The promise of the prefab house is that decent mass-produced homes can be delivered quickly and cost-effectively. Modular and manufactured housing — one of the fastest-growing segments of the US housing market in the 1990s — accounted for 25 percent of new house starts. Then, as now, housing needs are urgent: Studies document that every county in the United States faces a shortage of affordable housing.
The prefab-housing industry has produced complete, unsubsidized homes characterized by economy, ease of finance, and innovative techniques and materials. By these measures, manufactured housing stands out.
Evolving from “travel trailers” and “mobile homes,” manufactured housing was defined in 1976 by federal building code as a prefabricated structure on a permanent chassis. Compared to modular prefab or any other type of house, manufactured houses require the least amount of time for construction and on-site installation, and purchase costs typically exclude land, which, in mobile-home parks, is leased. Its characteristics make manufactured housing less expensive than other factory-built types of the same quality, providing housing access for many low-income earners.
What has changed since 2001? Housing, prefabrication, and me.
For the housing sector, the Great Recession began in 2007, and its aftereffects continue. While single-family construction has had a decade of weakness, multifamily and rental housing has expanded; 2015 was the fourth consecutive year that multifamily units accounted for more than 30 percent of housing starts. Within single detached housing, the market share of all factory-built homes has decreased: site-built homes constituted 97 percent of house starts in 2014, so the stock of manufactured housing has skewed older. Factory-built home technologies are well established but remain relatively uncommon, perhaps due to repetitive, bland design as well as the enduring stigma associated with “trailer parks.”
For prefabrication, the possibilities have exploded since 2001. New digital fabrication tools, from 3d printing and scanning to CNC milling and laser cutters, have spawned the “maker movement” and renewed interest in experimentation at many scales. Building Information Modeling (BIM) can lead to closer coordination between architects and contractors. New forms of “flat pack” off-site production of building systems and components might be less expensive, more sustainable, and offer tolerances and techniques not possible in on-site construction. Prefabrication now takes innovative directions unimagined in the predigital era of manufacturing houses and modular boxes for shipment on a highway.
After years of research, writing, and exhibiting on modular and manufactured housing, my firm was awarded a commission in 2007 to design modular housing in Sudbury. The project involves demolishing five single-family homes that, ironically, were prefab Alcon homes with aluminum metal structures that made them difficult to add to vertically and, sealing the demolition deal, had rusted connections. The practical innovations of my firm’s design were in the house type (duplex homes in a single-family zone) and the tenure form (an unprecedented mix of affordable and public housing).
Did this use of prefab modular housing save time or money? The answer, which depends on the choice of a comparative baseline, is yes and no. The modular installer “set” the boxes in just two days; the general contractor completed building and site work in 13 months, a time frame that a site builder with the right crew could match; the architectural commission encompassed five years, a very long time, much of it in securing affordable-housing financing; the Sudbury Housing Authority invested more than 10 years from conception to closeout, much longer than a for-profit private developer would dream of spending on a project this size.
The “hard” construction costs for 10 units of affordable housing totaled $285,000 per house, and total development costs were $357,000 per house: 14 percent lower than comparable construction in New England. But the hard-cost savings in the project, largely thanks to the modular delivery approach, offset only half the project soft costs, mostly due to the affordable/public-housing financing, legal, and other fees. (Soft costs in 18 line items, including one for architects and engineers, totaled $85.9K per house.)
Like much in life, this evaluation depends on context. As a final point of comparison, however, median housing prices in 2013 were $310,000 in Massachusetts overall but $675,000 in Sudbury. In this sense, the project was a dream come true.