Meet MDUs, a low-cost, fast-build, flexible solution for more homes now.
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 connect...

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