Just what IS a tiny house?
Well, functionally, they are most often designed to be used as a living space for one or two humans, though many other variants and uses exist. The way they relate to current building and siting rules in the US varies a bit by locale. Those rules are always changing (slowly, I might add).
Are they RVs (recreational vehicles)?
Tiny houses that are built on trailer frames with axles are sometimes considered RVs by those who are looking for a category to put them in. Functionally, they are not. RVs are meant for camping or weekend use. Zyl Vardos does not natively certify its structures as RVs, and is not an RV manufacturer, although you may be able to obtain an independent certification.
So what are they?
If they are on a foundation, tiny houses are simply a very small dwelling, studio, or auxiliary structure. Ability to build/site one of these depends on: local rules, utilities involved, intended use, and the type of permit needed from the local planning department. In some areas, structures under a certain square footage are allowed to be built without a permit, though they are typically not supposed to be a residential dwelling. Zyl Vardos will engineer and build structures to be placed on a foundation. If they cannot be shipped whole, we usually do the design, plus important parts: windows, doors, and rafter system. Then we would partner with a general contractor in your locale to complete the build.
If they are sitting on a trailer frame, tiny houses are really just a small form of manufactured housing. Currently, HUD (US Dept of Housing and Urban Development) defines manufactured housing as anything over 400 square feet. So, until HUD revises its threshold, or creates a new category, small tiny houses are not technically considered manufactured housing by HUD. This is a double edged sword: On the bad side, it is currently difficult (near impossible) to get a permit to site a tiny house on wheels as a 'full-time residence.' On the good side, it opens the door to many creative, resource efficient, and artistic building techniques for tiny houses. If tiny houses were regulated in the same way as manufactured housing (this is a big industry) - the ability to do unusual shapes, custom layouts, and handmade elements would be phenomenally difficult and/or expensive. Zyl Vardos designs thoughtful, functional, safe, lightweight, and incredibly strong structures.
Is it legal?
Of course it is! It is completely legal to own and use a tiny house on wheels that is less than 400 sq ft in every State. There are local rules, neighborhood covenants, city development plans, etc., that may restrict use in certain locations or situations. And these rules vary quite a bit by locale.
Zyl Vardos tiny houses are voluntarily built to IRC (International Residential Code) and NFPA (National Fire Protection Assn) building codes. We also use American Tiny House Association guidelines, although these are basically a subset of IRC, and NFPA. Even though many of our structures are not specifically RVs or permitted residences, we always choose to follow code voluntarily to create a structure that is safe to use on a long term basis.
Alternately, if we are building a structure on the ground (on a foundation) then it will be custom engineered to comply with the codes required at the location it will be sited (again, these vary from place to place.) Permits would be gotten, and inspections completed as required by your local municipality.
Zyl Vardos also builds totally unique art structures and houseboats, and these do not fall under either RV or residential code.
Abel Zyl is a founding member of the American Tiny House Association, and is currently their advisor for structural concerns and building code.