Wheels or foundation: the choice that rewrites the budget.

The standard that applies is decided by use, not by wheels.

Building codes do not care whether a home has axles. They ask what it is for. The International Code Council’s March 2023 guide, Navigating Certification and Regulation of Tiny Houses, written with the Tiny Home Industry Association, puts it this way: where federal manufactured-housing rules do not apply, the International Residential Code (IRC) or recreational-vehicle standards “cover the breadth of the requirements,” and if a home “is intended only for temporary or seasonal use and is on a chassis with wheels, then recreational vehicle or park model standards may apply.” The same guide notes that RV standards describe RVs as “intended for seasonal or intermittent use, not as permanent residences.” Checked October 27, 2026.

That is the fork. A wheeled home built to an RV or park-model standard is, on paper, a temporary dwelling, and where it may sit full time becomes a local zoning question. A home built to the IRC, with or without wheels, is a permanent dwelling that a local building department can inspect and approve. The IRC’s tiny-house appendix defines the category as “a dwelling that is 400 square feet (37 m²) or less in floor area excluding lofts,” and relaxes items such as loft ceiling heights and compact stairs; the 2021 Seattle Residential Code, Appendix AQ is one published adoption of it. Whether your jurisdiction has adopted the appendix is the first thing to ask.

What appears on the wheeled side.

Three lines exist only because the home can move: the trailer it sits on, the certification that stands in for a local building inspection, and registration. NOAH describes its certification as helping a tiny home’s “Placement, Insurance, Financing & Salability,” and presents those as benefits rather than guarantees. Treat certification the same way in your budget: a cost that opens doors, not a permit.

What the wheeled side does not have is a settled answer to “where may it stay.” That is why its open line is a place, not a price. RV-park pads, a relative’s backyard and a purchased lot produce three different projects, and in some jurisdictions full-time occupancy on wheels is simply not allowed. The land guide lists the six answers to get before paying a deposit.

What appears on the foundation side.

Permanence costs money up front: a real foundation, a building permit with its plan review and inspections, permanent water, wastewater and electrical service, and a crane if the home arrives in one piece. In exchange, the home becomes part of the real property, which is what lenders, insurers and assessors understand. The foundation subtotal here is $12,800 higher before contingency, and the extra is almost entirely site and utility work, which is why the contingency on that side is larger too.

Take three questions into your next call.

1. Which standard will this home be built to: IRC Appendix AQ, an RV or park-model standard, or something else, and who will inspect it?
2. If it is on wheels, where may it legally be occupied full time, and in writing from whom?
3. If it is on a foundation, what does the building permit require that the builder’s quote does not include?

Builders quote the home; the fork is yours to choose. Write both columns before you choose it.