Committee debates allowing mobile dwelling units on residential lots amid housing shortage

The Senate Housing Committee heard testimony on Senate Bill 5332, a proposal to allow mobile dwelling units (MDUs) — travel trailers, motor homes, tiny homes on wheels and similar vehicles — on residential lots in jurisdictions planning under the Growth Management Act, subject to specified utility hookups and inspections.

Staff summarized the proposed substitute by noting the bill defines a mobile dwelling unit broadly (travel trailers, folding camping trailers, truck campers, motor homes, fifth wheels) and requires that any city or county planning under the GMA "allow at least 1 mobile dwelling unit on each lot zoned for residential use" subject to conditions such as being inside an urban growth area, not in a critical area or shoreline, and having adequate water, sewer and transportation service. The substitute requires electrical connection via a dedicated outlet and plumbing hookups or access to toilets and showers in the primary dwelling; the bill permits inspection of utility hookups but not inspection of the dwelling unit itself. Staff cited a fiscal note: roughly $262,000 in one‑time state technical assistance and rulemaking costs for the Department of Commerce in the biennium, about $105,000 ongoing, and an estimated $5.8 million one‑time cost to local governments to adopt development regulations, with indeterminate additional inspection and enforcement costs.

Supporters argued MDUs can be a rapid, lower‑cost option to add housing. Dan Berdelet of the Siteline Institute said "MDUs can be far less expensive and far more rapidly deployed than any other type of housing," and described MDUs as an extension of accessory dwelling units that can add lower‑cost options. Cole Peterson, a licensed general contractor with experience in Portland’s policy, told the committee that hookups typically cost $10,000–$15,000 and that MDUs themselves commonly cost $50,000–$75,000; he said Portland’s legalization produced no major code enforcement problems since 2021.

Proponents from tiny‑home organizations, faith‑based and veteran housing advocates and manufacturers said MDUs can provide immediate, legal housing alternatives for people at risk of homelessness, multigenerational households, caregivers and caregivers who need nearby housing for elders.

Opponents and infrastructure witnesses raised concerns about public health, water and sewer systems and local costs. The Washington State Association of Counties said counties worry about verifying potable water access and the condition of onboard waste systems that the bill does not require inspectors to evaluate; the Snow King Water District Coalition warned of cross‑connection, back‑siphonage and risks from connecting to sewer cleanouts, saying allowing those direct cleanout connections "introduces risks like blockages, backups, and damage from irregular waste flows" and recommended below‑ground connections rather than cleanout hookups.

Committee members asked staff whether the bill addresses setbacks and lot placement; staff said the current draft does not explicitly set new setback rules and left those details to local development regulations. The lack of a uniform setback or detailed sewage inspection requirement was highlighted by multiple local government and water‑system witnesses as a subject for follow‑up.

The public hearing included both proponents and opponents; the committee closed the hearing and did not take a vote that day. Sponsors and several agencies signaled they will continue discussions about technical standards for hookups, cross‑connection controls and potential costs to local jurisdictions.