Comparisons
Tiny Homes in Washington: On a Foundation or On Wheels?
The short answer
In Washington a tiny house on a permanent foundation is a house. It gets a building permit and is treated much like any other home.
On wheels, it depends on what it was built as. Most tiny houses on wheels are sold as park model RVs, which are vehicles, not dwellings.
A few are built to the state building code and carry an L&I insignia. Those are legal dwellings, but your city or county still decides where one can go.
The 400 square foot line
Washington adopts the tiny house appendix of the residential code, Appendix AQ, statewide. It defines a tiny house as a dwelling of 400 square feet or less, not counting sleeping lofts.
The Pierce County guide adds that it must sit on a permanent foundation and be meant as a residence, with a living area, a kitchen and a bathroom. For the state definition it points to RCW 35.21.686.
Note what the exclusion does. A loft does not count toward the 400. So a well-designed tiny house has more usable space than the number suggests, which is exactly why the loft rules below matter so much.
On wheels, check what it was built as
This is the part that surprises people. Washington has three labels for a small home on wheels, and only one of them makes it a house.
- Park model RV. Built to ANSI A119.5 for recreational or seasonal living, up to 400 square feet when set up. Most tiny houses on wheels are this.
- Recreational vehicle. Built for camping or travel, and no wider than 8 feet 6 inches on the road.
- L&I tiny home. Built to the state building code somewhere other than where it will be used, inspected by L&I during construction and given the state modular insignia. L&I says this is the only one of the three it approves as a dwelling.
L&I is blunt about it in its own guidance. Park model RVs, RVs and HUD manufactured homes are not tiny homes, even when people live in them.
So before you buy one, ask to see the label. If the seller cannot show you an L&I modular insignia, you are buying a vehicle.
What follows from being an RV
Pierce County says park model RVs are not site built or factory built dwellings and need no building permit. They are reviewed the same way an RV is.
No building permit means no certificate of occupancy, the document that says a building is a legal place to live.
Lenders generally treat it as a vehicle, so financing usually means an RV loan at RV rates and terms. It is insured as an RV, not a home.
It generally does not add to your property value the way a permanent structure does.
An L&I approved tiny home on a chassis is different. It is a dwelling, but L&I only approves how it is built, not where it goes. Setting one up always needs a local permit, and the foundation system, a chassis included, has to be approved by your local building department.
Where a tiny house on wheels can actually go
For a park model RV, Pierce County lists three situations, and they are narrower than most people expect.
- Permanently, in a mobile home park.
- As a Temporary Housing Unit for Family.
- For recreational use, with a Temporary Use Permit, for a maximum of 120 days per year.
The Tacoma-Pierce County Health Department may also have its own requirements on top.
State law protects one route. Cities and towns cannot stop a recreational vehicle or a tiny house with wheels being used as a primary residence in a manufactured or mobile home community. They can require that it has at least one internal toilet and shower, or that the community provides them.
State law also lets cities allow tiny house communities, rented sites for tiny houses with or without wheels, under RCW 35.21.686. Whether there is one near you depends on your city.
So the honest summary is that living full time in a park model RV usually means finding a space in a mobile home park. It is not a way to put a second home in your own backyard.
If that is what you actually want, you want an ADU, and Thurston County changed those rules in December 2025.
The three routes compared
| Site built | L&I tiny home | Park model RV | |
|---|---|---|---|
| Legally a dwelling | Yes | Yes | No, an RV |
| Built to | 2021 IRC plus WA amendments | State building code, L&I inspected | ANSI A119.5 |
| Can be on wheels | No | Yes, on a chassis | Yes |
| Local building permit | Yes | Yes, to set it up | None required |
| Where it can go | Any lot that zoning allows | Where local zoning allows | In Pierce, a mobile home park, family unit or 120 days |
| Financing | Normal mortgage products | Depends on the lender and setup | RV lending |
Pierce County permits a factory built tiny home the same way it permits a manufactured home. Other cities may handle it differently, so ask your building department before you order one.
The loft rules almost nobody gets right
Washington writes its own loft section into the code, Section R333 (WAC 51-51-0333), and these numbers decide whether your design works before you draw it.
- A loft must be less than 70 square feet of floor area.
- Loft ceiling height must not exceed 7 feet over more than half the loft floor area.
- The ceiling below the loft must be at least 7 feet.
- The ceiling above the loft floor must be at least 3 feet.
- Sloped parts measuring less than 3 feet floor to ceiling do not count toward the loft area.
- A loft built to this section counts as part of the story below, so it does not make your tiny house a two story building.
There is a useful exception. A loft does not have to meet these rules at all if it is less than 3 feet deep, under 35 square feet, or has no permanent means of egress. That is the difference between a sleeping loft and a storage shelf.
One relaxation is worth knowing. A single loft in a sprinklered unit may be up to two thirds of the area of the room it sits in. That only holds if no other loft or mezzanine opens into the same room.
Guards in a low loft
Standard guard height is 36 inches, which in a loft with a 7 foot ceiling would leave you crawling under it.
So in loft areas with ceilings of 7 feet or less, Washington allows a lower guard. It can be 36 inches, or half the clear height from loft floor to loft ceiling, whichever is less.
It is a small clause and it is the difference between a loft that feels open and one that feels like a cot.
The ladder question
Every tiny house plan on the internet puts a ladder up to the loft. In Washington that is usually fine, because of an exception the state wrote into its egress rules.
Normally a habitable level has to be reached by a stairway or a ramp. Washington exempts ladders, ships ladders and alternating tread devices that serve an area of 200 square feet or less inside one dwelling. That area must be no more than 10 feet above the floor below, and the ladder cannot be the only way to reach a kitchen or a bathroom.
A code-compliant loft is under 70 square feet, so it sits well inside that limit. Where you do provide a permanent means of egress from a loft, the code wants at least 3 feet of ceiling height across its full width.
Two things to plan around. A loft that holds the only bathroom cannot rely on a ladder, and neither can one more than 10 feet up. Your building department has the final word on how it reads the exception, so show the ladder on the drawings you submit.
Ceiling heights and the airtightness test
Two more numbers from Appendix AQ as Washington adopts it.
- Habitable space needs a ceiling height of at least 6 feet 8 inches.
- Bathrooms, toilet rooms and kitchens need at least 6 feet 4 inches.
- Beams, girders, ducts and light fixtures must not hang below those heights.
Then the one that catches builders rather than owners. A tiny house must not leak more than 0.30 cfm at 50 Pascals per square foot of enclosure area. Testing is to RESNET/ICC 380, ASTM E779 or ASTM E1827. The building official can require an approved third party to run it, and the result comes as a signed written report.
That is a demanding target on a small building, because a tiny house has a lot of surface area for its volume. A house that tight also has to have whole-house mechanical ventilation. It is a real design constraint, not a formality.
Two sections Washington did not adopt
Appendix AQ sections AQ105 and AQ106 are not adopted in Washington.
AQ105 is the tiny house version of the emergency escape and rescue opening rules. Not adopting it means the ordinary requirements apply to your bedrooms and lofts, not a relaxed tiny house version.
If a plan you found online relies on Appendix AQ easing the escape window rules, it does not work here.
These are the rules in force today. Washington moves to the 2024 edition of the code on 3 May 2027, so if your build starts after that, check the current version.
What Pierce County will not allow
Pierce publishes a short guide, last revised in March 2024, and three things in it stop projects.
- Composting and incinerator toilets are not allowed, and neither is rainwater catchment for drinking water. A site built tiny home needs approved water supply and sewage disposal like any other house.
- Fire access applies in full. Same fire flow and emergency vehicle access as a full sized single family home. On a long rural driveway that can be the thing that decides the project.
- Your HOA can ban it outright. County code does not override that, and Pierce says so at the top of its own guide. Check before you apply for anything.
On permitting, a site built tiny home goes through the same process as a larger single family residence. Its drawings must meet the 2021 IRC with Washington amendments, specifically sections R301, R304, R311, R333 and Appendix AQ.
A tiny house as an ADU
This is where most of our tiny home work actually ends up.
A tiny house on a foundation can be a detached accessory dwelling unit, which is the thing most people picture when they imagine a small second home in the backyard. It is a dwelling, so it is financed, insured and valued like one.
The size limits are generous next to the 400 square foot tiny house definition. In the Thurston urban growth areas an ADU can be 1,000 square feet of gross floor area. A rural detached one is capped at 1,000, or 1,200 with a transferable development right.
Olympia, Lacey and Tumwater also offer a shortcut. The three cities share a set of pre-approved ADU plans, from a 480 square foot studio to an 800 square foot two bedroom, already reviewed for code compliance.
So if your reason for going tiny is cost rather than a love of 400 square feet, build a small ADU rather than a code-defined tiny house. You get the same simplicity without designing around loft ceiling heights.
Where to go from here
We build tiny homes and backyard studios on foundations, across Thurston and Pierce counties. We do not build homes on wheels, and if that is what you are after we would rather say so now than three conversations in.
Tell us the address and what you want the building to do. We will tell you whether it is a tiny house, an ADU or a studio. That is mostly a question of how you mean to use it and what your lot allows.
Common questions
Are tiny houses on wheels legal in Washington?
They are legal to own, but most are park model RVs certified under ANSI A119.5, not dwellings, and no building permit is issued for them. In Pierce County a park model RV can sit permanently in a mobile home park or serve as a Temporary Housing Unit for Family, and recreational use needs a Temporary Use Permit capped at 120 days a year. A tiny house built off-site to the state building code and approved by L&I is a dwelling even on a chassis, but it still needs a local permit to be set up.
How do I tell if a tiny house on wheels is an RV or a home?
Look for the label. An L&I approved tiny home carries the Washington State modular insignia, which L&I only applies after approving the plans and inspecting the build. A park model RV is certified to ANSI A119.5. If the seller cannot show an L&I modular insignia, treat it as a vehicle.
How big can a tiny house be in Washington?
Appendix AQ, which Washington adopts statewide, defines a tiny house as 400 square feet or less, excluding sleeping lofts. Because lofts do not count toward that figure, the usable space is larger than the number suggests.
Can I put a tiny house in my backyard?
On a foundation, usually yes, as an accessory dwelling unit subject to your local rules. A park model RV generally cannot be sited as a second home on an ordinary residential lot, because it is not a dwelling. An L&I approved tiny home on a chassis is a dwelling, so it comes down to what your city or county zoning allows.
What are the loft rules for a tiny house in Washington?
Under WAC 51-51-0333 a loft must be less than 70 square feet. Its ceiling must not exceed 7 feet over more than half that area. The ceiling below the loft must be at least 7 feet, and the ceiling above the loft floor at least 3 feet. A loft under 35 square feet, less than 3 feet deep, or with no permanent egress is exempt from those rules.
Can I use a ladder to reach the loft?
Usually, yes. Washington exempts ladders and ships ladders that serve an area of 200 square feet or less within a dwelling, as long as that area is no more than 10 feet above the floor below and the ladder is not the only access to a kitchen or bathroom. A code-compliant loft is under 70 square feet, so it normally qualifies.
Can I use a composting toilet in a tiny home in Pierce County?
No. Pierce County does not allow composting or incinerator toilets, or rainwater catchment for potable water. A site built tiny home needs approved water supply and sewage disposal like any other house.
Is a tiny house or an ADU the better option?
Usually an ADU, unless you specifically want a 400 square foot building. An ADU in a Thurston urban growth area can be 1,000 square feet of gross floor area, and a rural detached one can reach 1,200 with a transferable development right. Olympia, Lacey and Tumwater also share pre-approved ADU plans from 480 to 800 square feet.
Will a bank lend on a tiny house?
On a foundation it is real property and normal mortgage products apply. A park model RV is a recreational vehicle, so financing is usually RV lending at RV rates and terms, and it is insured as a vehicle rather than a home.
Code requirements on this page were last checked on 3 October 2026. Codes change, so confirm current standards with your city or county before you apply.
Planning a deck in Thurston County?
We handle the permit work as part of the build. Tell us the site and what you want.
Where this applies
We work in every one of these towns, and each has its own page covering the rules that apply there.