Modular Construction Glossary

ADU (Accessory Dwelling Unit)

An Accessory Dwelling Unit is an independent living space — with its own kitchen, bath, and entrance — on the same property as a main house. ADUs are popular for rental income, multigenerational living, and home offices. Modular and container construction is a fast, cost-effective way to add one.

Related: container home, modular construction

AHJ (Authority Having Jurisdiction)

The Authority Having Jurisdiction is the city, county, or state office responsible for code enforcement, plan review, and permitting where you build. Because requirements vary by AHJ, modular builders coordinate directly with the local office on each project.

Related: modular construction, wind load

Button-Up

Button-up refers to all the tasks needed to make separately delivered modules into one weather-tight, continuous building after they are set. This includes sealing roof and wall mate-lines, connecting electrical and plumbing across modules, patching drywall seams, and completing flooring and trim at the joints. Button-up scope and quality vary by manufacturer and directly affect how 'seamless' the finished home looks.

Certificate of Occupancy

A certificate of occupancy (CO) is issued by the local building department after final inspections confirm a structure meets applicable codes and is safe to live in or use. For modular and container projects, the CO depends on completed foundation, utility hookups, button-up, and site work passing inspection, not just factory completion. Without a CO, a building generally cannot be legally occupied, insured, or sometimes financed.

Chassis

A chassis is the structural steel base, typically with axles, wheels, and a hitch, that carries a manufactured unit over the road. In manufactured (HUD) homes the chassis stays permanently as part of the structure, while in modular construction a temporary carrier or flatbed is used and removed after placement. The distinction affects code classification, financing, and foundation requirements.

Container Home

A container home uses the durable steel structure of shipping containers as its frame, then adds insulation, finishes, windows, doors, and systems. The steel frame is strong and wind-resistant, making container homes well-suited to high-wind regions when properly engineered.

Related: modular construction, adu, tiny home

Corten Steel

Corten, or weathering steel, is an alloy that develops a protective oxide layer when exposed to the elements, slowing further corrosion without paint. Shipping containers are commonly made from Corten-type steel, which gives them durability but does not make them rust-proof, especially at cut edges and welds. Buyers modifying containers must protect newly exposed steel from moisture to prevent ongoing corrosion.

Draw Schedule

A draw schedule is the agreed sequence of partial payments, or draws, released to the builder as defined milestones are reached, such as deposit, factory completion, delivery, and final finish. It protects both buyer and lender by tying money to verifiable progress rather than paying everything upfront. Modular projects often front-load draws because much of the value is built in the factory before site work begins, which buyers should review carefully.

Egress

Egress refers to the means of safely exiting a building in an emergency, and codes require habitable rooms—especially bedrooms—to have a properly sized exit. An egress window must meet minimum opening dimensions and sill height so an occupant can escape and a firefighter can enter. Container and tiny-home conversions frequently need steel cut and reframed to add compliant egress openings.

Fabrication

Fabrication is the manufacturing phase where raw materials—steel, lumber, panels—are cut, welded, framed, and assembled into modules or building components. In container and modular work it covers structural modifications such as cutting openings, reinforcing the frame, and installing systems before delivery. Quality of fabrication directly determines structural integrity, weather-tightness, and ease of on-site assembly.

Factory QC

Factory quality control is the structured inspection and testing process applied to modules as they are built indoors, often by in-house inspectors plus an independent third-party agency. Because work happens in a controlled environment with repeatable processes, defects can be caught and corrected before the unit leaves the plant. This documented QC is a key reason modular construction can offer more consistent build quality than fully site-built work.

Foundation Types (Pier / Slab / Crawlspace)

A pier foundation uses isolated columns or piles to lift the structure off the ground, common for containers and modular units on sloped or remote sites. A slab is a continuous poured-concrete floor that sits at grade, while a crawlspace raises the building on perimeter walls with an accessible gap underneath for utilities. The right choice depends on soil conditions, frost depth, flood risk, and how the building will be anchored against wind and seismic loads.

Gray Water

Gray water is the relatively clean wastewater from showers, bathroom sinks, and washing machines, as opposed to black water from toilets. Where local code allows, it can be captured and reused for irrigation or filtered for non-potable uses, reducing freshwater demand in off-grid or sustainable builds. Gray-water systems are regulated separately from sewage and require approval from the local health or plumbing authority.

HVAC Mini-Split

A mini-split is a ductless heating and cooling system that pairs an outdoor compressor with indoor air-handling heads connected by refrigerant lines. It is popular in modular, container, and tiny-home builds because it needs no ductwork, fits compact spaces, and offers efficient zoned temperature control. Multiple indoor heads can run from one outdoor unit, allowing room-by-room comfort in small footprints.

IRC / IBC Code

The International Residential Code (IRC) covers one- and two-family dwellings, while the International Building Code (IBC) covers commercial buildings and most multi-family and larger structures. Jurisdictions adopt and amend specific editions of these codes, which set requirements for structure, egress, fire safety, and energy use. Modular buildings are typically built to the same IRC or IBC edition the local jurisdiction enforces, verified through a state modular program or third-party inspection.

Manufactured (HUD) Home

A manufactured home is built entirely in a factory to the federal HUD Code rather than to local IRC/IBC codes, and it retains a permanent steel chassis. This federal standard, financing treatment, and chassis distinguish it from modular homes, which are built to the same local codes as site-built houses. The classification affects where the home can be placed, how it is financed, and how it appraises.

Modular Construction

Modular construction builds a home or structure as finished sections in a controlled factory, then transports and joins them on a permanent foundation at the site. Because factory work runs in parallel with site preparation, projects finish faster than traditional construction while meeting the same local building codes.

Related: container home, prefab, adu

Modular Tiny Home

A modular tiny home is a compact factory-built dwelling, generally below roughly 400 square feet, designed as a permanent or semi-permanent residence. Built to applicable residential codes, it differs from tiny homes on wheels, which are often regulated as recreational vehicles. Tiny homes frequently serve as accessory dwellings, guest spaces, or affordable standalone housing on a permanent foundation.

Module

A module is a complete volumetric box-like section of a building assembled in a factory, including floors, walls, ceiling, and often finishes, wiring, and plumbing. Multiple modules are transported to the site and connected to form the finished structure. Module count and dimensions are usually limited by road-transport width and height restrictions.

Net-Zero

A net-zero building balances its total annual energy use with on-site renewable generation, typically solar, so its net energy purchase is around zero. Reaching net-zero relies on a tight, well-insulated envelope, efficient mechanical systems, and adequate renewable capacity. Compact modular and container homes can pursue net-zero relatively easily because their smaller conditioned volume lowers total energy demand.

On-Frame vs. Off-Frame

On-frame construction leaves the unit permanently attached to its steel transport frame, common in manufactured homes and some modular units placed on piers. Off-frame (or off-chassis) construction lifts the building off the carrier and sets it directly on a permanent foundation, allowing it to be treated more like site-built construction. The choice influences appraisal, code path, and the type of foundation required.

Panelized

Panelized construction prefabricates two-dimensional components—wall, floor, and roof panels—in a factory and ships them flat to be erected and connected on site. Unlike volumetric modular building, panels arrive open rather than as finished rooms, which lowers shipping cost but shifts more assembly and finishing work to the job site. It is a middle ground between full modular and traditional stick-building.

Prefab (Prefabricated)

Prefab, short for prefabricated, covers any building approach where parts are made in a factory and assembled on-site — including modular, panelized, and container construction. The shared benefit is the quality control and speed of factory production.

Related: modular construction, container home

R-Value / Insulation

R-value quantifies a material's thermal resistance, with higher numbers indicating greater resistance to heat transfer through walls, roofs, and floors. Building codes set minimum R-values by climate zone, and modular and container structures must meet these just like site-built homes. Because container and tiny-home walls are thin, achieving required R-values often relies on spray foam or rigid-board insulation rather than standard batts.

Seismic Design

Seismic design addresses the lateral and dynamic forces an earthquake imposes on a building, governed by seismic design categories tied to a site's ground-shaking risk. It dictates anchoring, bracing, connection strength, and foundation detailing, and is especially important for how modules attach to each other and to the foundation. In higher-risk zones, modular and container structures need engineered tie-downs and reinforced connections to comply.

Set Crew

A set crew handles the on-site placement of factory-built modules, operating or directing the crane, aligning units on the foundation, and bolting or welding them together. They typically work for a day or two during the 'set,' which is the most weather- and logistics-sensitive part of a modular build. Their work precedes the button-up and finishing trades.

Shipping Container (ISO / High-Cube)

Shipping containers are corrugated steel boxes built to ISO standards for global freight, most commonly 20 or 40 feet long and about 8 feet wide. A standard container is roughly 8 feet 6 inches tall, while a high-cube container is about 9 feet 6 inches, giving extra headroom that matters once floor and ceiling insulation reduce interior height. Container builders favor high-cube units and prefer one-trip or new containers to avoid contamination and rust found in heavily used boxes.

Snow Load

Snow load is the design weight, expressed in pounds per square foot, that a roof structure must support based on local climate data. Higher-snow regions require stronger roof framing, steeper pitches, or upgraded structural design. Buyers placing a modular or container building should confirm the unit's rated ground snow load matches their site's requirement set by the local authority.

Wind Load

Wind load is the pressure that wind places on a building. Areas like Tornado Alley and hurricane coasts require higher wind-load resistance. Steel-framed modular and container structures are engineered to the wind exposure of each specific site.

Related: container home, ahj