Table of Contents
- Things to consider before building a prefab ADU
- The benefits and drawbacks of ADUs
- The best backyard ADUs: our top picks
- Plant Prefab
- LightHouse ADU
- How it works
- Bensonwood / Unity Homes
- Bamboo Living
- Nomad Micro Homes
- Bert’s Boxes
- Honorable mention
Prefabricated homes can be extremely stylish and, if you choose the right company, very eco-friendly. No wonder, then, that prefab is popular for accessory dwelling units (ADUs) such as laneway houses.
Things to consider before building a prefab ADU
There are a lot of prefab home manufacturers out there, and not all are created equal. One good rule of thumb is to check to see how long a company has been in business. At minimum, you should be able to see a model home. Ideally, you’ll be able to see a home that’s been lived in for at least a few years, or be able to stay in a model home that operates as an Airbnb or hotel. If all a company has is plans and pretty computer-generated mock-ups, be very wary.
If you can, try to track down others who have recently built with the company you’re interested in. This way, you can get a feeling for how the process unfolds and whether your timeline, expectations, and your personality will mesh with the project managers and other team members.
In addition to having a good track record, it’s also important to feel good about a company’s future. This means that you need to get a good feeling about those running the company, so you are confident that the business will honor their projected building timeline and any warranties in subsequent years.
Look for companies with architects and building science experts on staff. Knowledgeable project managers are also a must. If a company doesn’t have a strong team of building science experts, consider going elsewhere. If you have multiple chemical sensitivities or mold sensitivities and the company can’t accommodate this, move onto another company.
Ask if you can tour the company’s factory. This will give you an idea of how the team operates and the quality of craftsmanship. Finally, before putting down any money, be sure you have details of the building and design, as well as a timeline for your project.
The benefits and drawbacks of ADUs
ADUs can be beneficial in that they make use of existing space in urban areas. This means less land disturbance and urban spread. It also allows people to live closer to where they work and to other amenities. This facilitates shorter commutes, meaning reduced associated greenhouse gas emissions. And, if you use a prefab ADU as a home office, you really reduce your commute!
By controlling urban sprawl, ADUs also help protect greenspace. Laneway houses and other ADUs allow for higher density in places where high-rises aren’t the right fit. This has benefits for the environment, wildlife, and for our mental and physical wellbeing, by keeping greenspaces green.
Now for the disadvantages. ADUs, including prefabricated ADUs, will likely cost you less than an average family home, but on a cost per square foot basis, these smaller dwellings can cost twice as much as a conventional home. There are just fewer efficiencies when building small, especially if your design needs to fit an awkwardly sized space or peculiar building code.
Some ADU designs are also super stylish but totally terrible when it comes to energy efficiency. After all, the higher the ratio of exterior walls to interior space, the harder it is to heat and cool the interior efficiently. The trick here is to build in efficiencies where possible, such as orienting the ADU to allow for passive heating and cooling, depending on your local climate, and using denser or lighter materials in darker or lighter colors to soak up or reflect energy from the sun. A clever designer will take all these factors into consideration when planning your dream ADU.
The best backyard ADUs: our top picks
Here are our top picks for the best prefab homes for a backyard ADU.
Highlights: A well-regarded B Corp making LEED Platinum homes that go beyond Passive House standards. Best for the west coast of the US.
Based in Rialto, CA, Plant Prefab is a B Corporation dedicated to sustainable building processes. In 2019, Plant Prefab committed to becoming fully carbon neutral by 2028. The company is clearly committed to sustainability and has a solid track record, winning it top billing at Leaf Score.
Plant Prefab uses the US Green Building Council’s LEED program as a baseline to verify the performance and sustainability of their LivingHomes builds. LEED includes water and resource conservation as well as energy efficiency and indoor air quality. All LivingHomes are designed to LEED Platinum standards for maximum energy-efficiency, with Plant Prefab’s LivingHome the first home to achieve LEED Platinum status. Since then, 27 LivingHomes have garnered LEED Platinum status and five LEED Gold.
The company’s own Z6 framework goes beyond LEED Platinum, however. The six zeros are: Zero Energy, Zero Water, Zero Emissions, Zero Carbon, Zero Waste and Zero Ignorance. Plant Prefab readily admits that no ADU achieves perfect scores on all these measures, but the aim is to get as close as possible to zero or negative impact on all fronts.
As an example of the Z6 approach to conserving water, all LivingHomes use low-flow water fixtures and are made grey water ready. This means that all waste water from the bath, shower, washing machine, and dishwasher can be used for landscape irrigation. Plant Prefab’s LivingHome 6 uses 40 percent less water for indoor uses than an average home. A whopping 80–90% of the waste generated during the construction of the LivingHome 6 is also reused, repurposed, or recycled.
Plant Prefab’s in-house design studio, Living Homes, uses fairly standard construction materials for most of the build, but the clever design process allows for significant efficiencies in construction and operation. You can choose one of the company’s designs or create your own. In fact, Plant Prefab began life in 2006 as a residential design studio (LivingHomes) and a decade later became Plant Prefab, so as to allow the company to build any design of prefabricated home for any architect.
Plant Prefab offers the LightHouse ADU in partnership with Alchemy Architects, and in 2020 launched its own ADU called LivingHome 10. The first LightHouse ADU was a 1 bedroom, 1 bathroom, 423 square foot design built and installed in October 2019. The design allowed for a one-day installation as the ADU is a single module that could be trucked whole to the site. This accessory dwelling unit (ADU) included a kitchen, bedroom, bathroom, and laundry room, with flexible storage space. It required just utility connections and minor exterior trim and detailing. This included the LightHouse LivingHome’s signature, hand-crafted entry light, stair, and awnings.
The LightHouse LivingHome ADU series now includes five size options and 13 floor plan variations with studio and 1-bedroom, 1-bathroom options ranging from 310-600 square feet. There’s even a 1-bedroom design atop a 2-car garage, ideal for an ADU in a laneway. Total costs range from around $265,000 to $533,000.
All millwork and engineered wood is free from added formaldehyde, and the company uses zero VOC paints and stains. High-performance heating and cooling systems are included, along with Energy Star appliances and smart energy monitoring. LED lighting is standard, as are low-flow water fixtures and drywall with a high recycled content that’s also mold-resistant and non-combustible. Solar-ready and net-zero design is also available.
How it works
The company serves the west coast of the US and a few other areas (“Plant is only cost-competitive outside of California for high-end projects shipping to areas with very expensive local labor”). You can visit the company’s factory in Rialto and see your ADU being built, and Plant Prefab offers a 10-year structural warranty and double warranty protection through years 2-10.
With Plant Prefab, you can choose your own architect and general contractor or work with those suggested by Plant Prefab. The general contractor will coordinate with the design team and work out any issues while conducting quality-control throughout the build.
Highlights: Best for smart home tech, with solar ready, net-zero, Passive House design, from a California-based company that can ship modules across the contiguous US.
Dvele was founded in 2016 and is based in California. Its founders previously enjoyed great success in Canada with a similar sustainability focused prefab company. Dvele is very transparent about the materials used in its designs, with an eye on reducing the environmental impact of its builds.
A Dvele build has an airtight envelope that allows homes to use 65-75% less energy than standard homes. All Dvele designs are solar ready and built to Passive House standards. The factory process allows for greater efficiencies and less waste than conventional builds, and the company plants 10,000 trees for each Dvele home finished.
The homes are smart tech enabled, with a mesh network, Wi-Fi running through the walls, and Sonos, Next, eero, and Lutron, embedded in the home to enable seamless home automation. This includes monitoring indoor air quality, moisture, and temperature changes inside the exterior walls. Each Dvele home can include in-line water filtration system at the water’s source point into the home.
The Dvele Mini offers between 460 and 650 sq. ft. and one to two bedrooms. It has the same smart tech as a standard-size Dvele home. Manufacturing costs for a Dvele are in the range of $200-75 per sq. ft., depending on finishings. This includes electrical, plumbing, mechanical, appliances, and other finishings ready to be shipped from the factory.
Dvele offers a fixed price contract for factory builds, so you don’t have to worry about overruns which are common with site builds. Homes are available to be shipped across the US, and the company may even be able to work with customers in Alaska and Hawaii. You can visit a Dvele home in Santa Rosa and Ventura, CA, and the company encourages factory tours.
Dvele do use some high VOC materials, such as epoxy, as a way to reduce potential mold growth. The whole home air filtration system appears to be effective at capturing any initial off-gassing, however, and the smart tech monitoring system can sense the presence of VOCs and take steps to bring in fresher air.
Dvele builds come with a 2-year warranty on fit and finish, a 5-year building envelope warranty, and 10-year structural warranty. The homes are seismically resistant, as you’d expect from a California company.
Highlights: A very well established company with decades of green building experience. The Nano model is an excellent ADU that is affordable, stylish, and sustainable.
Bensonwood is a certified Passive House builder based in Keene, NH. The company has a proprietary PHlexible Passive House wall panel system that it uses to create scalable enclosures that are air-tight and super energy efficient. Bensonwood has been around since 1973, and its sister company, Unity Homes, was founded in 2012 to offer a wider range of home designs, including the Nano, that are less expensive (but less customizable).
The Nano is a one- or two-story cottage offering 480-960 sq. ft. It starts at $85,000 for the Shell, with prices higher for a Tempo package where the company provides the assembled shell and all the materials to finish the home.
The Nano works great as an ADU as it offers 1-2 bedrooms and bathrooms in a well designed small footprint. The company is committed to sustainable design, using carefully chosen materials, fixtures and finishes to enhance livability while keeping the environmental impact low.
Like all Bensonwood builds, the Nano uses cellulose insulation instead of foam or fiberglass. This means that an otherwise wasted resource (discarded newspaper) can be repurposed, reducing carbon emissions. Cellulose is a highly advantageous insulation material which I’ve written about before on Leaf Score, so it’s great to see Bensonwood using this material in their panels and as blown insulation on-site where necessary.
The Nano also features heat pump technology for heating and cooling, and this can be hooked into a solar array to help make your ADU net-zero. The airtight factory build and mechanical air exchange makes for an excellent mold preventative design and is also great for anyone with chemical sensitivities (the company even lets you take some materials home to test for sensitivity).
Highlights: Based in North Carolina and established in 1968, Deltec is a B Corp making net-zero capable homes built to last.
Deltec is one of our favorite companies for general prefab homes, so it shouldn’t be a surprise to see this B Corp on our list of the best prefab homes for ADUs. Based in Asheville, NC, Deltec meets your where you’re at, with designs ranging from homes that are more energy efficient than standard to full-on net-zero ADUs. While the company doesn’t specialize in ADUs, their smaller c700 sq. ft. designs fit the bill nicely.
Deltec homes are prefabricated in a factory powered entirely by renewable energy (since 2007!) and the company won a US Department of Energy’s DOE Housing Innovation Award in 2019. The company’s recycle and reuse program helps divert more than three-quarters of manufacturing waste away from landfill.
The company has been building homes since 1968 and is a trusted name for green building. Deltec are particularly focused on building ‘legacy’ homes, i.e., those that stand the test of time and are good enough quality to withstand extreme weather events such as hurricanes and tornadoes.
Highlights: Seattle-based FabCab offers slightly pricier ADUs and tiny homes built with sustainable materials and low-toxicity finishes. Modules and panels can be shipped across the US.
FabCab offers timber cabin style prefab ADUs made predominantly with wood and featuring a lot of glass to help bring the outside in and create a spacious feeling even in a small footprint. The smallest FabCab design is a 621 sq. ft. tiny house suitable for use on a foundation. Costs range from about $375 per sq. ft., making these quite a bit more expensive than Plant Prefab, Deltec, and Bamboo Living.
The company is based in Seattle, WA, and sells home packages as kits comprising precision-cut and labeled components. It’s up to you whether you buy the parts only and put these together on site yourself, or if you purchase a complete home including assembly, finishes, foundation, site work, and permits. FabCab currently has a six month queue for builds.
The SIP panel construction and exposed Douglas fir timber framing makes for a straightforward build for most contractors. That said, builds can take a couple of weeks to create a weather-closed system, which is quite a bit longer than with Plant Prefab.
The FabCab approach offers efficient concept layouts to maximize usable space. Energy efficient appliances can be included, along with recycled carpeting and/or bamboo flooring, in-floor radiant heating (with optional solar water heating hook-up), low VOC paints and finishes, and water efficient plumbing features. You can also add in cisterns to catch and store rainwater.
FabCab is known for reducing construction waste through precision computerized cutting machines. This exact approach makes for high-quality, efficient assembly. The components can be shipped anywhere in North America, and the company has begun working with regional suppliers in some areas. This has the potential to make FabCab designs even more energy efficient (and cheaper) thanks to reduced transport costs.
Highlights: Pittsburgh-based company offering an ADU that can be net-zero and Passive House certified. The EcoTiny is designed with sustainable, low-toxicity materials and finishes, including rainwater capture systems.
Based in Pittsburgh, PA, EcoCraft has a range of home models that are all Passive House certified. EcoCraft claims that more than 95% of its construction waste is recycled, making it almost zero-waste.
The homes come with a 10-year structural warranty and 1-year cosmetic warranty. Prices start at around $285,000, or around $190 per square foot for a turn-key project at the low end.
The best ADU option is a 583 sq. ft. EcoTiny offering 1 bedroom and 1 bathroom. This design features a full-size kitchen, bathroom, and separate sleeping quarters, a large front deck, and high ceilings and large windows to create an open, airy feel.
The ADU can be designed to net-zero standards if solar is included. High-performance doors and windows help make the EcoTiny energy efficient, along with LED lighting and no thermal bridges (the builds are all airtight). EcoCraft strives to use recyclable materials that are low-toxicity, including formaldehyde-free insulation, low- or no-VOC paints, no-urea OSB, low- or no-VOC adhesives and sealants.
An energy recovery ventilation (ERV) system is included to keep the air fresh inside the home, though the company states that it focuses on “avoiding toxic materials so that our energy recovery ventilators don’t have to clean the air, too.” Oddly, there’s an option to have a suspended fireplace in this design, which I’d suggest avoiding given the potential for associated poor air quality.
The builds feature low-flow plumbing for water conservation, and can include whole house inline water filtration technologies. Rainwater capture can also be included, with cisterns and specially designed filtration rainwater allowing for water reuse for toilets and irrigation.
The company builds in modules rather than panels, and you can tour the factory and see your home being built. Standard builds use unfaced fiberglass batts for wall and floor insulation, with blown fiberglass in the roof space and spray foam to seal any air leaks. The company claims to use “ultra efficient mechanical equipment with small diameter ducts and variable speed blowers” to reduce energy use and ensure consistent temperatures.
Costs include excavation, basement, walls, roof, plumbing and electrical, finishes, appliances, delivery, and installation (if you’re in Pittsburgh or nearby). The full build option is available within 60 miles of Pittsburgh. Outside of that area, you can buy the panels but will have to find a contractor to complete the construction.
Highlights: Factory built multipurpose European minimalist microhomes now delivering to North America!
Kodasema has just started taking orders for shipping to North America and is a great option for a minimalist ADU. The KODA is a free-standing compact timber house that can be delivered in one piece and installed in a day. The company is a member of the United Nations One Planet Network, is based in Estonia, and creates factory constructed houses with a small environmental footprint.
The steel reinforced timber frame homes are delightfully minimalist and well insulated for comfortable, energy efficient year-round living. Any size of a Koda qualifies as an Accessory Dwelling Unit (ADU) in most jurisdictions. All you need is level ground and sewerage, plumbing, and electric hookups.
If you’re in Europe, you can order the Koda Micro (just 215 sq. ft.), which comes with a kitchenette, shower room, storage space, and separate sleeping and living areas. The Koda Loft is a little bigger at 278 sq. ft. and is also only available in Europe currently.
The Koda Loft Extended is the first model available for shipping to North America and offers 493 sq. ft. of space. It costs $159,000 USD and can be delivered within 24-30 weeks of payment (10% deposit, +70% to begin build, +20% to ship). The extended model is a great choice for an ADU as the design allows for separate living and work spaces. This means you can set up a laneway house with a separate entrance for a home office and guest suite.
Other KODA Models will start shipping in 2022 and shipping/delivery quotes are available on request. You will need to figure out land planning and preparation, engineering, permitting, utilities, cranes and transport.
KODA won in two categories at the 2021 DNA Paris Awards, for Interior Design/Sustainable Living and Architecture/Green Architecture. In 2019, a cluster of KODAs – a KODA Park – won the Outstanding Property Award London (OPAL) 2019 in the category Property Development / Sustainable/Environmental Development. The design has also garnered attention from various other sustainability groups largely because of its flexible design that allows for relocation and repurposing as housing, office, and retail space as desired.
The KODA itself is also eco-friendly, with light timber-construction housing estimated to consume half the amount of energy of a concrete or metal frame house (Adalberth, 2000). There’s also a potential 37% energy use reduction associated with building a timber-frame apartment block compared to a concrete build (Tettey et al., 2019).
Highlights: Highlights: A good option for a backyard ADU in a tropical location, especially if prone to hurricanes.
Bamboo Living is based in Pahoa, Hawaii, and provides durable, low-maintenance ADUs and homes made with bamboo. These buildings are seriously resilient as they are designed to withstand hurricanes and 200 mph winds. The company uses panelized construction with insulated and non-insulated wall options and offers a 20-year structural warranty.
Bamboo Living started out in 1995 and claims to have built more than 400 homes. The company’s range includes 100 sq. ft. single room designs to 3000 sq. ft. 5-bedroom villas and resorts. The Puna 352 is the company’s smallest bamboo home that qualifies as a full dwelling in most areas of the US. The company can help you get a building permit for this design and can also help you find a local contractor to put the home kit together.
Bamboo Homes offers tours of current builds, which is reassuring, and the Puna 352 was also featured on HGTV’s Tiny Paradise. The design features a u-shaped kitchen, walk-in closet, separate bathroom, and a loft for sleeping or storage. It has flying gables and triangular vents for air flow, making it a great choice for more tropical locations. The design also allows for adding an extra bedroom or two later on.
The Puna 352 measures 16 feet by 28 feet, providing 352 sq. ft. of interior space, 96 sq. ft. of porch space, and 81 sq. ft. of loft space. The Hybrid Bamboo Home Package starts at $27,200 and the Signature Package at $34,240. The Hybrid Home Package is for double-walled homes which are conventionally framed on site. You have to source the lumber and wall materials locally. The Signature Home Package comes with bamboo-framed single walls as a kit. This can be assembled onto your foundation out of the container. Not all jurisdictions allow for single-walled homes, and these are better suited to temperate climates, so be sure of local regulations before signing a contract.
The appeal of bamboo is that it grows incredibly quickly and stores a lot of carbon. It is also a great building material, if you know how to use it. The relative eco-friendliness of a bamboo home depends largely, however, on where it’s being shipped to, if it’s being framed conventionally on site, and if it is well suited to the climate (i.e., doesn’t require heating for much, if any, of the year.
Highlights: Easy to assemble micro homes ideal as an ADU for the minimalist DIYer.
Nomad Micro Homes aren’t my top choice for a full-size prefabricated house but are a good option for an ADU where you already have hookups for electric, sewerage, and so forth. Nomad Micro Homes are manufactured in BC, Canada, and come in two sizes: the 10.5’ x 10.5’ Micro and the 13’ x 13’ Cube. Some assembly is required on site, and it’s not clear that the company has a green building scientists on staff, so the sustainability of this ADU is questionable.
The microhomes are more affordable than most, but this is because you do a lot of the work yourself. Unlike with Bensonwood, Plant Prefab, etc., you’ll have to figure out things like permits, foundation, interior wall finishes, roofing membrane, appliances, electrical and plumbing hookups, and an ERV/HRV.
These ADUs are off-grid capable as they can be set up with rainwater collection, solar energy, and grey water treatment systems. They can also work with municipal sewer, septic field, or composting toiler set-ups. The ADU also has an optional water purification system and several options for renewable energy generation and back-up batteries and generators (propane powered, unfortunately).
The Nomad homes have triple glazing as an optional upgrade and are built with a 3D-printed steel frame, engineered wood panel, and pre-finished flashing and roof fascia. Ikea cabinetry is standard, with stainless steel and fiberglass sinks, a fiberglass shower base, and LED lighting.
R23 insulation is used in the walls and R28 insulation in the floors and roof (the company doesn’t specify what kind of insulation and hasn’t responded to questions via email). The outer insulated panel system is a continuous layer of insulation, with no thermal bridging.
You can buy your Nomad in just a few clicks, with options to ship the cube to you on a semi-trailer, flat-bed truck, or in shipping containers. You can even head to Vancouver, BC, to pick up your home yourself. The cubes can be shipped overseas and can be assembled within days. And if you decide to move or relocate the cube on your current property, no problem. Each cube weighs around 5,000 lbs, and can be unscrewed, stacked, moved, and reassembled.
Highlights: UK-based company making ADUs with contemporary modernist designs – just make sure to watch the materials used for construction.
Bert’s Boxes are potentially quite eco-friendly ADUs but it all depends on where you are and what materials are available for construction. The design is a fun combination of contemporary modernist architecture and rustic cabin. Best suited for those living in the UK, where the company is based.
Bert’s Boxes offers studio, 1- and 2-bedroom ADUs that can be made with some reclaimed materials such as wood flooring and exteriors, tiles and brassware. You can choose to add a green roof for extra insulation and promotion of biodiversity, and there’s also an option to add a basic photovoltaic array to cover 25-50% of energy needs. The solar array may qualify for subsidies in some areas, helping offset your building costs.
Bert’s Boxes are designed with Crittal windows and doors, double-glazed steel facades, insulated panels, and radiant heating throughout, as well as smart-technology for lighting, heating, and entertainment. Depending on where you live, you can add in a wood burning stove, choose traditional iron mongery and reclaimed oak cladding, and add on a pergola.
Made through a collaboration of two companies based in London, UK, these boxes come with a 10-year structural warranty. Unfortunately, the company offers little information on insulation and other building materials and seems largely focused on aesthetics. I contacted the company a year or so ago to ask for more information but never got a response.
The reason I’m including Bert’s Boxes is because the purchasing process is super simple, with easy add-ons for renewable energy products and quick completion (withing 14 weeks from order, typically). This is one of the fastest turnarounds, with occupation possible withing a few days of the box arriving on site.
Here are a few other companies I have my eye on for the best prefab homes for a backyard ADU.
Founded in 2016, with the first home built in 2019, this Pittsburgh-based company offers ADUs and larger homes that are net-zero ready. The panelized and modular construction takes place in the Bensonwood Tektoniks factory, offering reassurance of quality of construction. The company uses cellulose insulation and offers an upgrade option for finishes and materials that are more sustainable and healthy. Given how new Module is, I would wait until the company has built a few more homes before signing a contract.
Shelter Dynamics is another California-based company offering prefabricated housing options with sustainable features. These include Energy Star appliances, hybrid electric heat pump water heater, LED lighting, a “Mini-Split” heat pump HVAC system with separately controllable zones, solar panel arrays, and smart home automation. The company’s current range includes a 750 and 1000 sq. ft. models providing 2-3 bedrooms. A 450 sq. ft., 1-bedroom version is under design, so check back soon!
Pacific Homes is based in British Columbia, Canada, but can deliver all over North America. The company works with you to design your home based on one of Pacific’s plans or your own plan, using its Pacific SmartWall® System. This assembly is significantly more energy efficient that conventional construction and can be used in a wide variety of designs. The company uses sustainably sourced wood as well as recycled materials such as fiber cement siding and mouldings. You can tour the company’s factory and check on the claim that over 90% of production waste is recycled by local companies.