UPDATE: Washington state Representative Mia Gregerson (D – Seatac) and Senator Sharon Shewmake (D – Bellingham) have just introduced companion bills to legalize MDUs, HB 1443 and SB 5332.
Communities across Cascadia face a dire shortage of homes that is driving up prices and rents for everyone. From seniors wanting to downsize and age in place, to young people starting out and seeking apartments near their jobs, to families needing more space and nearby schools for their kids, people across the region need more homes, in all shapes and sizes, to suit their various stages of life, price points, and preferences.
Leaders in Washington state have listened to their constituents and answered by passing numerous bills to legalize or re-legalize more kinds of housing, earning 2023 the moniker “the year of housing” and 2024 “the year of housing 2.0.” Among these measures was one to boost the construction of accessory dwelling units (ADUs)—also known as backyard cottages, in-law apartments, or bonus homes.
ADUs have been a key element of adding “gentle density” in neighborhoods across the state, but their costs can still prove prohibitive. Enter: MDUs, or “mobile dwelling units”—a much less expensive, more flexible, and faster-built option that could be another integral tool in the toolbox for bringing much-needed homes online fast for Washingtonians. (We’re using the term “MDU” in this article to align with a legislative proposal concerning this housing type in Cascadia. Elsewhere, they’re known by terms including “tiny homes on wheels,” “movable tiny homes,” and “vehicular residential facilities.” Same general idea, though.)
And Washington wouldn’t be blazing any trails if it legalized this housing form. Other jurisdictions, from Oakland, California, to Portland, Oregon, have done so in recent years and seen a modest but meaningful uptake in their construction.
Below, Portland-based Kol Peterson, an early proponent of MDUs, and Sightline Housing and Cities Senior Director Dan Bertolet answer common questions about them. Peterson’s general contracting company, TinyHookups LLC, specializes in electrical and plumbing hookups installation for MDUs and has installed them for about 25 properties in the past few years, making these homes a reality for more of his neighbors.
What are MDUs?
A mobile dwelling unit, or MDU, is a movable home with utility connections located on a residential lot that also has a conventional home. An MDU could be a tiny house on wheels, a campervan conversion, a park model RV, or a standard RV. MDUs are a very inexpensive form of housing that can be rapidly deployed. MDUs address a gap in the low end of the cost spectrum that no other housing type can fill.
Why do we need state legislation for MDUs?
Most jurisdictions have tight restrictions or outright bans on the use of MDUs as legal residences. Washington state has a pressing need for more housing—especially homes that can be created quickly and that are affordable to lower-income residents without the need for public subsidy. Legislation is needed to set statewide standards that will enable homeowners to add MDUs to their properties predictably and safely.
What would Washington’s proposed MDU bill do?
SB 5332 would legalize one MDU on any residential lot that already has a primary dwelling on it and is located within an urban growth area. Consistent with standard regulations in RV parks, it would require locally permitted connections to the MDU for water, sewer, and electricity but would not require any permitting or inspections of the MDU itself.
How affordable are MDUs?
MDUs with full bathrooms and kitchens sell for anywhere from $10,000–$200,000 and are commonly available new and used throughout the state. Connections for sewer, water, and electrical, plus site preparation, typically cost in the range of $15,000. For comparison, stand-alone accessory dwelling units (ADU) typically cost $250,000 to $500,000.
How quickly can MDUs be deployed?
Once the infrastructure has been installed, an MDU can be set up in as little as a day or two. Getting the utility connections and site prepared might take one or two weeks if professionally managed, or perhaps a few months, if not. Best case, deployment from start to finish can be as short as three weeks. In contrast, ADUs commonly take a year or more to permit and construct.
How many people live in MDUs now?
According to the 2023 Census American Community Survey, nearly 7,500 households live in MDU-style housing in Washington state. Because most local laws are so restrictive, residents often live in MDUs illegally on both private and public property, in potentially unsafe or unsanitary conditions, but reliable data on how many is lacking. An estimated one million Americans currently live full-time in MDU homes, reflecting a growing need.
What is the potential for MDU homes in Washington?
The theoretical upper limit is vast: Washington has about 1.8 million house lots that could potentially add an MDU. If just one out of 100 did, that’s 18,000 new MDU homes. It’s reasonable to expect that MDU legalization could increase the state’s supply of housing by roughly the same order of magnitude as ADU legalization. But MDUs are far less expensive than ADUs, providing homes in the state’s most needed low-cost range.
Who would live in MDUs?
The flexibility and affordability of MDUs can meet the needs of people in a wide variety of life situations. First and foremost, they are by far the least expensive type of market housing and provide homes for those who could not afford anything else unless subsidized.
MDUs are multigenerational living solutions, from aging parents needing a watchful eye to young adults seeking affordable independence. Their movability makes them ideal for family housing needs that may not be permanent. At the same time, if needed, they can comfortably serve as long-term residences.
Like ADUs, MDUs can be “mortgage helpers” that generate income, helping homeowners stay financially solvent and letting them age in place. One option for owners is to rent out an MDU pad with utility connections to a tenant who brings their own MDU. This arrangement could help MDU residents who have lost a place to station their MDU, as when an RV park gets sold off and closed down.
Are MDU tenants protected?
Washington state has legal protections for MDU occupants to ensure that they have the same safeguards as traditional renters do, as codified by House Bill 1227 (2009), which established rights and responsibilities of landlords and tenants of MDUs. The Washington Department of Revenue classifies full-time occupants of MDUs as legal residents.
Have other states or cities adopted MDU legislation?
In 2023, Oregon passed SB 1013 to allow counties to legalize one utility-connected MDU per lot in unincorporated areas. Oregon legislators are currently developing a bill to legalize MDUs on lots within urban growth areas. In 2021, Portland, Oregon, legalized one utility-connected MDU on all residential lots in the city.
Read more from Kol Peterson, including a breakdown of MDU installation costs:
- One year in, are tiny houses on wheels the new ADU in Portland? | Accessory Dwellings
- The birth and evolution of mobile ADUs | Building an ADU
