One of the first big decisions homeowners face when planning an Accessory Dwelling Unit is how it will be built. Should the home be constructed primarily on site, or should it be factory built and installed on the property. Both approaches can lead to successful outcomes, but they offer very different experiences along the way. Understanding those differences helps homeowners choose the path that best fits their property, timeline, and priorities.

In California, interest in factory built ADUs has grown quickly, while site built construction remains familiar and widely used. Neither method is universally better. The right choice depends on access, site conditions, design goals, and how much predictability a homeowner wants during the build. At Joy Line Homes, we help homeowners evaluate both options with clarity, so the decision feels informed rather than overwhelming.

What Site Built ADUs Look Like in Practice

A site built ADU is constructed primarily at the property itself. Materials are delivered to the site, and trades complete the work step by step, from foundation to framing to finishes. This approach is familiar to many homeowners because it mirrors traditional home construction.

Site built construction can be well suited for properties with limited access, unusual shapes, or steep slopes. When delivery constraints make it difficult to bring in a factory built unit, on site construction may offer more flexibility.

Design customization is often cited as a benefit. Because the home is built piece by piece, it can be easier to adjust layouts or details to fit very specific site conditions. That said, customization also requires careful coordination to avoid delays.

Challenges of Building Entirely On Site

Site built ADUs rely heavily on jobsite conditions. Weather, labor availability, and scheduling between trades can all influence the timeline. In busy construction markets, delays are common and can add stress for homeowners.

Extended construction also means extended disruption. Noise, dust, and daily activity can continue for months, which can be challenging for households living on the property during the build.

Quality control varies depending on coordination and oversight. While many site built projects are executed beautifully, inconsistency between trades can lead to uneven finishes if the process is not carefully managed.

How Factory Built ADUs Are Constructed

Factory built ADUs are constructed largely in a controlled environment. Walls, floors, and systems are assembled indoors, protected from weather, and completed using repeatable processes. Once the unit is ready, it is delivered to the site and installed on a prepared foundation.

This method allows much of the construction to happen in parallel with site preparation. While foundations and utilities are being completed on site, the home itself is being built in the factory.

For many homeowners, this overlap shortens the overall project timeline and creates a more predictable sequence.

Consistency and Quality Control

One of the strongest advantages of factory built ADUs is consistency. Controlled conditions support precision, repeatable detailing, and standardized quality checks throughout the build.

Materials are not exposed to prolonged moisture, and work is completed with stable tools and setups. This often results in cleaner finishes and tighter tolerances.

Homeowners may not see the factory process firsthand, but they feel its impact in how solid and complete the finished home feels.

Timeline Predictability

Factory built construction tends to offer greater timeline predictability. Because production follows a defined sequence, it is easier to estimate completion dates.

This predictability can be especially valuable when the ADU is needed by a certain date, such as for family housing or planned rental use.

While no construction process is completely immune to delays, factory built workflows reduce many common variables.

Reduced Disruption at Home

Because much of the work happens off site, factory built ADUs often involve a shorter on site construction phase. Homeowners typically experience site prep, delivery, installation, and final connections rather than months of daily trade activity.

This can make a meaningful difference for families living on the property. Reduced disruption often leads to a more positive overall experience.

Neighbors may also appreciate a shorter construction window, which can help maintain goodwill within the community.

Design Flexibility in Factory Built Homes

A common misconception is that factory built ADUs lack design flexibility. In reality, experienced builders offer a wide range of layouts, finishes, and architectural styles.

Design decisions are often finalized earlier in the process, which supports smoother production. While late changes are more limited, early clarity leads to fewer compromises.

For many homeowners, this structured approach reduces decision fatigue and creates confidence in the final outcome.

Site Access and Delivery Considerations

Factory built ADUs require sufficient access for delivery and installation. This includes driveway width, turning radius, overhead clearance, and sometimes crane placement.

Properties with tight access or significant elevation changes may face limitations. Early site evaluation is essential to determine feasibility.

When access works, delivery day is often a highlight of the project, marking rapid progress.

Cost Clarity and Budget Control

Factory built ADUs often provide clearer upfront pricing for the unit itself. This transparency can help homeowners plan with greater confidence.

Site built projects may allow more incremental decisions, but that flexibility can also introduce budget uncertainty if changes accumulate.

In both approaches, site work and utilities remain significant cost factors. The key difference is how predictable the building portion of the budget feels.

Which Approach Is Right for You

The best choice depends on priorities. Homeowners who value predictability, reduced disruption, and consistent quality often lean toward factory built options.

Those with complex sites or highly customized design goals may prefer site built construction.

There is no single correct answer, only the right fit for a specific property and lifestyle.

A Balanced Decision

Both factory built and site built ADUs can result in beautiful, livable homes. The difference lies in the journey.

At Joy Line Homes, we help homeowners evaluate access, design goals, budget, and timeline before recommending an approach.

With the right guidance, choosing between factory built and site built becomes a confident step rather than a confusing one.