Tiny House Building Permit: What the Code Requires

The code does not use the word "tiny" loosely. Appendix Q was approved for the 2018 IRC and was renamed Appendix AQ in the 2021 IRC. It defines a tiny house as a single dwelling unit of 400 square feet or less, excluding lofts.

Three points matter before you draw anything:

• It is optional. The appendix text says its provisions are not mandatory unless the adopting ordinance specifically references them. A model code appendix has no force of its own.
• It relaxes, it does not replace. Appendix Q relaxes various requirements in the body of the IRC for small houses. Everything it does not mention still comes from the main code.
• It is being rewritten as a standard. The ICC and the Tiny Home Industry Association are developing ICC/THIA Standard 1215, which will continue to appear in the IRC as Appendix AQ. Check which edition your jurisdiction actually enforces.

Adoption varies, and the local answer controls. Three real examples show the range:

The Austin example is the one to plan for. When the appendix is not adopted, you are not exempt from the code. You are asking the building official to accept the appendix's relaxed provisions as equivalent. Austin's guidance says a tiny house there must still meet the fire separation distances in IRC R302 and sit on a permanent foundation that complies with Chapter 4 of the 2021 IRC.

To find your own answer, ask the building department three things. Which IRC edition do you enforce? Do your local amendments include Appendix AQ? If not, do you accept it under the alternate materials and methods section of the administrative chapter (R104.11 in the model code)?

The appendix is about fitting a house into 400 square feet without failing the standard dimensional rules. These are the provisions people ask about most. The figures below come from adopted local versions of the appendix, so confirm them against the text your jurisdiction enforces.

Ceiling height. One town's summary of the 2018 appendix lists a 6 ft 8 in minimum ceiling height, with 6 ft 4 in allowed for bathrooms and kitchens. That is a secondary summary, so read the adopted section itself before relying on it.

Loft stairs. Seattle's adopted version sets these limits for stairs that serve a loft:

Source: Seattle Residential Code, 2021 Appendix Q.

Run the formulas before you buy plans. A 9¾ in riser needs a tread of about 7 in under the first formula. An 8 in tread paired with that riser satisfies neither formula, which is the kind of mismatch that gets caught at plan check.

Loft guards. Loft guards must be at least 36 in high or one-half of the clear height to the ceiling, whichever is less.

Ladders. Appendix Q allows ladders in some loft cases, and each rung must support 200 lb. Rung spacing and angle figures differ between summaries I found, so take them from the adopted code, not from a blog.

Emergency escape. The appendix points back to the main code's emergency escape and rescue opening rules in IRC R310. Look there for opening sizes. A sleeping loft is where reviewers look first.

The IRC does not care how the house is moved. It is silent on whether a tiny house sits on a foundation or on wheels and focuses on how the structure is used. In practice, local departments do care, and they split into two camps.

• Permanent foundation. Austin requires a permanent foundation under Chapter 4 and permanent utility connections. Lee County, Alabama allows a light-frame tiny house of 400 sq ft or less to use the prescriptive IRC foundation, with footings at least 12 in below undisturbed soil, or a pier option under set criteria (Lee County policy).
• On wheels. A unit built to a vehicle standard is treated differently. Recreational vehicles are built to the ANSI A1192 code and park model RVs to ANSI A119.5, and neither is inspected to the standards for permanent dwellings. Goose Creek, SC says a site-built tiny house must fully comply with the state residential code, while an ANSI 119.5 unit is registered with the state DMV as a vehicle. Greenville County, SC uses a separate application for RV and park model units.

Some departments accept a third-party certification as proof of what the unit was built to. Coconino County, AZ asks owners to show documentation that a tiny house was built to a recognized code or standard, or a special inspection applies (Coconino County matrix). Whether your department accepts such a label is a local call.

IRC R105.2 exempts one-story detached accessory structures, such as tool and storage sheds, playhouses and similar uses, when the floor area does not exceed 200 sq ft. People sometimes try to stretch that to a small cabin. It rarely works, and the shed guide covers why: see do you need a permit for a shed.

Local conditions make it tighter. Coconino County, AZ requires the structure to stay single-story, unoccupied, and under a 15 ft ridge height. The county also says that if electrical, plumbing or mechanical work is added, the whole structure needs a building permit. A house someone sleeps in, with a kitchen and a bathroom, fails each of those tests.

A building permit says the structure is safe. Zoning says whether it can be on the lot. Treat them as two applications with two answers.

Denver University's law review notes that a tiny home can be built to ANSI 119 standards, but zoning treats manufactured and modular houses like other code-compliant dwellings. The practical risk is a lot where a second dwelling is not allowed. That is an accessory dwelling unit question, and in California the state rules are covered in the garage conversion guide. If you are in a city with published local pages, start with the jurisdiction page, such as Los Angeles.

Requirements differ, but expect a department to ask for most of this:

1. A site plan showing setbacks and fire separation distances. The site plan guide walks through one.
2. Floor plans with the loft, stair or ladder, and egress openings dimensioned.
3. Foundation plans, or evidence of the vehicle standard if the unit is on wheels.
4. Structural, plumbing and electrical details for a unit that will be connected to utilities.
5. For Austin-style jurisdictions, the alternate method request with your justification.

The general sequence for any permit is in how to get a building permit.