Factory-built ADU construction has gone from a niche alternative to a significant share of all ADU permits issued, and the cost argument is a large part of why. But "factory-built is cheaper" is too blunt a claim to plan a project around. The honest version is narrower: certain cost categories shift meaningfully with panelized construction, and others don't move regardless of method.
What actually shifts
- Field labor duration — the biggest lever. A shorter field-assembly window means fewer billable days for crane, crew and site supervision, which is where panelized systems create the most direct savings versus a full stick-built framing sequence.
- Weather-related delay risk — every rain day or extreme-heat stoppage during a conventional framing schedule adds cost through crew downtime and schedule slip. A shorter enclosure window reduces the number of days exposed to that risk, though it doesn't eliminate it.
- Repeatable-unit efficiency — if you're building the same ADU plan across multiple lots or sites, panel fabrication gets more efficient with volume in a way that on-site framing crews generally don't.
What doesn't shift
- Land, permitting and impact fees — identical regardless of construction method, and in many jurisdictions this is a larger share of total project cost than the shell itself.
- Foundation work — a panelized shell still needs a code-compliant, inspected foundation ready before assembly starts. This cost doesn't disappear; it just moves earlier in the schedule.
- Interior finish, MEP and fixtures — plumbing, electrical, HVAC, cabinetry and finish work run on largely the same cost basis whether the shell was framed on site or assembled from panels.
- Transport and site access — panel systems ship flatter and more efficiently than complete modular units, but delivery logistics still cost money and still depend on site access.
Why a generic cost comparison misleads
Published cost ranges for "factory-built ADUs" often bundle wildly different products — everything from a bare steel-frame DIY kit to a fully finished, delivery-ready turnkey unit — against an equally generic "site-built" baseline. That's why PanLbilt's own comparison framework deliberately avoids a flat dollar figure: enclosure schedule, field labor, weather exposure, material handling and installed shell economics are modeled against your current bid or estimate, not a generic industry average.
How to build a real comparison
The inputs that actually determine whether panelized construction saves money on your specific ADU project:
| Input | Why it matters |
|---|---|
| Current framing bid or estimate | The actual baseline you're comparing against — not a published industry average |
| Number of units / repeatability | Panel fabrication efficiency scales with repeat units; a single custom ADU sees less benefit |
| Site access and crane logistics | Determines whether the field-assembly time savings are fully realized on your lot |
| Local labor market conditions | Where skilled framing labor is scarce or expensive, the field-labor-duration savings are larger |
Want a real number instead of a generic estimate? Send your floor plan, target unit count and current construction method for a project-specific shell comparison.
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