Easy-To-Assemble SIPs Engineering

Prefab ADU Shell Kits

Build a Prefab ADU from Urban Cottage

Now for the fun part! Once you’ve purchased one of our detached ADU plans and obtained a permit, you can begin the construction of your prefab ADU.

Purchase a Shell Kit

The architectural plan you purchase will determine which shell kit you purchase. Each shell kit correlates with one of the pre-approved plans and includes the panels, integral wood, and metal components to put the shell structure together. You (or your contractor) can buy the other components you will need — like the doors, windows, siding, etc. — from your local supplier.

Regarding pricing, because of the quickly changing lumber and materials pricing, please call Urban Cottage at (206) 617-4455 or submit the form below for a current budget estimate.

Build or Contract Out Construction

While we can deliver a SIPs package to your driveway for your own assembly, we find that most folks opt for some help from a general contractor to build their prefab ADU. Whether you would like to have your cottage just “shelled up” (to where it’s weathertight but has no finishes) or you want it completely finished (move-in ready), we can help you find a general contractor experienced in building our cottages to take care of that for you.

And we also offer consulting services to assist with any of the above approaches or even something in between!

Quick note: We currently only ship kits within Seattle city limits.

Our Shell Kits

Our shell kits have been designed with construction in mind. Once the foundation is laid, there is no heavy machinery required — meaning, the units can be assembled with only hand tools. Additionally, the SIPs that make up the shell and roof of the unit are liftable by a team of just 2–3 people. Depending on your skill level and the complexity of your lot, it is entirely possible to assemble one of our shell kits on your own.

In most cases, however, we recommend hiring one of our preferred construction companies to do the work for you — at least through the shelled-in stage, but possibly for the entire project. Our preferred contractors are familiar with the process of building Urban Cottage detached ADUs and can typically construct the unit quicker than other contractors. Additionally, they manage the delivery schedule, coordinating with us directly to ensure each package is delivered when needed. Hiring one of a professional means you’ll be in your backyard cottage in no time at all!

Why Choose to Build With Urban Cottage?


Our cottages are compact, efficient, and hand-buildable. All of these factors help us keep the cost down for you. We also buy the panelized exterior wall system (SIPs) in volume and can pass those savings on to you!


While we have four set plans available for purchase, each plan can be phased up in size if you decide you’d like to expand. Start with our studio model, then add on additional bedrooms in the future — we can help you plan the location of your cottage to make it possible.

Opportunity for DIY

Our SIPs panelized system comes with full installation instructions and can be assembled by you and a few of your friends. No special tools are required and we provide you with all the pieces. If you can put together a chest of drawers, you can put together one of our cottage panel SIPs packages. Let us show you how!

SIPs Construction

We can deliver the installation packages on a schedule that works for you. You may choose to hire a crew to complete the project in just a few weeks; alternatively, you may decide to take your time, putting the pieces together as time and funding allow. No matter the pace, we are here to support you!

All our cottages are built with structural insulated panels (SIPs). This system not only simplifies construction, it also provides a super insulated and soundproof living space.

SIPs allow for more insulation inside a thinner wall, granting an additional 12 square feet of livable space inside your base unit (24 square feet & 48 square feet in the one- and two-bedroom, respectively) than with standard framed walls. And, to top it off, SIPs are environmentally friendly.


To ensure the look of your cottage complements the homes in your neighborhood, you can choose from a variety of roofing and exterior design options. Get the cost benefits of a production-oriented cottage while retaining the ability to easily and inexpensively customize it to fit your yard and design goals!

Interested In Purchasing A Drawing Plan? Let's Get Started!

Fill out this form below to contact us for more information on purchasing and building.

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