Modular homes are built in sections inside a factory, transported to the building site, and assembled on a permanent foundation. Factory workers usually install the structural framing, floors, walls, roof sections, wiring, plumbing, insulation, cabinets and interior finishes before the modules leave the factory. At the same time, contractors prepare the land, foundation and utility connections at the property.
A modular home may consist of one or more three-dimensional sections, also called volumetric modules. The rules are different from those for manufactured homes. HUD says manufactured homes built in the United States after June 15, 1976, must comply with the HUD Code and display a certification label.
Modular Home Construction at a Glance
| Stage | What happens | Main location |
|---|---|---|
| Design and engineering | Plans are adapted to the site, transport limits and local building codes | Design office |
| Site preparation | Land is cleared and the foundation, driveway and utility connections are prepared | Building site |
| Module construction | Floors, walls, roof, wiring, plumbing and finishes are installed | Factory |
| Quality inspections | Construction is checked during factory production and after installation | Factory and site |
| Transportation | Finished modules are secured to trailers and delivered | Road network |
| Crane setting | Modules are lifted onto the foundation and joined together | Building site |
| Final completion | Connections, exterior finishes, stairs, porches and inspections are completed | Building site |
1. The Home Is Designed for the Site and the Factory
The process starts with a floor plan, site survey and structural design. The manufacturer must account for:
- Local zoning and building requirements
- Soil conditions and foundation type
- Snow, wind, seismic and flood requirements
- Module dimensions and road transportation limits
- Plumbing, electrical and HVAC routes
- How the sections will be lifted and connected
- Exterior features such as porches, garages and roof designs
Modular homes generally follow the building codes adopted by the state or local jurisdiction where they will be installed. The International Code Council identifies modular housing as a type of off-site construction subject to the code requirements for the final location, which may include the International Residential Code.
Most design decisions need to be settled before factory production starts. Moving a wall, window or plumbing line after construction has begun can be harder than making the same change during conventional site-built construction.
2. The Building Site Is Prepared at the Same Time
The site contractor or general contractor prepares the property while the factory builds the modules. Site work can include:
- Clearing and grading the land
- Excavation
- Installing the foundation
- Connecting water, sewer, septic, gas and electricity
- Preparing drainage
- Building access roads or driveways
- Arranging permits, inspections and crane access
The foundation may be a crawl-space foundation, basement, slab or another approved system. The right choice depends on the home design, soil, climate, local codes and buyer requirements.
This parallel schedule can reduce the time between groundbreaking and occupancy. Factory construction and site preparation happen at the same time rather than one after the other.
3. Workers Build the Modules Inside a Factory
Workers build each module on a production line or at a designated factory workstation. The module is constructed as a complete structural section instead of being assembled from individual materials at the property.
Floor Systems Are Assembled First
Workers construct the floor platform with engineered lumber, floor joists or other approved structural materials. The floor must support the completed home and withstand lifting, transport and placement forces.
Walls Are Framed and Installed
Workers build and attach the exterior and interior walls to the floor platform. They create openings for:
- Doors
- Windows
- Electrical outlets
- Plumbing fixtures
- Heating and cooling equipment
- Stairways and mechanical systems
The walls may receive insulation, sheathing and some interior finishes before the module is closed.
Plumbing, Electrical and HVAC Systems Are Installed
Factory workers can install much of the modular home's mechanical, electrical and plumbing systems before delivery. This work may include:
- Electrical wiring and breaker panels
- Water supply lines
- Drain and waste pipes
- Heating and cooling ductwork
- Ventilation systems
- Light fixtures and switches
- Bathroom fixtures
- Kitchen connections
The International Code Council and Modular Building Institute publish standards covering the planning, fabrication, inspection and code compliance of off-site construction, including mechanical, electrical and plumbing systems.
Interior and Exterior Finishes Are Added
Depending on the manufacturer and design, the factory may install:
- Drywall
- Paint
- Flooring
- Cabinets
- Countertops
- Doors and trim
- Appliances
- Bathroom fixtures
- Siding
- Roofing
- Windows
The more work completed in the factory, the less work remains after the modules reach the property.
4. The Modules Are Engineered for Transportation
A modular home is designed as a system that can be built, lifted, transported and joined at the property. Each section must remain stable while it is:
- Supported during factory production
- Stored or stacked if necessary
- Lifted by a crane
- Transported by truck
- Joined to other sections at the site
The ICC/MBI off-site construction standard addresses structural connections, lateral force resistance, temporary supports, stacking and weather protection.
Manufacturers may use extra framing, lifting points, bracing or structural connections to protect the modules during delivery. These transportation requirements can affect the module layout and structural design.
5. Inspectors Check the Home During and After Construction
Modular home inspections can involve factory inspections, third-party inspections, state modular programs and local building officials. The exact process varies by state and jurisdiction.
Inspections may cover:
- Structural framing
- Electrical work
- Plumbing
- Mechanical systems
- Fire safety
- Insulation and energy requirements
- Factory workmanship
- Module connections
- Foundation attachment
- Final site installation
The ICC/MBI Standard 1205 addresses inspection and regulatory compliance for off-site construction. It covers permitting, in-plant inspections, on-site inspections and the role of the local authority having jurisdiction.
6. Finished Modules Are Transported to the Property
After the modules pass the required factory checks, workers secure them to specialized trailers. The delivery route may need to account for:
- Bridge clearances
- Narrow roads
- Low overhead wires
- Sharp turns
- Traffic restrictions
- Police or utility escorts
- Delivery timing
- Crane arrival
The transportation company, foundation contractor and crane operator coordinate the delivery so the property is ready when the modules arrive.
7. A Crane Places the Modules on the Foundation
A large crane lifts each module from the trailer and sets it on the prepared foundation. Workers then:
- Align the modules
- Secure them to the foundation
- Join floors, walls and roofs
- Connect plumbing and electrical systems
- Seal the joints
- Complete roof and exterior connections
- Install stairs, decks, porches or garages
The main structure can become weather-tight soon after the sections are joined. NAHB states that modular homes are transported with major components already installed and placed on a permanent foundation by crane.
The crane setting may take around two days on some projects. The full schedule depends on design approvals, site conditions, foundation work, utility connections, weather and the amount of finishing required at the property.
8. Contractors Complete the Remaining Site Work
Factory construction does not remove the need for site work. Contractors may still need to complete:
- Module-to-module connections
- Utility hookups
- HVAC commissioning
- Foundation backfilling
- Siding and roofing details
- Exterior trim
- Porches, decks and steps
- Garages and outbuildings
- Landscaping and drainage
- Final painting or flooring repairs
- Local inspections
After the home passes the required final inspections, the local authority can issue approval for occupancy.
Modular Homes Versus Manufactured Homes
The two terms describe different types of factory-built housing.
| Feature | Modular home | Manufactured home |
|---|---|---|
| Main regulation | State and local building codes for the final site | Federal HUD Manufactured Home Construction and Safety Standards |
| Foundation | Installed on a permanent foundation | May be placed on a permanent or non-permanent foundation, depending on the installation |
| Chassis | Not generally built around a permanent transport chassis | Built on a permanent chassis |
| Construction method | Built in sections and assembled at the site | Built in transportable sections for installation as a manufactured home |
| Appearance | Can be designed to resemble site-built housing | Available in many designs, but follows manufactured housing requirements |
HUD regulates manufactured homes through the HUD Code. Modular homes instead follow the applicable building codes for the location where they will be installed. Manufactured homes built in the United States after June 15, 1976, must comply with the HUD Code and display a certification label.
Why Can Modular Homes Be Built Faster?
Modular homes can be built faster on some projects because factory production and site preparation happen at the same time. Other factors include:
- Factory production is protected from rain and snow
- Materials are stored in a controlled environment
- Workers repeat standardized construction tasks
- Much of the interior work is completed before delivery
- The home arrives in large finished sections rather than individual materials
Modular construction is not automatically faster for every property. Difficult access, complex designs, slow permitting, foundation problems and long utility connections can delay the project. The final schedule depends on both the factory work and the conditions at the site.