Prefab vs site-built ADUs: honest tradeoffs

True modular: full 3D box modules built in a factory, trucked to site, craned into place. Panelized: flat 2D wall and floor panels built in a factory, assembled on site. Pre-cut/pre-engineered: components cut and labeled in a factory, framed conventionally on site. Each has a different LA cost profile.

Fine Homebuilding has good coverage of the technical distinctions Fine Homebuilding — prefab construction coverage and JLC's commercial-grade construction reporting JLC — construction supply chain reporting is useful for understanding the supply-chain economics.

Hard cost savings of 5–15% are realistic for full modular ADUs in LA, driven by factory labor productivity and bulk material purchasing. Schedule compression on the structure itself is real — a modular ADU can be set in a single day after months of factory build time.

What modular doesn't save: foundation construction, utility hookups, finish work like decks and landscaping, and permits. The Terner Center has published research on California modular adoption Terner Center — California modular research and the LA Times has covered local modular ADU projects LA Times — modular ADU coverage.

Crane access. Many LA lots — Westside flats, hillside, narrow Eastside parcels — cannot accommodate the crane swing radius needed to set a modular ADU. The crane logistics ($8,000–$25,000 just for the crane day, more for street closures) frequently erase the factory savings.

Site-built construction also produces a more customized result. Modular ADUs tend toward standard floor plans because the factory tooling is the constraint. ArchDaily's coverage of modular vs site-built tradeoffs is worth reading ArchDaily — modular vs site-built and Dwell has good case studies of both Dwell — prefab and site-built case studies.

Panelized construction (2D wall panels assembled on site, no crane needed) gets most of the factory-precision benefit without the crane logistics. Panels arrive on a flatbed truck, get carried into the backyard, and are stood up by a small framing crew. For LA lots with poor crane access, panelized often pencils better than either pure modular or pure stick-build.

Curbed LA has reported on panelized ADU adoption in LA neighborhoods Curbed LA — panelized ADU adoption and ArchDaily covers the design implications well ArchDaily — panelized residential.

Make sure the quote includes: foundation, crane day(s), site prep, utility connections, finish carpentry, exterior decking and entry stairs, landscaping restoration, and contingency. Many prefab marketing prices exclude all of these — the bare module price is often only 55–65% of the actual project cost.

Compare apples to apples by getting a site-built quote on the same scope from a local contractor and looking at total project cost, not module cost. Maxable maintains good comparisons Maxable — prefab comparison guides and AARP's small-footprint home guides are useful for spec definition AARP — small-footprint home guides.