A prefab ADU is built in a factory, trucked to your property, and set on a foundation. A stick-built ADU is built right where it will stand, from the foundation up. Prefab can move faster when you want a standard model and have an easy, open site. Stick-built gives you more control over the design, the fit with your home, and how the ADU handles local weather.
More homeowners are adding ADUs and tiny homes for family, rental income, or a simpler way to live, and how the home gets built is one of the first big decisions. Choosing between a prefab vs. stick-built ADU shapes your budget, your timeline, and what the finished home can do. The price on a prefab unit, for example, often leaves out the foundation, utilities, delivery, and permits. Cost, timeline, design, and site fit each tip the scale a little differently depending on your property.
What Is a Prefab ADU?
“Prefab” is short for prefabricated. Some or all of the home is built in a factory, then shipped to your property. You’ll see three main types:
- Modular: Whole sections of the home, called modules, are built in the factory. A crane sets them on the foundation, and a crew joins them together. A modular ADU is one of the most common types of prefab ADU marketed to homeowners.
- Panelized: Wall, floor, and roof panels are built in the factory. They are shipped flat and put together on site.
- Kit: You get a package of pre-cut materials and plans. A builder (or a very handy owner) assembles everything on site.
*You’ll also see smaller units marketed as a prefab tiny home. They’re built using the same methods, just at a smaller size.
Prefab does not mean the work on site goes away. Someone still has to prepare the ground, pour the foundation, run water, sewer, and power, and finish the connections. In Washington, factory-built modular homes also need state plan approval and an L&I insignia before they leave the factory. The local building department still reviews and permits the site-specific work (such as the foundation, placement, installation, and applicable utility connections) and enforces local zoning and building requirements.
Prefab vs. Stick-Built ADU: How the Two Types Compare
First, a quick note on terms. “Stick-built” and “site-built” mean the same thing: a home framed and finished on the property where it will stand. So whether you call it a prefab vs. site-built ADU or prefab vs. stick-built, the comparison below is the same. Here’s how the two build methods compare on the things most homeowners care about:
| Prefab ADU | Stick-Built ADU | |
| Cost factors | Unit price plus foundation, delivery, crane, utilities, permits, and finish work | One project scope that covers the full build |
| Timeline | Factory work can run while the site is prepared | Each phase happens on site, in order |
| Customization | Limited to the maker’s models and options | Designed around your needs and your property |
| Site fit | Needs truck and crane access | Works on tight, sloped, or hard-to-reach sites |
| Climate fit | Built to the maker’s standard specs | Built for local snow, cold, and frost |
| Financing | Some lenders treat factory-built homes differently | Often financed like a typical construction project |
| Resale | Depends on build quality and how it’s installed | Blends with the main home and neighborhood |
Neither column wins every row. The right choice depends on your property, your plans, and how much control you want.
What the Prefab Price Tag Leaves Out
Ads for prefab ADU homes often show a clean, simple number. That number usually covers the unit itself. It rarely covers what it takes to turn that unit into a finished home on your property.
Here’s what often sits outside the advertised price:
- Foundation: Your ADU needs a permanent foundation built for local soil and frost depth.
- Delivery and crane: Getting a large module from the factory to your backyard takes trucks, a crane, and a crew.
- Utility connections: Water, sewer or septic, and power all need to be run to the unit.
- Permits and plans: Local permits, site plans, and inspections still apply.
- Finish work: Decks, steps, skirting, landscaping, and sealing the seams between modules.
None of this means prefab is a bad deal. It means the fair comparison is finished project to finished project. When you get a prefab quote, ask what’s included and what isn’t. That one question can save you a hard surprise later.
Timeline: Is Prefab Faster?
Prefab’s biggest selling point is speed. The factory can build the home while your site is being prepared. In theory, that saves time.
In practice, the factory is rarely what slows an ADU down. Permits, site work, and utility hookups set the pace for both build methods. A factory-built unit can’t be delivered until the foundation is ready and the permits are approved. And factory schedules can fill up, too.
In Spokane County, ADU permit and zoning rules differ between the City of Spokane, Spokane Valley, and unincorporated county land. Those reviews often shape your timeline more than the build method does.
Design Freedom and Fitting Your Property
This is where the two paths split the most.
With prefab, you choose from the maker’s models. Some offer finish options or a few layout changes. But the shape and size are mostly set by what the factory builds and what fits on a truck.
A stick-built ADU starts with your property and your life. Do you need a ground-floor bedroom for a parent? Big windows facing the view? A roofline that matches your main house? Those choices are part of the design from day one.
Site conditions matter, too. A modular ADU needs room for a truck and a crane to reach the spot where it will sit. That can rule out:
- Sloped or uneven sites
- Narrow side yards or tight backyard access
- Sites with mature trees, power lines, or fences in the way
A stick-built ADU can be framed almost anywhere a crew can walk. Each of our floor plans can be adjusted to fit your yard and your preferences.
Why Stick-Built Often Wins in the Inland Northwest
Spokane and North Idaho winters ask a lot of a home. Heavy snow, deep cold, and frozen ground are part of life here. A home built for a milder climate may not be ready for that. Stick-built construction lets every part of the ADU be planned for these conditions from the start.
Built for Snow and Frozen Ground
Roof snow load requirements are set locally, and they can climb with elevation. A stick-built roof is engineered for the snow load where your home stands. The foundation is set below the local frost line, so freezing and thawing ground won’t shift it.
Warm, Tight, and Efficient
A small home can be comfortable and efficient when its shell is built well. That means solid insulation, careful air sealing, and quality windows placed to catch winter sun. For heating and cooling, many small homes use a ductless mini-split heat pump.
ENERGY STAR says certified mini-splits can use up to 60% less energy than standard electric radiators, and many cold-climate models are designed to provide reliable heat through cold winters when they are properly selected and sized for the home and local climate.
Water That Keeps Flowing
Frozen pipes are a real worry in a cold climate. In a stick-built ADU, plumbing is kept inside the heated part of the home, and utility lines are buried below the frost line. Planned ventilation brings in fresh air without losing heat, which helps prevent winter condensation and mold.
One Local Team From Start to Finish
With a stick-built ADU, one team is responsible for the whole project: plans, permits, foundation, framing, and the final walkthrough. You aren’t stuck between a factory, a delivery company, and a local contractor when something needs fixing.
That one-team approach is how we work as a custom ADU home builder in Spokane and across North Idaho, from Coeur d’Alene to Sandpoint. It’s also how every home in our finished projects was built.
When Prefab Might Be the Right Call
Prefab is a good fit for some projects. It may be the right call if:
- Your timeline lines up with the factory’s schedule
- You’ve found prefab ADU homes built for snow and cold like ours
- You’re comfortable managing the site work and local contractors yourself
If that sounds like your situation, prefab is worth a close look. Just compare finished-project costs, not unit prices, and confirm what the maker includes. Knowing how to find a good ADU contractor matters just as much with prefab, since someone local still has to handle the foundation, utilities, and permits.
Choosing the Right Path for Your ADU
When it comes to prefab vs. stick-built ADU options, both can become a comfortable, lasting home. Prefab works best for standard designs on easy, open sites. Stick-built shines when you want a custom layout, have a tricky site, or need a home built for Inland Northwest winters.
Not sure which fits your property? As tiny home builders who work across Spokane and North Idaho, we’re happy to walk your site with you and talk through your options. No pressure, just clear answers.