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A Prefab Homes Primer

A Prefab Homes Primer
Modern design never looked so good.
Today’s prefab house looks about as much like your great-grandmother’s Sears kit home as a new Tesla 3 looks like a Ford Model T. And yet, prefab homes are skyrocketing in popularity these days. I recently sat down with Matthew Coates, president and principal architect of Coates Design Architects, a sustainable design and green building firm on Bainbridge Island, Washington, and Brian Abramson, co-founder of Method Homes, a pre-designed and custom prefab home builder in Seattle, to find out what you need to know about prefab homes if you’re thinking about building one.

Suit the Particular Situation
Don’t get your heart set on building a prefab house solely based on the coolness factor. Selecting a house based on the delivery method is “the tail wagging the dog,” Coates says. “Instead, look at your project constraints and ask, ‘What is the best method to fit my constraints?’” Speaking of building constraints, Abramson says it’s crucial to buy your property first and familiarize yourself with any issues before contacting a prefab builder. Why? “Every prefab house needs to be customized and engineered to a specific site,” he explains. “We can’t help until we have a specific piece of property and the issues, such as wetlands, steep slopes and zoning concerns, are identified.”

Consider the Costs
Prefab homes are not cheaper than stick-built homes just because they arrive preassembled. “If you think you’re going to save money building prefab, you might be surprised once all the costs are added up,” Coates says, though he adds that building a prefab house can, in fact, have a substantial financial benefit, particularly if time is money to you. In Coates’ experience, the cost to build a custom prefab home (including everything) starts at around $275 per square foot, in part because the cost of shipping prefab modules is significant, foundation costs vary, and any complications on the road between the factory and your site (such as low bridges, etc.) that cause problems for the trucks delivering your house may limit design options and/or cost you extra.

Method Homes divides prefab home building costs into three categories: soft, modular and site costs. Soft costs, including the feasibility study, site survey, soils analysis, architectural design, structural engineering, permit coordination, utility connection fees and permit and impact fees, can be between $15,000 and $30,000 and up. Modular costs (Method includes siding, cabinets, flooring, tile, plumbing, plumbing/lighting fixtures, countertops and the HVAC system in the building costs, although other prefab home builders may cost things out differently) run $136–$194 per square foot and up.

And site costs, (which are separate from the cost of building the prefab home and include site prep work [excluding excavation], utilities, the home’s foundation, transportation of the modules by semi-trailer trucks from the factory to the building site, installation [including the use of a large crane to move the modules from the semi-trailer trucks to the foundation], utility connections, all on-site finish work [including connecting the home’s modules at the seams], and any site-built components like decks, exterior stairs and paint) typically run an additional 30 to 70 percent of the modular costs. According to Abramson, on average, general contractors take approximately two months to ready a home site for installation, and Method works closely with them during the site-prep process and visits the job site prior to delivery to ensure that everything is ready for installation. Below is an example from Method Homes' Cabin Series, by 
Prentiss Balance Wickline Architects.

When Speed Matters and Custom Modifications Are Not Required
Building a prefab home is the right decision under certain circumstances—but not all the time. Coates explains that prefabs make sense when time is money (e.g. you need to build the house fast). Method’s website says it can build a custom prefab home 60 percent faster than the traditional site-built construction cycle. And a National Association of Home Builders (NAHB) case study of 36 projects completed during Superstorm Sandy recovery efforts showed that building modular homes increased construction speed by 31 percent over that of building traditional homes.

Coates adds that prefab homes also make sense when you don’t need significant custom modification or personalization and your site is easily accessible. Coates and Abramson both say that prefab homes may be more sustainable than stick-built homes, so factor that in as well. On the other hand, according to Coates, prefab homes are meant to sit on flat foundations, are not cost-effective for sloped lots with difficult access, and it’s somewhat common to find small cracks or damage to the drywall due to the stresses put on the modular units during shipping and while craning them into place (these cracks are usually simple to fix). In addition, the NAHB points out that prefab construction can face challenges from labor shortages and construction-speed issues to job-site security and construction volume. The key is to weigh the plusses and minuses and determine whether a prefab home hits your project sweet spot.

It’s also important to understand clearly what level of finish you are getting when you buy a prefab house. According to Coates, prefab can mean anything from a wall panel built in a factory to an entire home. If you’re expecting a finished home with all the interior work done and what gets delivered to your site is bare modules in which the drywall still needs to be taped and painted, cabinets hung, etc., this can cause problems. Communicating clearly with your builder and the general contractor can help you avoid unpleasant surprises.

And finally, it’s important to understand that a prefab home is not a manufactured home. “There’s still a misconception that prefab homes are manufactured homes (or trailers) built to HUD standards,” Abramson says. “Method homes are built to the homeowners’ local building codes; in fact, we build our houses above local code requirements, and they appraise no differently than a stick-built home.”

Design Variety
Prefab homes come in a wide range of designs and sizes. Method homes typically range between 2,000 and 3,000 square feet, although the company builds everything from small homes like this one, part of

Method's Cabin Series, designed by Prentiss Balance Wickline Architects, to large custom 
hybrid prefab homes like this one 

at Martis Camp near Lake Tahoe designed by SageModern. According to Abramson, hybrid prefab and site-built homes are becoming more popular, especially in larger homes where homeowners want a great room that’s larger than can be easily built in a factory setting. Along with complete custom prefab home design, Method also offers eight series of predesigned prefab models (created by five architects), which can be customized to individual homeowner needs and specific building sites. With the predesigned models, the building process is streamlined, and design revisions are limited to two rounds.

Some designs on can be built as prefabs. Take for example, Coates Design’s 2,052-square-foot Plan 498-4
with its expansive, light-filled living room and kitchen, large fireplace and wall of windows that invites

indoor-outdoor living. Coates says that this plan can be modified easily for prefab construction because it’s a single-story design with simple shapes that are easily configurable for shipment. He explains that the multiple units could be joined together at the indoor-outdoor entry (see the floor plan inset above), which would be 

built on-site. Also, Coates Design’s 3,056-square-foot, two-story Plan 498-6 with its open floor plan and large window wall that unites the living area with the adjacent deck to form a single indoor-outdoor

entertaining space, would do well as a prefab home.

Coates explains that building a home in a controlled environment leads to less damage to building materials, meaning less construction waste; a tighter, more energy-efficient building envelope (giving you lower heating and cooling costs); and the reduction or elimination of harsh chemicals to remove mold and fungus from wood building materials exposed to rain (improving your indoor air quality). According to Abramson, Method’s in-factory building process reduces construction waste to less than 10 percent. Method Homes also include locally harvested lumber, low or no-VOC paints and finishes, above-code insulation, FSC-certified hardwood floors, dual-flush toilets, low-flow fixtures and pre-wiring for solar. They are free of UA formaldehyde and are eligible for LEED and other environmental certifications.

What To Expect On Installation Day
First, semi-trailer trucks arrive at your property and deliver the pieces of your new home. Method ships their modules between 85 and 90 percent complete to minimize on-site construction. Second, your general contractor and crew bolt the modules to the foundation, bolt them to each other and then connect them with drywall, flooring and siding. Coates explains that prefab homes are bolted to the foundation in a similar manner to stick-built houses; however, with prefabs, the anchor bolts are drilled and epoxied into the foundation after the modules are set in place, thereby attaching them to the foundation. Coates recommends patching the joints after the house is set, and Abramson recommends verifying that the anchor bolts are flush with the sill plate. Coates also suggests giving advance thought to which way the seams run in the house and how to handle flooring and wall materials to hide the seams on-site. Third, the roof, interior stairs, exterior decks, utility connections and HVAC system are installed. Method estimates post-delivery on-site construction time at between two weeks and two or more months.

Why the Increased Interest Now? 
“There’s a desire for a streamlined, shortened process combined with limited labor availability for on-site construction,” Abramson says. Coates adds that the popularity is also due to prefabs getting a lot of attention in design magazines and online publications right now—for good reason. “Prefab construction offers the right homeowner a simple, cost-effective housing solution that removes most of the guesswork and construction time typically involved in a custom stick-built project,” Coates says. “If you’re someone who becomes overwhelmed with the thousands of decisions and choices that are part of a custom house project, or if you need it yesterday, prefabrication might be a great solution for you.” 

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