Park Model RVs And ADUs: Knowing The Real Difference
Both add living space to your property. They follow completely different rulebooks. Here is how they compare, when a park model RV is the easier path, and when it can be placed as an ADU instead.
Jump to the side by side comparison ↓The Simplest Way to Add Living Space
If you want the simplest and most affordable way to add living space to your property, a park model RV is one of the best options available. You get a real home without the heavy construction process that comes with building a traditional ADU.
Here is why people choose them:
- No foundation required in most setups
- No residential building permit in many areas
- Built in 8 to 12 weeks, with some units available right off the lot
- One of the easiest, most cost-effective ways to add a guest house, rental income, or multigenerational housing
What Makes a Park Model RV Different
At NW Tiny Homes, every home we sell is a park model RV. People sometimes call these tiny homes on wheels, or THOWs. Here is the distinction that matters most.
What a park model RV actually is
A park model RV is a factory-built recreational vehicle. It is federally classified as an RV, built to a national standard, and titled with the DMV. It is not a manufactured home and not a travel trailer. It is its own legal category. Because it is titled as a vehicle but lived in like a home, you will sometimes see it called a mobile dwelling unit, or MDU.
- Built to ANSI A119.5 and RVIA certified by a third-party inspector
- Up to 400 square feet in setup mode, before lofts and porches
- Built on a single steel chassis, with the wheels staying on
- Titled and taxed as personal property in most states
ANSI A119.5 covers the same systems you expect in a house: structural integrity with snow and wind loads, fire and life safety with smoke and CO alarms and two means of egress, electrical to NEC, plumbing with pressure-tested gas, and full insulation and weather protection.
Park Model RV vs ADU
An ADU, or accessory dwelling unit, is a second home built on your lot under residential building code. A park model RV reaches the same goal, a second living space, through a completely different category, usually for about a third to a quarter of the cost. Here is how they line up.
How it compares to other categories
It also helps to see a park model RV next to its neighbors. The differences are sharper than most people expect.
| Park Model RV | Manufactured Home | Travel Trailer | Site-Built ADU | |
|---|---|---|---|---|
| Standard | ANSI A119.5 | HUD Code (24 CFR 3280) | ANSI A119.2 / RVIA | Local IRC building code |
| Federal class | Recreational vehicle | Manufactured home | Recreational vehicle | Site-built dwelling |
| Max living area | 400 sq ft | No federal cap | ~320 sq ft typical | Varies by jurisdiction |
| Title | DMV vehicle title | HUD label | DMV vehicle title | Real property deed |
| Tax | Personal property (most states) | Real or personal | Personal property | Real property |
| Permitting | RV placement and utility permit | Foundation and full building | Placement, often by-right | Full residential building permit |
Scroll the table sideways on mobile to see all four categories.
Cities Treat These as Two Different Things
A lot of homeowners assume that if ADUs are allowed in their city, a park model RV must be allowed too. Cities separate these into two categories:
- ADUs follow residential building rules
- Park model RVs follow occupied RV rules
Just because an ADU is permitted does not guarantee an occupied RV is. The reverse is also true. A city that does not allow an occupied RV on a residential lot may still allow that same unit once it is placed as an ADU on a foundation. That is exactly why the categories matter.
Our Zoning Guide breaks this down county by county and city by city across Oregon and Washington, with email templates you can send your planner.
Can a Park Model RV Be an ADU?
Sometimes, yes, and this is changing. We are seeing more areas open up to it. A park model RV is not an ADU on its own, but a growing number of cities and counties will permit one to serve as an ADU or detached ADU when it is set up to residential standards on the parcel.
When a jurisdiction allows this, it usually asks for some combination of:
- A certified unit: ANSI A119.5, the RVIA seal, and a Washington L&I insignia for Washington placements
- A permanent, engineered foundation the local building official will accept
- Tie-downs and permanent utility connections, inspected locally
- A residential permit that gets issued and finaled for the unit as a dwelling
It varies heavily from one jurisdiction to the next. Some areas have a clear path, many do not yet, and the only way to know is to ask your local building official before you buy.
A real Washington example: Lynnwood
Lynnwood approved a Washington L&I inspected park model RV as a detached ADU once it was permanently attached to a foundation, and accepted Diamond Piers as the engineered foundation system. Here is the pathway in three steps.
Certified unit
An ANSI A119.5 park model RV with a Washington L&I insignia, manufacturer data plate, RVIA seal, and engineering documentation.
Permanent foundation
An engineered foundation the building official accepts. Diamond Piers is a code-listed alternative to a poured stem wall and was accepted in Lynnwood.
Residential permit and final
The building department issues and finals a residential permit for the unit as a DADU. Once finaled, it is a permitted accessory dwelling, the same as any other DADU.
Where Park Models Fit in Oregon
Oregon has been steadily opening up placement, and two spots stand out for our customers.
Clackamas County
Since September 2024, Clackamas County allows a park model RV as a second dwelling on urban and rural residential property. That makes it one of the cleanest placement options in the Portland metro right now.
Portland
Portland treats park model RVs as occupied recreational vehicles, not ADUs, which keeps permitting simple. Long-term rental on a residential lot is allowed. Short-term rental under 30 days in an occupied park model is not.
These are two examples, not the whole map. Our Zoning Guide covers every county and city in Oregon, with the rules and a question template for each.
Washington Is Opening the Door for Park Models
Washington has spent the last few years widening where a second home can legally go, and 2026 is the biggest step yet. Two laws set the stage.
HB 1337
Requires cities and towns to allow up to two ADUs on residential lots in single-family zones, with no owner-occupancy requirement. It opened ADUs up across Washington, but it stopped at city limits.
HB 1345
Lets counties allow a detached ADU on rural residential land outside the urban growth area, on property owners already have. Signed in March 2026, effective June 11, 2026.
Here is where a park model RV fits in. When a county allows a detached ADU under these rules, a unit set up to residential standards can qualify. That typically means:
- A Washington L&I insignia on the unit
- An approved foundation, either ANSI A225.1 or an engineered system such as Diamond Piers
- Permanent water, sewer, and electrical connections
- One detached ADU per parcel, on a lot of at least one acre, within 150 feet of the main home and sharing its driveway (up to 1,296 square feet, well above a park model's 400)
This is where our lineup has an edge. Every Cavco and Champion model we sell is built to the qualifying spec: a Washington L&I insignia, ready for an ANSI A225.1 foundation, with permanent water, sewer, and electrical. Some counties, such as Mason, require that L&I certification specifically, and our units carry it.
We track this county by county. Our Zoning Guide shows where each Washington county and city stands today.
Which Path Is Right for You
For most people, it comes down to two paths. The good news is that you do not have to figure out which one applies on your own.
Park Model RV on Your Land
Placed under RV rules. No foundation in most setups, no residential building permit in many areas, and the fastest way to get a home on site. Best when your city or county allows an occupied RV or park model RV on private property.
Place It as an ADU
Set on a permanent engineered foundation with tie-downs and a residential permit. More steps and more cost, but it can unlock placement in areas that allow ADUs but not occupied RVs on private land.
The Quickest Way to Check With Your City
Here is a message you can send straight to your city planner or zoning department. The same answers are often posted on your city or county planning website.
Hi there,
I am considering placing a Park Model RV built to ANSI 119.5 on my property. Can you help confirm the following:
- Is an occupied RV allowed on my property or in my zoning district?
- Are there any restrictions on using a Park Model RV as a long-term or full-time residence?
- Does the city require a specific type of RVIA certification or inspection?
- Are there setback rules or placement requirements for RVs?
- Is a permit required for delivery or connection of utilities?
- If RV living is not allowed, does the city allow it as an ADU on a foundation, or offer any temporary stay, seasonal use, or caretaker exemptions?
Thank you for your help.
Final Takeaway
Park model RVs are one of the best ADU alternatives available. They are faster, more affordable, and far simpler to place. They fall under RV laws, not residential housing laws, which is why your first step is always to confirm the rules for an occupied RV on your specific property.
And if the RV route is not open in your area, a park model RV can often still be placed as an ADU on a foundation. Either way, there is usually a path, and we are happy to help you find the one that fits your land.
Not sure which path fits your property?
Come walk through our models and talk it through. We will help you understand placement, foundations, utilities, and the next steps for your city.
Schedule a Tour Want to check your county first? See the Zoning Guide. Ready to compare layouts? Browse our Models.