One of the most common points of confusion for homeowners exploring Accessory Dwelling Unit is the terminology. Prefab, modular, factory built, site built, traditional construction. These words are often used interchangeably, even though they describe very different building methods. Understanding what each term actually means can make the entire planning process feel clearer and far less intimidating.

Choosing the right type of ADU is not about chasing trends or labels. It is about understanding how each construction method affects timeline, quality, cost control, and daily life during the build. At Joy Line Homes, we spend a lot of time helping homeowners decode these terms so decisions are based on substance rather than marketing language.

What “Prefab” Really Means

Prefab is a broad umbrella term. It simply means that some portion of the home is built off site rather than entirely on the property. Within that category, there are many different levels of quality, customization, and process.

Some prefab homes are highly refined, design forward residences built in controlled environments. Others are more basic, focusing primarily on speed or cost. The word prefab alone does not tell you much about how the home will actually perform.

When homeowners hear prefab, they should ask deeper questions. How much of the home is built off site. What standards are used. How are systems integrated. Prefab describes a method, not a guarantee of quality.

Understanding Modular ADUs

Modular ADUs are a specific type of prefab construction. They are built in sections, or modules, in a factory setting. These modules are transported to the site and assembled on a permanent foundation.

Modular construction follows the same building codes as site built homes. The difference is the environment in which the work happens. Controlled conditions allow for consistent processes, protected materials, and repeatable quality checks.

Because modules are designed to fit together precisely, planning and coordination happen early. This can reduce surprises later in the project and create a more predictable timeline.

Factory Built Does Not Mean Temporary

A common misconception is that factory built homes are somehow less permanent. In reality, factory built ADUs are designed to be long lasting structures installed on permanent foundations.

Once installed, a modular or factory built ADU functions like any other home. It connects to utilities, meets local codes, and becomes a permanent part of the property.

The difference lies in how the home is assembled, not in its longevity or legitimacy.

What Traditional Site Built ADUs Involve

Traditional site built ADUs are constructed almost entirely at the property. Materials arrive in stages, and trades complete work sequentially, from foundation to framing to finishes.

This approach offers flexibility, particularly for complex or constrained sites. It can accommodate unusual layouts, tight access, or steep slopes that make delivery difficult.

Site built construction is familiar, but it also exposes the project to weather, labor availability, and scheduling challenges that can affect timelines.

How Construction Method Affects Timeline

One of the biggest differences between these approaches is how long construction takes. Site built ADUs typically follow a linear timeline, with each phase depending on the previous one being completed.

Modular and factory built ADUs allow for overlap. While the unit is being built in the factory, site preparation and foundations can happen simultaneously.

This overlap often shortens the overall project duration and creates a clearer sense of progress.

Quality Control and Consistency

Quality control looks different depending on the build method. In site built projects, quality depends heavily on coordination between trades and conditions on the jobsite.

Factory environments support repeatable processes and inspections at multiple stages. Materials are protected, and work is completed with consistent tools and setups.

This often results in tighter finishes and more uniform results, especially for smaller homes where details are noticed more quickly.

Design Flexibility Across Methods

Another misconception is that modular or prefab homes lack design flexibility. While late changes are more limited, many factory built ADUs offer a wide range of layouts, finishes, and architectural styles.

Traditional site built construction can allow for more customization during the build, but that flexibility often comes with added complexity and potential cost increases.

The key difference is when decisions are made. Factory built approaches require earlier clarity, while site built approaches may allow more changes along the way.

Disruption During Construction

Living on a property during construction can be challenging. Site built ADUs often involve months of daily activity, including noise, dust, and staging.

Factory built and modular ADUs typically reduce on site time. Homeowners experience site prep, delivery, installation, and final connections rather than continuous construction.

For many families, this reduced disruption is a major factor in choosing a factory built option.

Cost Transparency and Budget Control

Cost clarity varies by method. Factory built ADUs often provide more defined pricing for the unit itself, which helps with early budgeting.

Site built projects may offer flexibility, but that flexibility can introduce uncertainty if changes accumulate during construction.

In all cases, site work, utilities, and permitting remain significant cost factors that must be planned carefully.

Choosing What Fits Your Property

The right choice depends on access, site conditions, timeline goals, and personal tolerance for disruption. No single method is best for every property.

Homeowners who value predictability and efficiency often lean toward modular or factory built options. Those with highly complex sites may find traditional construction more suitable.

A clear site evaluation helps narrow the options quickly and confidently.

Clarity Leads to Better Decisions

Understanding the real differences between prefab, modular, and traditional ADUs empowers homeowners to ask better questions.

At Joy Line Homes, we believe clarity is the foundation of a good building experience. When homeowners understand the process, they feel more confident and more in control.

With the right information, choosing a construction method becomes a thoughtful decision rather than a confusing one.