Process · 10 min read · April 12, 2026 · 249 words
Prefab ADU vs site-built: the honest 2026 comparison
Prefab promises speed and a fixed price. Site-built promises flexibility. Here's what each actually delivers in California.
Key takeaways
- Flat lots in the Valley or South Bay with 25 ft+ driveway access for a delivery truck.
- Owners who want a near-fixed price and are comfortable with a catalog floor plan.
- Schedules sensitive to weather — modular sets in one day, dries in immediately.
Answered in this guide
Jump straight to the question you came in with — every answer is on this page, with links onward to the deeper guide.
- How long does an ADU project take in Los Angeles?
- Can I move faster with a pre-approved standard plan?
- What slows projects down most often?
- What is the typical week-by-week breakdown?
- Are you licensed and insured?
- Do you use one crew or subcontractors?
More across the studio · the full FAQ map · the reference desk
Prefab ADUs (modular and panelized) get pitched as faster and cheaper. The reality in 2026 is that prefab delivers a 4–8 week schedule advantage at roughly the same all-in cost as site-built, and only when your lot has good crane access and a forgiving slope. The cost saving disappears into transport, crane day rates, and the same site work (foundation, utilities, hardscape) that any project carries.
Where prefab actually wins
- Flat lots in the Valley or South Bay with 25 ft+ driveway access for a delivery truck.
- Owners who want a near-fixed price and are comfortable with a catalog floor plan.
- Schedules sensitive to weather — modular sets in one day, dries in immediately.
Where site-built wins
- Hillside, tight-access, or tree-protected lots where a crane cannot stage.
- Owners who want custom rooflines, materials, or layouts beyond a catalog.
- Projects where matching the primary house architecture is non-negotiable.
The cost myth, decoded
Catalog-published prefab prices ($180K–$240K for an 800 sq ft 1-bed) routinely exclude foundation, utility connections, permits, design, site work, finish upgrades, and delivery. By the time you're holding a certificate of occupancy, the all-in is $290K–$370K — within $10–$20K of a comparable site-built project from a reputable design-build studio.
Sources
- California HCD — Manufactured Housing · California HCD
Next chapter · 01 of 02
Cost · 9 min read
The anatomy of an LA ADU build, line by line
The cost anatomy applies whether you choose prefab or site-built — both carry the same site work.
Where every dollar of a $325,000 detached 800 sq ft ADU actually goes — soft costs, hard costs, contingencies, and the line items that surprise homeowners most.
FAQ · Process
Common questions on process
The questions readers send us most after this guide.
How long does an ADU project take in Los Angeles?
Most detached ADUs run 9–13 months from contract to certificate of occupancy. Garage conversions are typically 4–6 months. Permitting alone is usually 60–120 days at LADBS depending on whether we use a pre-approved standard plan or a custom design that needs full plan check.Can I move faster with a pre-approved standard plan?
Yes — meaningfully. LADBS Standard Plan ADUs skip the design review portion of plan check and often clear permitting in 30–60 days instead of 90–120. We carry a curated library of standard plans from 480 to 1,200 sq ft.What slows projects down most often?
Three things, in order: (1) utility upgrades — LADWP service upgrades can add 8–14 weeks; (2) sewer capacity studies in older neighborhoods; (3) owner-driven design changes after permit submittal. We flag all three at the feasibility call.What is the typical week-by-week breakdown?
Weeks 1–3: feasibility, survey, schematic design. Weeks 4–8: construction documents and Title 24. Weeks 9–20: plan check at LADBS or your local department. Weeks 21–24: site mobilization and foundation. Weeks 25–40: framing, MEP rough-in, drywall, finishes, and inspections. Final two weeks are punch list and closeout.Are you licensed and insured?
Yes — CSLB #1098432, fully bonded, $2M general liability, and workers' comp on every site. BBB A+ accredited, member of NAHB and the LA chapter of AIA.Do you use one crew or subcontractors?
We carry our own foreman, framers, and finish carpenters in-house. Specialized trades — MEP, roofing, glazing, solar — are long-standing subcontract relationships, the same crews on every project. You will know every face on your site by week three.
Reference desk · Process
More answers from the California reference desk
City-specific questions pulled from our 5,000-answer FAQ corpus — every link opens a deeper desk page.
How long does a ADU take in Beverly Hills from start to finish?
End-to-end for a ADU in Beverly Hills: feasibility + design 6–10 weeks, City of Beverly Hills Community Development plan check 120–180 days, construction 16–22 weeks, inspections + closeout 2–3 weeks. Realistic total: 9–14 months with a competent team and clean submittals. Add 6–12 weeks for ladwp service upgrades common on pre-1970 panels — budget 8–12 wks..What is the actual construction schedule for a ADU in Beverly Hills?
Once permits issue in Beverly Hills, a ADU runs roughly: mobilization 1 wk, foundation 2–3 wks, framing + dry-in 3–4 wks, MEP rough 3 wks, drywall + finish 5–7 wks, inspections + punch 2 wks. The largest single source of slippage is inspector scheduling; the second is finish-material lead time (60–90 days on premium tile, 8–12 weeks on imported windows).Is there a fast-track path for a ADU in Beverly Hills?
City of Beverly Hills Community Development doesn't run a paid fast-track for residential work, but submittal quality compresses the timeline the same way: a fully bound set with stamped structural, Title 24, and verified site plan typically clears in 120 days rather than the upper end. Express service exists at LADBS for certain scopes — confirm eligibility before paying the fee.Does weather affect ADU schedules in Beverly Hills?
Most Beverly Hills sites lose 5–10 working days a year to weather — Pacific atmospheric-river storms January–March, and red-flag wind days that pause crane work and roof tear-offs. Plan to dry-in (roof on, windows in) before December if the schedule allows; once the building is weather-tight, the finish trades run year-round.How many inspections are required for a ADU in Beverly Hills?
Expect 8–14 inspections from City of Beverly Hills Community Development on a ADU: foundation, underground plumbing, framing, rough electrical, rough plumbing, rough mechanical, insulation, drywall, energy/HERS testing where required, final building, final electrical, final plumbing. Add utility company inspections for the panel upgrade and meter set. Building inspectors prefer 48–72 hours notice; scheduling lag is the most common late-stage delay.How long does a garage conversion take in Beverly Hills from start to finish?
End-to-end for a garage conversion in Beverly Hills: feasibility + design 6–10 weeks, City of Beverly Hills Community Development plan check 120–180 days, construction 16–22 weeks, inspections + closeout 2–3 weeks. Realistic total: 9–14 months with a competent team and clean submittals. Add 6–12 weeks for ladwp service upgrades common on pre-1970 panels — budget 8–12 wks..What is the actual construction schedule for a garage conversion in Beverly Hills?
Once permits issue in Beverly Hills, a garage conversion runs roughly: mobilization 1 wk, foundation 2–3 wks, framing + dry-in 3–4 wks, MEP rough 3 wks, drywall + finish 5–7 wks, inspections + punch 2 wks. The largest single source of slippage is inspector scheduling; the second is finish-material lead time (60–90 days on premium tile, 8–12 weeks on imported windows).Is there a fast-track path for a garage conversion in Beverly Hills?
City of Beverly Hills Community Development doesn't run a paid fast-track for residential work, but submittal quality compresses the timeline the same way: a fully bound set with stamped structural, Title 24, and verified site plan typically clears in 120 days rather than the upper end. Express service exists at LADBS for certain scopes — confirm eligibility before paying the fee.Does weather affect garage conversion schedules in Beverly Hills?
Most Beverly Hills sites lose 5–10 working days a year to weather — Pacific atmospheric-river storms January–March, and red-flag wind days that pause crane work and roof tear-offs. Plan to dry-in (roof on, windows in) before December if the schedule allows; once the building is weather-tight, the finish trades run year-round.How many inspections are required for a garage conversion in Beverly Hills?
Expect 8–14 inspections from City of Beverly Hills Community Development on a garage conversion: foundation, underground plumbing, framing, rough electrical, rough plumbing, rough mechanical, insulation, drywall, energy/HERS testing where required, final building, final electrical, final plumbing. Add utility company inspections for the panel upgrade and meter set. Building inspectors prefer 48–72 hours notice; scheduling lag is the most common late-stage delay.How long does a JADU take in Beverly Hills from start to finish?
End-to-end for a JADU in Beverly Hills: feasibility + design 6–10 weeks, City of Beverly Hills Community Development plan check 120–180 days, construction 16–22 weeks, inspections + closeout 2–3 weeks. Realistic total: 9–14 months with a competent team and clean submittals. Add 6–12 weeks for ladwp service upgrades common on pre-1970 panels — budget 8–12 wks..What is the actual construction schedule for a JADU in Beverly Hills?
Once permits issue in Beverly Hills, a JADU runs roughly: mobilization 1 wk, foundation 2–3 wks, framing + dry-in 3–4 wks, MEP rough 3 wks, drywall + finish 5–7 wks, inspections + punch 2 wks. The largest single source of slippage is inspector scheduling; the second is finish-material lead time (60–90 days on premium tile, 8–12 weeks on imported windows).
Sources & further reading
- California Government Code §65852.2 — statewide ADU framework (ministerial review, 60-day clock).
- LADBS — Accessory Dwelling Unit information bulletins and current permit fee schedule.
- HCD — California Department of Housing & Community Development, ADU handbook (2024 update).
- Internal data: 120++ ADU projects delivered across Los Angeles County, 2018–2025.
Continue your read · the editorial path
We chained these chapters in the order LA homeowners actually need them. Each one picks up where the last one left a question open.
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