House Feature

Sustainable in Sebastopol: A Family Quickly Outgrows the First Prefab lightHouse ADU

By Camille LeFevre, Home Feature Editor

Last Updated: Apr 13, 2025

Amos and Lindsay Schallich had a plan. First, they would build a lightHouse, a prefab accessory dwelling unit designed by Alchemy Architects in St. Paul, MN, on their lot in Sebastopol, CA. After living in the ADU for a couple of years, they'd build a weeHouse (also designed by Alchemy) on the same property and turn the lightHouse into a rental unit or guest house.

"We love Alchemy's designs and wanted to work with them," says Amos Schallich. The couple also "liked the idea of prefab," he continues, "something that was built carefully and sustainably in a controlled environment, and then just showed up ready to go. The appeal was to get a smaller house built on the property quickly to then live in it while building the main house."

Then, The Kids Came Along.

The Schallichs only lived in their lightHouse for a couple of months. "Ultimately, it was smaller than what we needed with two small kids," he explains. "We'd started the project before having kids, and while they were in the plan, we didn't fully understand what having them in a studio-sized space would entail."

In re-evaluating their initial plan, the couple decided to sell the property and move elsewhere. Nonetheless, they remain enthusiastic about their lightHouse, the first built by Alchemy. The prefab ADU was constructed at Plant Prefab in California due to a unique collaboration between Alchemy and Plant.

How Big Is The lightHouse ADU?
Sustainable in Sebastopol
"A Ton of Character"

Home

One of the most surprising aspects of prefab that the couple learned was "how truly custom you can make a prefab house and still have all of the control over it that you would with a house built on-site," he says. "I can't think of anything that we wanted to do that we weren't able to do due to prefab constraints."

How Big Is The lightHouse ADU?

The Schallichs' one-bedroom, one-bath, 423-square-foot lightHouse was built in October 2019. Constructed as a single Plant Module, Plant Prefab had the ADU trucked to the site and installed—in one day—on a simple foundation. The on-site finish work included utility connections and minor exterior trim and detailing—the latter included the lightHouse's signature entry light, stair, and awnings.

Architect Geoffrey Warner, Alchemy's founder, and principal, meticulously designed the ADU to include a custom kitchen. The lightHouse also has a bedroom, bathroom, laundry room, and flexible storage.

It's bigger than a shed but smaller than a house

Warner says of the Schallichs' ADU. "It's a nicely designed, simplified structure, so we can concentrate on just what does go into it."

Sebastopol ADU Bedroom Alchemy Architects — Sebastopol ADU Bedroom. Photo Credit: Alchemy Architects

The lightHouse is available in five sizes (small, medium, large, extra-large, and 2X), ranging from 310 to 600 square feet. It has a dozen floor plans, from a one-room studio to a one-bedroom unit above a two-car garage.

The lightHouse ADU has three exterior plans that respond to the site: an end porch, a side porch--or an entry 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.

The entrance to the Sebastopol lightHouse is one end. "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 explains. The property also has a slight slope, so Warner lowered the ADU down into the grade.

Sustainable in Sebastopol

The ADU's small footprint and factory construction (key to decreasing construction waste) aren't the lightHouse's only sustainable aspects.

The Sebastopol lightHouse has a high-performance structure or envelope that's thermally broken with a high-quality air and weather barrier. Warner says the ADU very nearly qualifies as "a passive house with a lot of additional sustainable aspects, even though we're not certifying them as passive houses."

Plant Prefab built the Sebastopol lightHouse with 2x4s, R-50 12-inch neo-core SIPs (structural insulated panels), and a plywood wall interior. The passive-house windows are triple pane, as are the insulated sliding doors. Per California code, the ADU's roof insulation and white roofing were also part of the construction.

The ADU is clad in cedar slats placed in random channels over a liquid Sip-Seal liquid membrane. The vapor-permeable, flexible weather barrier is often used for high-performance building envelopes. "We used the membrane because it simplifies window installation and detailing for keeping the house dry," Warner says, "and does a great job of keeping water out." The membrane is fire rated, as Sebastopol is in fire country.

LEDs for lighting, low-flow plumbing, no-VOC paint, ENERGY STAR appliances, and energy-efficient windows add to the package. A low-temperature Fujitsu mini-split provides both electrical heat and cooling.

Alchemy Sebastopol Kitchen Alchemy Architects — Sebastopol ADU Kitchen. Photo Credit: Alchemy Architects

The lightHouse has a kitchen with a full-size refrigerator and storage areas, separate from the sleeping area. It also includes another signature element: A built-in 30-inch-wide oak bench with removable backs to transform the structure into a bed (cushions and bedding can be kept in storage underneath).

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Sebastopol ADU Bathroom Alchemy Architects — Sebastopol ADU Bathroom. Photo Credit: Alchemy Architects

In every lightHouse, the bathroom is located in the same place, regardless of the plan. It's part of a bathroom/storage core that contains all of the utilities, a washer/dryer, and a full sprinkler system. The tiled bath has a zero-threshold shower, frameless glass panels, and a touchless faucet on an enameled Dornbracht Alape sink.

"We consider the lightHouse a whole, complete, well-integrated package, in which the components work well together," Warner says.

"A Ton of Character"

The product of decades of research and design thinking, the lightHouse demonstrates Alchemy's diligence in creating small projects. "Sustainable housing with a small footprint is the right thing to do," Warner says. "Even these small projects inform our other efforts. We also love our adventurous clients, who allow us to guide them toward innovative solutions in customizing prefab to meet their needs."

The Schallichs weren't thrilled to leave their lightHouse. "It had so many unique features that made it extremely livable for being a small space," says Amos Schallich. "It was full of light, had plenty of storage, and really had everything that we needed. I loved the hardwood floors, the plywood interior walls, and the bench along the giant front window."