01
Side-by-Side or Stacked?
| Side-by-side duplex | Stacked (up/down) duplex | |
|---|---|---|
| Layout | Two mirrored homes sharing one wall | One home per floor |
| Lot width needed | Wider — usually 50 ft+ | Works on narrower lots |
| Sound separation | Wall only — easier to isolate | Floor/ceiling assembly needs careful detailing |
| Separate sale later | Straightforward with a unit lot subdivision | Typically requires a condominium |
| Best for | Owners who may sell one side | Owner living on one floor, aging-in-place on ground level |
02
Three Ways Owners Use a Duplex
Live in one, rent the other
The rent from the second unit offsets a meaningful share of the mortgage. FHA and conventional loan programs allow owner-occupied two-unit purchases and construction-to-permanent financing.
Two households, one property
Parents and adult children, or two related families, share the land while keeping separate homes.
Build and sell one side
A side-by-side duplex on a unit lot subdivision gives each half its own parcel. Selling one half can return a large part of the construction cost.
03
What Shapes the Cost of a Duplex
Existing house
Building next to a house you keep costs less than demolition, but limits layout.
Utilities
Each unit normally needs its own water meter and electrical service. Sewer capacity and the location of the main often decide where the building sits.
Site and soils
Much of Snohomish County sits on glacial till that drains poorly, so stormwater detention is common.
Finish level
Rental-grade and owner-grade finishes can differ by 15–25% of construction cost.
City fees
Impact fees (school, park, traffic) and utility connection charges vary widely from city to city.
Run your own numbers → Build Cost & Return Calculator
04
The Duplex-Specific Details We Get Right
- Fire-rated separation between units — wall or floor/ceiling assembly built and inspected to code.
- Sound control: staggered studs or double walls, resilient channel, insulation in party walls, and solid-core entry doors.
- Separate mechanical systems so each tenant controls and pays for their own heat.
- Two clear addresses and entries that do not feel like a "front" and "back" unit.
- Parking within the state cap: no more than one space per unit on lots up to 6,000 sq ft and two per unit on larger lots, and none required within a half-mile walk of a major transit stop.
05
Timeline
| Phase | Typical duration |
|---|---|
| Lot review & feasibility | 2–4 weeks |
| Design & engineering | 2–4 months |
| Permit review | 3–6 months |
| Construction | 7–10 months |
06
Duplex FAQ
QCan I keep my existing house and add a second unit?
Sometimes. If the house can be converted (for example, a daylight basement turned into a separate unit) or a new attached unit can be added within setbacks, the result is a duplex. A structural and code review decides whether conversion beats new construction.
QDoes a duplex need two driveways?
No. A shared driveway is common and often required to limit curb cuts.
QCan a duplex later become townhomes?
A side-by-side duplex designed with a full fire wall and separate utilities can be split into two fee-simple homes through a unit lot subdivision.