Which Multiplex Fits Which Lot
| Type | Units | Typical lot | Notes |
|---|---|---|---|
| Triplex | 3 | 5,000–7,500 sq ft | Often a stacked flat over a two-story unit |
| Fourplex | 4 | 6,000–9,000 sq ft | Classic two-up/two-down; strong rental layout |
| Fiveplex / Sixplex | 5–6 | 7,500 sq ft+, near transit | Usually stacked flats; elevator rarely required at this scale |
| Stacked flats | 3–6 | Varies | One-level homes, good for older renters |
The Code Path Changes at Three Units
Duplexes and townhouses are generally built under the residential code. A building with three or more stacked homes usually falls under the commercial building code instead. That shift brings:
- Automatic fire sprinklers throughout the building.
- Fire-rated separations between every unit and at exit paths.
- Accessibility: under the federal Fair Housing Act, buildings with four or more units must make ground-floor units accessible (and all units if there is an elevator).
- Commercial energy code provisions in some configurations.
We confirm the code path with the city's building official at the pre-application stage, before design is locked, so the budget reflects the real requirements.
Designing a Multiplex That Rents and Holds Value
- Separate entries: exterior stairs or a shared stair with a short corridor — never a long double-loaded hallway.
- One- and two-bedroom mix tuned to the local rental market near transit.
- In-unit laundry in every home.
- Durable exterior: fiber cement or metal panel cladding, rain-screen detailing for the Puget Sound climate.
- Individually metered electricity; water submetering where the utility requires a single tap.
- Bike storage when parking is reduced or waived near transit.
Financing Notes
A fourplex is the largest building that most residential mortgage programs will treat as "residential" for an owner-occupant. Five and six units move into commercial or construction-to-DSCR financing. See → How to Finance a Middle Housing Project
Multiplex FAQ
QDo I need parking for every unit?
Not near transit. Within a half-mile walk of a major transit stop, a city may not require off-street parking for middle housing. Elsewhere the state caps the requirement at one space per unit on lots up to 6,000 sq ft and two on larger lots.
QCan I sell the units of a fourplex separately?
Only as condominiums, which brings additional legal requirements and warranty exposure in Washington. Most owners keep multiplexes as rentals, or choose townhomes when individual sale is the goal.
QIs a sixplex allowed on my lot?
In Tier 1 cities within a quarter-mile walk of a major transit stop, or where the city's own code allows it. Check → What Can I Build?