House Feature

Alchemy Architects Designs for Prefab ADU’s

By Camille LeFevre, Home Feature Editor

Last Updated: Feb 9, 2025

When architect Geoffrey Warner started his firm Alchemy Architects 26 years ago in St. Paul, Minnesota, he was designing furniture and lighting, renovating bungalows (including his own) and innovating a typology known as a “modern farmhouse.” All of these endeavors received critical acclaim for their fresh modernist aesthetic and inventive use of materials. They were also widely published.

Variations on a Theme
Designing for Durability
Research-Driven Design

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Then, in 2003, a violinist with the Minnesota Orchestra asked Warner to create a small, modern cabin she could put on some land in Wisconsin as a rural getaway. The weeHouse® was born. The original 336-square-foot weeHouse was built “in-factory,” Warner says, and shipped to the site on a truck. A crane placed the structure on top of a pier foundation. The cementitious siding was coated with oxidizing paint that provided a rustic, hardy finish. The interior included wood-wrapped walls, IKEA cabinetry and accessories, and floor-to-ceiling Andersen windows.

Since then, Alchemy has designed and constructed weeHouses (and not-so-weeHouses) in various iterations throughout the country. The Sonoma weeHouse, built for an Apple Senior Design Director of Real Estate and Development, won the 2018 Small Projects Award from the American Institute of Architects. For Warner, the weeHouse, a form of modular construction, was developed as a way of delivering efficient yet elegant design; a small, sustainable option for houses, offices, rooftop studios, and multi-unit developments, as well as cabins.

Next, Warner and his team have moved on to designed the barnHouse, which draws from the rural vernacular and celebrates lofted interior spaces. Barn Houses are constructed with 12” SIP (Structural Insulated Panels) floor, wall and ceiling panels; passive-house windows and window shrouds; and a central utility core in which the kitchen, bathroom, sleep, and laundry areas are located.

Then came the lightHotel art project, a tiny sustainable motel room on wheels that moved around Minneapolis and St. Paul—including to museums, a community garden, and a city park. “A beacon for ecotourism and sustainable living,” the lightHotel was available for rent via Airbnb. Constructed from an 8′ x 20′ shipping container, the lightHotel has a spray-foam shell, triple-glazed windows, and doors, solar-powered HVAC, LED lighting, and hydronic in-floor heat. A 250-gallon water tank of well-water supplies the water for an on-demand hot water heater, and a bio-filter produces re-usable greywater. Guest experiences and the lightHotel’s itinerary were mapped online, educating guests on conservation and sustainability. (For a video on the lightHotel, go here.) In 2018, Tom Puzak, an entrepreneur and mountain-bike enthusiast, bought the lightHotel as the centerpiece of a sustainable, off-grid “Getaway Circle" he’s building in Minnesota’s Cuyuna Lakes area.

Alchemy’s lightHotel helped inform the firm’s research for its newest prefabricated lightHouse (modular or SIP kit), a low-energy ADU (accessory dwelling unit). Currently, in beta-testing, the first lightHouse ADU is under construction in Sebastopol, California. This low-energy ADU can serve as a small house, cabin, office, guest quarters, mother-in-law apartment, aging-in-place space, or rental unit. The Sebastopol couple who commissioned the lightHouse intend to live in the ADU while Alchemy is designing their 3-bedroom, 2-bath weeHouse on the property. The lightHouse will then be used as a rental or guest house.

Variations on a Theme

Knowledge gained from design iterations of the weeHouse, barnHouse, and lightHouse, as well as custom commercial applications, has informed the designers at Alchemy around developing and refining a sustainable structure that’s easy to construct and deliver—regardless of location. “With the weeHouse, we originally thought clients would choose a plan, and we could execute it with a factory partner in about three months,” Warner says. “It didn’t work that way. The weeHouse modules we showed were just starting points for customized homes.”

Moreover, states have different climates, not to mention variations in state and city codes, requirements, and restrictions. Sites vary in terms of slope and grade, vegetation and views, shade and sun. Clients have an array of needs and aesthetics.

Over the years, and in collaboration with various clients, Alchemy has built weeHouse iterations that include decks, stairs, garages, upper and lower levels using stacked modules, cantilevered modules, and “skyway” or underground connections between modules. They’ve planned weeCommunities and weeHouse family compounds. They’ve also designed a weeHouseBoat.

Still, Warner wanted “to develop a living unit that minimizes the number of variations, and we settled on the lightHouse, an ADU,” he says. “It’s bigger than a shed but smaller than a house. It’s a nicely designed, simplified structure, so we can concentrate on just what does go into it, while the shell stays the same size.”

Alchemy’s creation of the lightHouse has dovetailed with new regulations in cities throughout the U.S. that allow stand-alone ADUs between 300 and 600 square feet. The cost? Approximately $125,000 in the Midwest for a completed 450-square-foot lightHouse on a simple foundation.

Alchemy’s lightHouse ADU has three exterior plans that respond to the site: an end porch, a side porch, or it can be placed on top of a garage. “We figure out which solution works on each client’s lot, and provide variations with different entrances and views,” Warner says.

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The entrance to the Sebastopol lightHouse is from the end, with the view to the west. “We needed to temper the sun coming in from the west, and there wasn’t a view to the south, so we put the access on the east side,” Warner says. Because of the Sebastopol lot slopes, Alchemy also lowered the ADU down into the grade. A slight slope in the roof brings the ceiling height down over the kitchen for a greater sense of intimacy and to “give the lightHouse the feeling that it’s not just a box,” Warner says.

A lightHouse can be stick-built or constructed in a factory and delivered as a module or SIPs kit-of-parts, depending on on-site access. The Sebastopol lightHouse is being built out of 2x4s, with R50 12-inch neo-core SIP insulated panels and a plywood wall interior. The lightHouse also has passive house windows that are triple pane and tilt-turn, as well as insulated sliding doors. Roof insulation and white roofing, per California code, are also part of the construction.

Sheathing can vary, from a barn-wood-style siding to metal slats. The Sebastopol version will have cedar slats in random channels over a liquid Sip-Seal liquid membrane—a vapor-permeable, flexible weather barrier for high-performance building envelopes.

“Liquid membrane is used a lot in commercial buildings,” Warner says. “It’s also used around windows to seal them up. We’re using it because it simplifies window installation and detailing for keeping the house dry, and does a great job of keeping water out.”

Designing for Durability

“We always want to design for durability,” he adds. Because Sebastopol is located in fire country, the membrane is also fire-rated. “The siding is sacrificial; it’s cedar,” Warner says. “But because we’re putting on the cedar as a rain screen that let’s air in, it will dry faster and last a long time. And yes: you can see the membrane, which is black, through the rain-screen siding. It’s an animated skin.”

The Sebastopol lightHouse has a kitchen with a full-size refrigerator and storage areas, separate from the sleeping area. It also includes another Alchemy invention: A kitchen bench 30 inches wide with backs that can be removed so a guest can sleep on it (cushions and bedding can be tucked into storage underneath). In every lightHouse, the bathroom is located in the same place, regardless of the plan.

The ADU uses only LEDs for lighting. A low-temperature Fujitsu mini-split that provides both electrical heat and cooling will provide optimal comfort. “The low-temperature version has excellent performance,” Warner says. “We don’t even need an HRV or ERV, which is what brings in fresh air for a tight house and needs to heat when you bring it in. We can get away with a mini HRV bath fan.”

The last layer or feature, Warner says, is “laser-cut, CNC-folded cor-ten ‘jewelry’ on the outside of the house, as awnings or a custom outdoor light, for example.” The total cost for the Sebastopol lightHouse will come in around $250,000. “It’s expensive to build anything in California,” Warner says. “Even a small, sustainable, passive house.”

For all intents and purposes, the lightHouse is a “practical passive house with a lot of sustainable aspects,” Warner adds, “even though we’re not certifying them as passive houses. We’re incorporating more insulation than is usually used in California, for instance. We look at the lightHouse as a process of designing and building a whole, complete, well-integrated package, in which the components work well together.”

Research-Driven Design

Like any product research lab, Alchemy continually references past trials and successes when designing a new form of housing. “We’re constantly building on what we’ve done in the past,” Warner says. “Even though these are small projects, everything we do informs our other efforts. We also love our flexible, curious clients, who allow us to shepherd them toward innovative solutions.”

One of those solutions is the Envision Community, a community housing project for the homeless in Minneapolis, MN, that utilizes concepts Alchemy has developed through its work on the weeHouse and lightHouse. Another is the lighterHouse, a smaller ADU version without a kitchen.

“Our hopes for the lightHouse are to get to the point where we can plug and play the solution for clients,” Warner says. “A kind of ‘You want one, we ship it to you’ solution. We work so diligently on these small projects because sustainable housing with a small footprint is the right thing to do. We keep learning. It’s all good.”

Alchemy Architects Designs for Prefab ADU’s

Table of Contents

Variations on a Theme

Building Materials