State modular programs and codes: a practical overview

One of the most common questions from first-time modular buyers is "who approves this thing?" The answer surprises people. Part of the inspection happens in the factory, sometimes hundreds of miles away, and part happens on site through the local building department. This overview explains how that usually works and where to look for your state's rules.

Modular is not HUD-code manufactured housing

First, a distinction that matters for financing, zoning and resale. Manufactured homes are built to a federal standard, the HUD Manufactured Home Construction and Safety Standards in 24 CFR Part 3280. Modular buildings are built to the state and local building codes where they will stand, just like site-built buildings.

Maryland's building codes office explains the difference simply: manufactured homes are built to the federal HUD code, while modular buildings must meet the same state building codes as site-built construction. The ICC/MBI off-site standards make the same split and do not apply to HUD manufactured housing.

How state programs usually work

Many states run a modular or industrialized building program. The details vary, but the pattern is similar:

• Plan review. The building design is reviewed and approved, by the state or by a third-party agency the state certifies.
• In-plant inspection. An approved inspection agency checks construction in the factory, often while walls and ceilings are still open.
• Label or insignia. Each approved module or building gets a state label showing it was built to approved plans under inspection.
• Local site work. The local building department usually permits and inspects the foundation, site utilities, and the installation and completion on site.

The Modular Building Institute keeps a directory of state program administrators, which is the fastest way to find the right office for your state.

Two examples

California. The Department of Housing and Community Development runs the Factory-Built Housing program. According to HCD's handbook, every model needs plan approval from an HCD-certified design approval agency, the manufacturer must maintain quality control, in-plant inspection is done by an HCD-certified quality assurance agency, and each unit gets an HCD insignia of approval before it ships. Local building departments handle permits and inspections for the on-site installation. More detail is on HCD's program page.

Texas. The Texas Department of Licensing and Regulation runs the Industrialized Housing and Buildings program. Texas rules require in-plant inspection by approved third-party inspection agencies, and inspections may be announced or unannounced. TDLR also publishes guidance on site inspections of the installation, which can be done by council-approved inspectors.

Other states use their own names and procedures. Some rely mostly on local building departments. Always check the program for the state where the building will stand, not the state where the plant is.

What the local building department still controls

Even with a state label, plan on local involvement. Typical local items include:

• Zoning, setbacks and site plan approval
• Foundation and anchoring
• Site utilities and connections
• On-site work at module joints, stairs, roofs and anything built in the field
• Fire department access and review
• The certificate of occupancy

Bring the local building official in early. Some departments see modular projects often. Others may need time to understand how factory inspection and site inspection fit together.

The ICC/MBI standards

In 2021 the International Code Council and the Modular Building Institute published two consensus standards: ICC/MBI 1200 for planning, design, fabrication and assembly, and ICC/MBI 1205 for inspection and regulatory compliance. A third, ICC/MBI 1210, covers MEP systems and energy. These standards don't replace your state program, but they give owners, manufacturers and officials a shared reference for roles and inspection.

Practical steps for owners

• Confirm the manufacturer is approved in your project's state, or ask how long approval will take.
• Ask the state program office how plan review timing works, and add it to your schedule.
• Meet with the local building department before design is final.
• Get a written list of which inspections happen in the plant and which happen on site.
• Keep copies of state labels, approved plans and inspection reports. You will need them at closeout.

Rules change, so treat this as a starting point. Check with the state program office and your local building department before relying on any of it for your project.

Common surprises

• The plant's home state doesn't matter much. A building is approved under the rules where it will stand.
• Labels are permanent records. Losing track of state labels or approved plans can hold up the certificate of occupancy.
• Site-built parts are inspected locally. Anything built in the field, like stairs, porches or a podium, follows local inspection.
• Changes after approval need review. Altering an approved design, in the plant or on site, may need new approval before work continues.