A 4-plex (also written as fourplex or quadplex) is a single residential building that contains four separate living units under one roof. Each unit has its own kitchen, bathroom, and entrance, but they all share one structure, one lot, and typically one owner. Think of it as a small apartment building that still looks and feels like a large house.
How a 4-Plex Is Laid Out
The most common configuration is a two-story building with two units on the ground floor and two above. The building is usually 2 to 2.5 stories tall, and from the street it resembles a medium-sized single-family home. Units may have shared entries through a common hallway or individual doors that open directly to the outside. Most 4-plexes include a rear yard, and some designs place all four units side by side on a single level, though stacked layouts are far more typical in urban and suburban areas.
Unit sizes vary widely depending on when the building was constructed and where it’s located. Older 4-plexes often have units ranging from 600 to 900 square feet, while newer builds can push past 1,000 square feet per unit. Each unit functions as a fully independent home, with its own lease and its own tenants.
Residential, Not Commercial
One of the most important things about a 4-plex is where it falls on the legal line between residential and commercial property. Buildings with 2 to 4 units are classified as residential. Once a property hits 5 or more units, it crosses into commercial territory. That single-unit difference changes everything: financing terms, insurance requirements, building codes, and the type of loan you can use to buy it.
Local zoning laws and the presence of commercial features like retail spaces can also influence classification, but in most jurisdictions, the 4-unit threshold is the bright line. A 4-plex sits right at the upper edge of residential, which gives it a unique set of financial advantages.
Financing a 4-Plex
Because a 4-plex qualifies as residential, you can purchase one with the same types of loans used for single-family homes. If you plan to live in one of the units, FHA loans allow a down payment as low as 3.5%. Conventional owner-occupied loans typically require 3% to 5% down. Compare that to a 5-unit building, which would require a commercial loan with 20% to 25% down, stricter underwriting, and shorter repayment terms.
FHA loans on 3- and 4-unit properties come with a specific requirement called the self-sufficiency test. The estimated fair market rent from all four units (including the one you live in) must equal or exceed your total monthly mortgage payment, including principal, interest, taxes, and insurance. Lenders use an appraiser’s estimate of rent, then subtract a factor for vacancies and maintenance. You also need three months of mortgage reserves in the bank after closing, and that money can’t come from a gift.
Conventional and FHA loans on 4-plexes offer 30-year fixed terms and competitive interest rates. Lenders may count projected rental income from the other units toward your qualifying income, making it easier to get approved than you might expect for a property at this price point.
House Hacking With a 4-Plex
The strategy that makes 4-plexes popular with first-time investors is called house hacking: you live in one unit and rent out the other three. This lets you use owner-occupied financing (low down payment, better rates) while generating rental income that can cover most or all of your housing costs.
A common example looks like this: buy a 4-plex for $400,000 with an FHA loan at 3.5% down, which means $14,000 out of pocket. Rent the three vacant units at $1,200 per month each, bringing in $3,600 monthly. That rental income can offset or fully cover the mortgage payment plus operating expenses, effectively letting you live for free or close to it.
Beyond the cash flow, house hacking a 4-plex offers tax benefits. You can deduct mortgage interest, property taxes, depreciation, and operating expenses tied to the rental units. And because you have three income-producing units instead of one, a single vacancy doesn’t wipe out your revenue the way it would with a duplex.
Maintenance and Operating Costs
A 4-plex has meaningful cost advantages over owning four separate rental houses. There’s one roof to replace, one foundation to maintain, one lawn to mow, one driveway to repair, and one sidewalk to shovel. Those shared elements create economies of scale that reduce per-unit maintenance costs significantly. You also pay one set of closing costs, carry one mortgage, and file one property tax bill.
Most 4-plexes have individual electric and gas meters for each unit, so tenants pay their own energy bills. Water is a different story. Many older buildings have only one water meter, which means the owner either absorbs the cost, splits it evenly among tenants, or estimates each unit’s share and bills accordingly. If you’re buying a 4-plex, checking the metering setup before closing can save you from unexpectedly covering water bills for all four households. Some owners add a flat utility fee to the lease; others collect a proportional share monthly and reconcile annually.
Zoning Changes Opening New Areas
4-plexes are part of a housing category increasingly referred to as “middle housing,” the range of building types between single-family homes and large apartment complexes. A growing number of cities and states have passed laws allowing duplexes, triplexes, and 4-plexes on lots that were previously restricted to single-family homes.
Arizona’s HB2721, passed in 2024, is one example. The law required cities like Scottsdale to permit middle housing on single-family lots meeting certain location criteria, such as being within one mile of a designated urban center. Under these rules, a property owner can build one duplex, triplex, or fourplex on an eligible lot, subject to specific development standards. Oregon, California, Montana, and several other states have enacted similar reforms, reflecting a broader push to increase housing density without building large apartment blocks.
These zoning changes mean 4-plexes can now be built in neighborhoods where they were previously illegal, expanding both the supply of rental housing and the opportunities for small-scale investors. If you’re considering building rather than buying, checking your city’s current zoning code is the essential first step, since these rules are changing rapidly in many markets.