Quick Answer: Permit-ready ADU plans are pre-designed architectural drawings that have been pre-approved by a local jurisdiction — you purchase the plans, submit them with a simple site plan showing your lot, and the city approves them faster (often 4-8 weeks vs 4-12 months for custom plans) because the structural and architectural elements were already reviewed. Prefab ADUs are factory-built structures (either modular — built in sections and assembled on site, or manufactured panel kits assembled by a crew) — the building itself is fabricated off-site and delivered, but still needs a foundation, utilities, and a building permit. Permit-ready plans can be used with site-built construction; prefab units use factory construction. Key difference: permit-ready is about the drawings; prefab is about the construction method.
Permit-Ready vs Prefab ADU — Full Comparison
Helpful Resources:
| Factor | Permit-Ready Plans | Prefab / Modular ADU |
|---|---|---|
| What it is | Pre-approved architectural drawings; site-built construction | Factory-built structure delivered to your lot |
| Permitting timeline | 4-8 weeks (pre-approved) | 4-12 weeks (state building certification + local site permit) |
| Customization | Limited — select from pre-designed floor plans | Limited — manufacturer’s standard models; some options |
| Cost (turnkey) | $180,000 – $350,000 (site-built with permit-ready plans) | $150,000 – $300,000 (unit + delivery + site work + foundation) |
| Quality control | Depends on general contractor quality on site | Factory quality control, consistent construction |
| Site prep required | Yes — foundation, utilities, grading | Yes — foundation, utilities, grading (same as site-built) |
| Best for | Standard lots without unusual conditions | Same — but faster construction phase once site is ready |
| Examples (California) | San Jose’s pre-approved ADU library; LA’s Standard Plan program | Abodu, Mighty Buildings, Cover, Villa Homes |
San Jose’s Pre-Approved ADU Plan Program
The City of San Jose maintains a library of pre-approved ADU plans (called “pre-approved standard plans” or “permit-ready plans”) that homeowners can select and submit with a simplified application. These plans have already passed architectural, structural, and energy code review — the city only needs to review the site-specific items (setbacks, utilities, grading). This can reduce permit review time from 4-8 months to 6-10 weeks for the right lot and plan combination.
Quartz Construction builds both site-built ADUs (with permit-ready or custom plans) and coordinates prefab/modular ADU installations throughout San Jose. Learn about ADU construction services. CSLB #1003436 | Call (650) 749-7436.
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