Cost
Modular vs Site-Built ADU: Real Cost and Timeline Comparison
A side-by-side comparison of modular and site-built ADUs in the Bay Area, price per square foot, timeline, finish quality, and where each one actually wins.
In the Bay Area, the choice between modular and site-built ADUs comes down to three things: time, predictability, and what your lot can physically accept. Here's the honest comparison.
Cost per square foot
Site-built ADUs run roughly $700 – $900 per sq ft all-in in 2026 Bay Area conditions. Modular ADUs from SHAPE come in at $500 – $700 per sq ft all-in. The delta comes from labor running in a controlled facility and from work happening in parallel rather than in sequence.
Timeline
Site-built ADUs typically take 12 to 18 months from permit. Modular ADUs typically take 2 to 4 months from permit. The reason is simple: while modules are being built in the factory, the foundation and site work are happening on your property, not after.
Finish quality
Factory builds are squarer, drier, and tighter at the envelope than anything assembled outdoors in Bay Area winter weather. Site-built has the edge only on truly custom architectural moves that don't survive transport, radical curves, very large single panes, certain stone applications.
When site-built still wins
A handful of cases: extremely tight urban lots with no crane access, additions that physically attach to the main house's structure, and projects where the homeowner has a specific architect they want to use and that architect doesn't work modular.
For everything else, and that's most Bay Area ADU projects, modular wins on time and cost without giving up design or quality.
Let's build it.
A free 30-minute call with our team. You'll leave with a feasibility read on your lot and a real sense of what the project costs and how long it takes.
