Prefab vs. Custom ADU: Which Should You Build?
By Eric Dexter, Owner & Builder ·
Prefab ADUs are marketed as the fast, cheap option; custom, site-built ADUs promise a home tailored to your lot. Both can be great — but the right choice depends heavily on your property. Here's the honest comparison.
What “prefab” means
A prefab (or modular) ADU is built in sections at a factory, then trucked in and craned onto a foundation on your lot. A custom ADU is designed for your property and built on-site from the ground up.
Cost: look past the sticker price
Prefab's advertised base price can look low — but it usually excludes the site work every ADU needs: foundation, utility connections, permits, delivery and a crane. Add those and the gap with a custom build narrows, sometimes to nothing. A custom quote should be all-in from the start; see real numbers on our realistic costs page and the cost calculator.
Site fit — this is the big one
Prefab modules are standard sizes and need crane access and a relatively simple site. On a tight, sloped, or tree-lined Seattle lot — or one with Environmental Critical Areas — getting a module in can be difficult or impossible. A site-built custom ADU is designed around your exact lot, slope and access, which is why it so often wins in the Pacific Northwest.
Quality & customization
Custom means the layout, finishes and details are built for how you'll actually use the space. Prefab gives you a fixed catalog of designs. If you're matching the ADU to your main house or a specific need, custom has the edge.
Timeline
Prefab can be faster — but only if your site is simple, since the foundation, utilities and permitting still take time regardless. On a complex lot, the site work is the long pole either way.
Which is right for a Seattle lot?
For simple, open lots, prefab can be worth pricing. For the constrained, sloped, tree-protected lots common around Seattle, a site-built custom ADU usually delivers a better fit and fewer surprises. Not sure which your lot favors? Talk to us and we'll walk both paths for your property.
