What Is the Minimum Number of Units Required to Call a Property an Apartment Complex?

There’s no single nationwide rule that defines the smallest “apartment complex.” Instead, definitions depend on local building codes, zoning ordinances, and legal classifications. Common practical thresholds you’ll see across U.S. jurisdictions are 2, 3, 4, and 5+ units — each carries different labels and regulatory consequences.

Basic unit labels (common usage)

Duplex — a building with two separate dwelling units (often treated like single-family/residential for some rules).
Triplex — three units; sometimes the point at which buildings become “multiple dwellings.”

“Multiple dwelling” and small multifamily

Many codes define a multiple dwelling as a building with three or more dwelling units. For example, New York’s building regulations classify a “multiple dwelling” as a building with three or more units — a designation that brings additional safety, inspection and code requirements.

Fourplex and 4+ unit thresholds

Several state and local planning rules treat fourplexes and buildings with four or more units as “multifamily projects,” which can change allowable uses, permitting paths, and financing classifications. Washington state guidance and some municipal codes explicitly use a four-unit cutoff when defining multifamily projects.

Five units and above — often treated as apartment buildings / commercial

In industry statistics and many lender/insurance classifications, a building with five or more units is commonly considered an “apartment building” or commercial multifamily property (with different financing, tax, and insurance rules). National construction datasets and real-estate lenders frequently use 5+ as the dividing line.

Local examples that show variation

• Los Angeles: municipal code language treats an “apartment house” as a residential building designed or used for three or more dwelling units, showing how a 3+ cutoff is used in a major city.
• New York City / State: multiple dwelling rules apply at three or more units and trigger different DOB oversight and safety requirements.
• Typical lender/market practice: 2–4 units are often treated as residential while 5+ units are classified and financed as commercial multifamily.

Why the threshold matters

The minimum-unit threshold is important because it affects:

• Which building and safety codes apply (egress, fire separation, sprinklers).
• Whether a property must be registered or licensed as a rental/apartment building.
• Financing and insurance classifications (residential vs. commercial multifamily).
• Zoning and allowable uses, parking and density requirements.

Practical guidance for renters and small landlords

If you need a definitive answer for a specific property, check three places: (1) local zoning code and municipal building code, (2) the county or city's rental-registration rules, and (3) the property's certificate of occupancy (which states the building's legal use and permitted number of units). For example, a fourplex in Seattle/King County may be treated differently from a fourplex in a small rural county — so local rules matter.

Short takeaway

There isn't a single nationwide minimum: 2 units = duplex (residential), 3+ units often triggers “multiple dwelling” rules, 4+ commonly treated as multifamily, and 5+ frequently considered an apartment building/commercial multifamily in industry practice. Always verify local definitions for precise legal and regulatory consequences.