Once you've decided to build an accessory dwelling unit, the next big fork in the road is how it gets built. Broadly, Washington homeowners choose between two approaches: a prefab ADU (built in sections at a factory and delivered to your lot) or a site-built ADU (framed and finished in place by a general contractor, the traditional "stick-built" method). Both produce a permanent, code-compliant dwelling. The difference is where most of the work happens, and that single fact ripples into your timeline, budget, customization options, and site prep. Here's how to think it through.
What "Prefab" Actually Means
"Prefab" is an umbrella term that covers a few distinct construction methods, and they are not interchangeable:
- Modular: The unit is built as one or more three-dimensional boxes at a factory, trucked to your site, and set on a permanent foundation with a crane. Modular homes are built to the same state building code as site-built homes and, once installed, are essentially indistinguishable from a conventional structure.
- Panelized / kit: Walls, floors, and roof sections are fabricated flat at a factory, then shipped and assembled on your foundation. This sits between modular and fully site-built.
- Manufactured (HUD-code): Built to the federal HUD code rather than the state/local building code. These are regulated differently and may not qualify the same way as an ADU in every jurisdiction, so confirm local acceptance before assuming a HUD-code unit counts.
When people say "prefab ADU" they usually mean modular or panelized. The distinction matters because it affects permitting, financing, and resale, so ask any vendor exactly which method they use.
The Core Trade-Offs
Timeline
Factory construction runs in parallel with your site work. While the unit is being assembled indoors, your contractor can be pulling permits and prepping the foundation, which can compress the overall schedule. Factory builds are also insulated from weather delays, a real advantage in the Puget Sound's wet season.
That said, prefab is not automatically faster end-to-end. You still need design, permitting, a foundation, utility connections, and final inspections on-site, and those steps run on the same local clock as any project. Delivery logistics (crane access, road permits, set day) add their own coordination. For a realistic picture of either path, see our guide on how long it takes to build an ADU in Washington.
Cost
Neither method is universally cheaper. Prefab can reduce labor hours and waste through factory efficiency, but you add delivery, crane, and set costs, plus the same site work (foundation, utilities, permits, connections) that a site-built project requires. Site prep and "everything below the box" frequently surprise homeowners who assumed the quoted unit price was the all-in number.
Site-built projects give you more control to value-engineer as you go, but are more exposed to local labor rates and change orders. The honest answer is that the two often land in a similar range once all site costs are included. For current ranges by ADU type, see our Washington ADU cost guide, and use our ADU budgeting guide to build a realistic all-in number rather than comparing a factory unit price to a full turnkey bid.
Customization
This is usually the clearest dividing line. Site-built wins on flexibility — odd lot shapes, tight setbacks, sloped sites, custom layouts, and unusual finishes are all easier when everything is built in place. Prefab trades some flexibility for predictability: you typically choose from a set of floor plans and finish packages, which is exactly what some homeowners want. If your lot is flat and a standard layout works, prefab's constraints may not cost you anything.
Site Requirements
Prefab lives or dies on access. A modular box has to physically reach your backyard and be lifted into place, which means crane clearance, truck access, and enough room to maneuver. Tight urban lots, overhead power lines, mature trees, and narrow side yards can make a modular set difficult or expensive, and sometimes impractical. Site-built crews, by contrast, can work in cramped conditions that a crane simply cannot reach. Walk your lot honestly before assuming a box can be delivered to it.
Permitting Is the Same Gate Either Way
Regardless of method, an ADU in Washington goes through your local permitting authority, and HB 1337 — the statewide law that removed owner-occupancy requirements and streamlined ADU rules — applies the same to a modular unit and a stick-built one. Your city or county reviews zoning, setbacks, height, and utility connections regardless of where the structure was assembled. A modular unit also carries state approval of its factory-built components, but you still need local site, foundation, and utility permits. Don't assume prefab skips the permit process; it doesn't.
Quick Comparison
| Factor | Prefab (Modular/Panelized) | Site-Built |
|---|---|---|
| Build speed | Factory work runs parallel to site prep; weather-proof | Sequential; weather-exposed |
| Cost | Efficient unit build, but add delivery/crane/set + full site work | More exposed to local labor and change orders |
| Customization | Plan and finish packages; less flexible | Highly flexible on layout, lot, and finishes |
| Site needs | Requires crane and truck access | Works on tight, sloped, or constrained lots |
| Permitting | Local permits still required; factory components state-approved | Standard local permitting |
How to Decide
Start with your lot and your priorities. If you have good access, a flat site, a layout that fits a standard plan, and you value schedule certainty, prefab is worth a serious look. If your lot is tight or sloped, you want a custom layout, or you're converting or adding onto an existing structure (see garage conversions and the difference between an attached ADU and a DADU), site-built is usually the better fit.
Whichever way you lean, get apples-to-apples bids: make sure every quote includes design, permitting, foundation, utility connections, delivery/crane (for prefab), and finishes, not just the structure. Our list of questions to ask an ADU builder will help you pressure-test each bid, and if you're weighing how to pay for it, start with financing an ADU in Washington.
The method matters less than the team. A good builder will tell you honestly whether prefab or site-built makes sense for your lot — and several of them are in our directory, ready to quote your project.