Modular Homes, Manufactured Homes, RVs, Oh My! What do all these categories mean?

The housing world has a terminology problem. Ask someone what is the difference between a manufactured home and a modular home, or between a park model RV and a tiny home on wheels, and you’ll usually get a shrug. But these distinctions matter practically, because they determine how a structure is regulated, where it can be placed, whether you can get a mortgage, and whether you can legally live in it full time. Here’s a plain-language map of the landscape.

Modular Homes: Factory-Built, Site-Finished

A modular home is built in a factory, sometimes in large sections, sometimes nearly complete, and then transported to a permanent foundation where it’s assembled on site. Despite being factory-built, a modular home is governed by the same International Residential Code (IRC) that applies to a conventionally built house. It sits on a permanent foundation, it’s classified as real property, and it finances exactly like a site-built home — conventional mortgage, FHA, VA, the works.

Size is not a defining characteristic modular homes. They can be 800 square feet or 3,000 square feet depending on what the manufacturer is building. The factory origin is a construction method, not a housing category.

For most practical purposes, a modular home is a house that happens to have been built indoors. The regulatory and financial treatment is essentially identical to a home built on site from the ground up. Think of buying a modular home like buying the materials to build you house. The materials are just much further along in the assembly process.

Manufactured Homes: The HUD Standard

A manufactured home is also built in a factory, but here the federal government steps in directly. Manufactured homes are built to the HUD code, a national standard established in 1976 that covers structural integrity, fire resistance, energy efficiency, plumbing, and electrical systems. Before 1976 these were called mobile homes; the terminology changed along with the standards.

Manufactured homes are built on a permanent steel chassis and can technically be moved, though in practice most are placed semi-permanently. They can be financed through specialized mortgage products, and when permanently affixed to land they’re sometimes eligible for conventional financing depending on the jurisdiction.

The key distinction from modular: the HUD code is a federal standard, separate from local building codes. A manufactured home meets federal requirements but may not meet the IRC standards that a site-built or modular home would need to satisfy. This matters for placement because some jurisdictions restrict where manufactured homes can be sited.

RVs: Built for the Road

A recreational vehicle is designed primarily for mobility and temporary occupancy: road trips, camping, seasonal use. Standard RVs are built to NFPA 1192, the RV Industry Association’s safety standard, and how they are regulated depends on the state. Some of them do it through DMV, while others, like Washington, regulate them through Labor & Industries. They require registration, tags, and insurance like any vehicle.

RVs are not designed or intended for full-time permanent habitation, which is reflected in their construction standards. Insulation, systems sizing, and build quality are calibrated for intermittent use rather than year-round living. That said, an estimated 300,000 to 500,000 Americans live in RVs full time.

Park Model RVs: The RV’s More Settled Cousin

A park model RV is a specific subcategory of RV built to ANSI 119.5 standards. The defining characteristics: it’s built on a trailer chassis, it cannot exceed 400 square feet of interior space (lofts and porches don’t count toward that limit), and it’s technically classified for temporary or recreational use. Like a standard RV it requires registration and insurance.

In practice, park models are designed for longer-term placement rather than frequent travel. You’ll find them in RV parks, tiny home communities like Sea Breeze in Port Townsend, and increasingly in backyard ADU situations. They’re sturdier and more residentially finished than a standard RV, but the regulatory classification as a recreational vehicle means permanent full-time occupancy exists in a legal gray zone in most jurisdictions.

The 400-square-foot limit on park model size is a hard boundary. Above that threshold, the structure moves into manufactured home territory under HUD standards.

Tiny Homes on Wheels: The New Kid On the Block Still Looking for Respect

A tiny home on wheels — THOW — is where things get complicated, and where most of our builds at Mount Baker Tiny Homes live.

A THOW is a custom or semi-custom home built on a trailer chassis, typically under 400 square feet, designed for full-time habitation. THOWs can be built to ANSI 119.5 standards (the same as park model RVs) or to NFPA 1192 (the standard RV code). Our builds are certified to NFPA 1192. It’s a rigorous standard that covers everything that matters for safety and livability.

But note that although THOWs are built to the RV code, this is a kind of bottom limit, not a top limit. THOWs are built using residential materials and like 2×4 framing, residential window and doors, R13 insulation, and residential flooring. Because of this they are much more durable than RVs. For example, have you noticed how often RV roofs leak? A THOW roof, typically made from metal residential roofing panels, lasts as long as the roof on a regular house. Maybe longer. This affects portability— it takes more to drive it down the road, but that’s OK because they are moved infrequently. So technically speaking THOWs are built to the RV code, in practice they are mostly built to the International Residential Construction standard.

The THOW’s central challenge is regulatory recognition. The structure itself can be built to an excellent standard. The systems can be every bit as good as a conventional home. But because it sits on wheels and is classified as personal property rather than real estate, most jurisdictions don’t have a clear legal framework for full-time occupancy. You can’t get a conventional mortgage. Zoning codes often prohibit permanent residency in a wheeled structure. But since enforcement is typically complaint-driven rather than proactive, this means many thousands of people live in THOWs without incident. Still, the legal foundation is shakier than it should be.

This is changing. Portland legalized occupied residential vehicles in all residential zones in 2021. Bellingham’s City Council is currently considering a proposal to do the same. Washington State has had bills in the legislature to formally recognize THOWs as a permanent housing type. The direction of travel is clear. The pace is, as with most housing policy, frustratingly slow. In fact, we may soon see a new designation emerge that tries to cut through the regulatory confusion entirely. Housing advocates and local policymakers are beginning to use the term Movable Dwelling Unit — or MDU. The MDU category would umbrella both THOWs and RVs used as permanent housing, giving jurisdictions a single clean term to legalize, regulate, and track. The idea is to stop trying to retrofit THOWs into existing categories — recreational vehicle, accessory dwelling unit, manufactured home — none of which quite fit, and instead create a category that describes what these structures actually are: affordable, movable, permanent homes. Whether MDU becomes widely adopted terminology remains to be seen, but it represents a meaningful shift in how policymakers are beginning to think about this housing type. We’re watching it closely.

Quick Reference

• Modular home: Factory-built, IRC code, permanent foundation, conventional mortgage. Any size.
• Manufactured home: Factory-built, HUD code, steel chassis, specialized mortgage. Typically larger than 400 sq ft.
• RV: NFPA 1192, DMV-registered, designed for temporary/mobile use. No mortgage.
• Park Model RV: ANSI 119.5, DMV-registered, under 400 sq ft, designed for semi-permanent placement. No mortgage.
• Tiny Home on Wheels: NFPA 1192 or ANSI 119.5, DMV-registered, custom-built for full-time habitation, under 400 sq ft. Personal loan or RV loan only. Legal status for permanent occupancy varies by jurisdiction — and is actively being reformed.
• Movable Dwelling Unit (MDU): An emerging policy term — not yet a formal regulatory category in most places — that would umbrella THOWs and permanently-occupied RVs under a single housing designation. Watch this space.

Mount Baker Tiny Homes builds custom tiny homes on wheels and van conversions in Bellingham, Washington. Questions about how our builds are certified or what that means for your situation? Get in touch.