Manufactured and Modular Structures

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Factory-constructed structures include both residential and commercial structures that are built off-site in a factory and are later transported to a building site for installation. The State Department of Labor and Industries (L&I) regulates and approves the construction of all factory-constructed structures that are sold, leased, or used in Washington State. Site installation requirements and inspections vary with the type and use of the factory-constructed structure and are generally the responsibility of the local jurisdiction where the structure is to be located.

This tip sheet describes the types of factory-constructed structures allowed in Tacoma, along with their permitting and inspection requirements.

Modular Homes.
Subject to the requirements for single-family dwellings.

Manufactured and Mobile Home/Trailer Courts. 
Only allowed with a conditional use permit in R-4-L and C-2 Districts.

All manufactured homes shall be comprised of at least two fully-enclosed parallel sections, each of not less than 12 feet wide by 36 feet long.

TMC 2.02.620 Manufactured Homes
Manufactured homes, as defined by Title 46 of the Revised Code of Washington (“RCW”) (“Motor Vehicles”), shall be permitted to be installed in the City, subject to the following conditions:

Manufactured homes to be installed in the City shall be new, which means any manufactured home required to be titled under Title 46 RCW which has not been previously titled to a retail purchaser and which is not a “used mobile home” as defined in RCW 82.45.032(2), which states:

(2) “Used mobile home” means a mobile home which has been previously sold at retail and has been subjected to tax under Washington State RCW chapter 82.08, or which has been previously used and has been subjected to tax under Washington State RCW chapter 82.12, and which has substantially lost its identity as a mobile unit at the time of sale by virtue of its being fixed in location upon land owned or leased by the owner of the mobile home and placed on a foundation (posts or blocks) with fixed pipe connections with sewer, water, and other utilities.


Building Permit Requirements
In order to install a factory-constructed structure on a site, you must:

  1. Application. Apply for a building permit online at If you need assistance, you can call (253) 591-5030 to speak with a staff member. There are different requirements for each of the three factory-constructed structures. (See Table 1 and Table 2 on page 7 and 8)
  2. Site Plan. A site plan of the site showing the location of the factory-constructed structure with setbacks and yard requirements noted is necessary to ensure compliance with the Land Use Code (Title 13, Tacoma Municipal Code).

Installation permit does not cover any additions or accessory structures, unless specifically included in the L&I approvals, such as a deck, porch, carport or garage, since these elements are typically constructed on-site. If you are planning to add accessory structures on the site, you will need to include information relating to these in your application to erect the additional structure(s) or subsequently obtain an additional building permit from Planning and Development Services. If accessory structures are partially supported by factory-constructed structures, such additional loads must be included in the L&I approvals.

The Building Official shall be responsible for issuing all permits for alterations, remodeling, or expansion of manufactured housing which has been converted to real property and is located within City limits.

Links for fees and inspections:
Fee Estimator

Two types of residential structures built in factories are permitted in the city of Tacoma if they comply with local code requirements, such as structure setbacks, side and rear yards, environmentally critical areas, and other development standards, including:

  1. Red Seal Insignia (Manufactured). Factory-built structures (typically described as manufactured housing) that are approved by L&I with issuance of a red seal insignia certifying that they meet standards of the federal Department of Housing and Urban Development (HUD)
  2. Gold Seal Insignia (Modular). A type typically described as modular construction (used for residential and non-residential purposes) is approved by L&I with issuance of a gold seal insignia certifying that it meets the standards of the Washington State Building Code.

Factory-constructed housing with a red seal insignia (typically referred to as manufactured housing or “red seal housing”) is regulated through the National Manufactured Home Construction and Safety Standards Act of 1974 as administered by HUD. The act sets minimum construction standards that preempt local and state building codes for all manufactured housing built after June 15, 1976. The red seal is issued through L&I, which inspects the factories assembling manufactured housing to certify compliance with HUD standards. This approval also applies to the plumbing, heating, electrical and other components within the structure.

Red seal housing has two separate information plates. The first, a red seal insignia, is discussed below. The second is a data plate, located near the main electrical panel, which lists important information about the home itself, including where and on what date the home was built, a list of major equipment installed in the home, and the structural earthquake zone and wind loads for which the home was designed.

The red seal insignia must be attached to each transportable section of manufactured housing, located at the tail light end of each section. It is a 2” x 4” label on aluminum, stamped with a number composed of three letters followed by six digits (e.g., ABC – 000001). The three letters identify the inspection agency responsible for inspecting a manufactured housing factory.

All building plans, subsequent alterations to building plans, and construction of red seal housing must be approved by L&I. If alterations to the housing structure are made prior to or during installation or after occupancy without L&I approval, the red seal insignia will be invalidated. Manufacturers who wish to alter red seal housing must revise the building plans and resubmit them to L&I for re-approval. If an owner wishes to alter a red seal structure after occupancy, and applies for a permit to make such alterations, the Tacoma Planning and Development Services Department (PDS) requires either proof that such alteration has been L&I-approved prior to permit issuance or that the plans have been approved and stamped by a licensed engineer or architect.

Additional Requirements for Red Seal Housing

As of July 1, 1995, red seal housing may not be installed in Washington without a certified manufactured housing installer providing on-site supervision whenever installation work is being performed.

Exceptions are allowed for homeowners performing installation work on their own homes. Installation is defined as all on-site work necessary for the installation of red seal housing, including, but not limited to, construction of the foundation (if any), support piers, anchors, skirting and connections to the water and sewer systems. It does not include site preparation or accessory structures that are constructed, such as garages, carports and decks, which are items subject to PDS inspections. PDS will inspect the foundation when it meets International Building Code standards.

When the foundation does not meet International Building Code standards, PDS may require an independent inspection by a person knowledgeable with the federal standards (an architect, engineer or other licensed professional certified by Labor & Industries). After the inspection, this independent inspector will be required to submit a letter to PDS attesting that the installment meets federal standards, as administered by L&I.

Red seal housing must be installed per the manufacturer’s instructions, which are approved by HUD. A building permit application must include either the name and registration number of the contractor or licensed manufactured home dealer or the name and certification number of the certified installer.

Is a foundation required?
Manufactured homes shall be set upon a permanent foundation, as defined by the HUD handbook Permanent Foundation Guide for Manufactured Housing, which is sufficient to resist wind and seismic lateral forces, as well as the gravity loads as specified in the IRC, as adopted and amended in TMC 2.02. The Building Official shall be responsible for issuing all permits for foundations for manufactured homes.

“Permanent Foundation” for manufactured homes is defined in the HUD handbook as:

Definition of Permanent Foundation
Permanent foundations must be constructed of durable materials; i.e., concrete, mortared masonry, or treated wood – and be site-built. It shall have attachment points to anchor and stabilize the manufactured home to transfer all loads, herein defined, to the underlying soil or rock. The permanent foundations shall be structurally designed for the following:

Vertical stability.

  1. Rated anchorage capacity to prevent uplift and overturning due to wind or seismic forces, whichever controls. Screw-in anchors are not considered a permanent anchorage.
  1. Footing size to prevent overloading the soil-bearing capacity and avoid soil settlement. Footing shall be reinforced concrete to be considered permanent.
  2. Base of footing below maximum frost-penetration depth.
  3. Encloses a basement or crawl space with a continuous wall (whether bearing or non-bearing) that separates the basement or crawl space from the backfill, and keeps out vermin and water.

Lateral Stability.
An anchorage system with a tested and rated or engineered load capacity to prevent sliding due to wind or seismic forces, whichever controls, in the transverse and longitudinal directions.

  1. The space from the bottom of the manufactured home to the ground shall be enclosed by concrete or an approved concrete product. Such concrete product may be designed to support the manufactured home for gravity and lateral loads, or may be decorative.
  2. All manufactured homes shall be originally constructed with a composition or wood shake or shingle, coated metal, excluding zinc galvanized metal, or similar roof of not less than 3:12 pitch.
  3. All manufactured homes shall have exterior siding similar in appearance to siding materials commonly used on conventional site-built, IRC-compliant, single-family residences.
  4. The roof shall be designed to support 25 pounds per-square-foot snow load, in conformance with TMC 2.02.600.
  5. Manufactured homes installed within the City shall meet the Washington State Energy Code, as adopted and amended by TMC 2.10.
  6. Light and ventilation in manufactured homes shall meet the requirements of Section R303 of the IRC.

Factory-constructed structures with a gold seal insignia (typically referred to as modular or “gold seal buildings”) are regulated by state standards for housing body and frame construction as determined by state law – Washington Administrative Code (WAC) 296- 150M.

Gold seal buildings may be used for commercial or residential uses. The structure must comply with the Washington State Building, Me­chanical, Plumbing, and Energy codes, and must be placed on a permanent foundation conforming to the International Building Code (IBC). The gold seal insignia must be placed on or near the meter base or near the location where electrical service enters the structure.

If alterations to the gold seal structure are made before occupancy without the prior approval of L&I, the gold seal insignia will be invalidated. Manufacturers must resubmit a building plan and design for approval. Approved alterations will be added as a data plate on the gold seal building. The additional data plate will state what alterations have been made and the date of approval. Alterations made after the gold seal building is occupied are subject to the review and approval of the local jurisdiction.

In summary, gold seal buildings differ from red seal housing in three important ways:

  1. Gold seal buildings must be installed on a permanent foundation.
  2. Gold seal buildings are built to the Washington State Building, Mechanical, Plumbing and Energy codes.
  3. Gold seal buildings are built according to the state L&I construction standards, as opposed to HUD construction standards.

A factory-constructed structure built for nonresidential use (typically referred to as a commercial coach) is approved by L&I with issuance of a black seal or gold seal insignia certifying that the structure was manufactured in compliance with state standards.

Black Seal Insignia
Nonresidential factory constructed structures with a black seal insignia are called commercial coaches. The black seal insignia certifies that the structure is manufactured in compliance with state standards.

Commercial coaches must comply with the Washington State Building Code (for occupancy determination), the State Mechanical, Plumbing, and Energy codes, as well as other standards outlined in Washington State Law (WAC 296-150). Coaches must be placed on a permanent foundation conforming to the Building Code. This insignia can be found adjacent to the entry door, at least 12 inches above the floor line.

Commercial coaches can only be used as temporary structures designed for human occupancy for industrial, educational, assembly, professional or other commercial purposes. They can include temporary office buildings and portable school classrooms.

Commercial coaches that are portable classrooms have additional requirements for ventilation, heating, cooling and temperature-control systems. These systems must be designed by a professional engineer registered in the state of Washington and experienced in the design of heating, ventilating and air conditioning systems.

In addition to the black seal insignia, commercial coaches have an identification plate which lists important information about the non-residential structure, such as where and on what date the structure was manufactured and an assigned identification number for the structure.

The tip sheets listed below may be helpful to understand the permitting requirements related to your project.
They are available online at: or in the Permitting Center located at 747 Market Street, 3rd floor, Tacoma, WA 98402


Table 1: Comparison of the Three Types of Factory-Constructed Structures
Insignia Required Use Construction Code Requirements Construction Location[1] Who Inspects Construction in Factory? Who Inspects the On-Site Structure?
Red Seal (Manufactured Housing)[2] Residence Federal Housing and Urban Development (HUD) Factory State Labor and Industries (L&I) for HUD[3] City of Tacoma
Gold Seal (Modular Construction)2 Residence or Non-Residence[4] Washington Building Code Factory L&I City of Tacoma
Black Seal (Commercial Coaches) Non-Residence (Temporary Only) Washington State Administrative Code – Commercial Factory L&I City of Tacoma

[1] Construction of that portion of the structure above the foundation.
[2] Manufactured housing and modular construction for residential purposes are frequently used terms to describe certain types of factory constructed housing, but these terms are often misused and not consistently defined. Due to this problem, the L&I insignia system is used in this Tip to distinguish between types of factory-constructed housing.
[3] L&I inspects the assembly of factory-constructed structures for manufacturers in Washington State. Factory-constructed structures assembled in other states would be inspected by the state agency acting for HUD.
[4] Gold Seal could be commercial if constructed to commercial requirements.


Table 2: Installation Requirements for Factory-Constructed Structures

City Permits and Information Required
Types of Factory-Constructed Structures
Red Seal Insignia Gold Seal Insignia Black Seal Insignia
Type of Building Permit Required from PDS Residential Building for New Construction Permit (BLDRN) Building Permit for either Residential or Commercial New Construction (BLDRN or BLDCN). Not Allowed
Site Plan (see PDS Tips #G230) Yes Yes N/A
Site Drainage Plan Yes Yes N/A
Site Grading Permit (Excavation Plan) If exceeding grading threshold, or if located in an ECA If exceeding grading threshold, or if located in an ECA N/A

Structure Foundation Plan

Copy of L&I Approved Factory Constructed Structure Plans

Yes Yes N/A
Electrical Permit from TPU Yes Yes N/A
Plumbing Permit from PDS Yes Yes N/A
Site Inspection by PDS Site Inspector Yes



Foundation Inspection by PDS Inspector Yes, except special inspections required for HUD foundations Yes N/A

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