. Token diversification (one distant property you can't oversee) adds risk while feeling prudent. And real estate concentration may be fine in context — if your net worth includes index funds and a pension, your rentals can afford to be a focused, concentrated sleeve of a diversified life.