Are prefab ADUs cheaper than custom?

Prefab often provides more predictable mid-range pricing due to factory efficiency; custom can be cheaper or pricier depending on design and site complexity.

How long does an ADU build take?

About 5–8 months for prefab and 8–14+ months for custom, depending on permitting and site conditions.

Do builders handle permits?

Prefab and design-build firms often manage submittals; independent GCs may rely on your architect—confirm in scope.

Will a builder help with utilities and trenching?

Yes, but confirm inclusions: lateral distances, panel upgrade, sewer tie-in, and any utility fees.

What warranties should I expect?

Commonly: 1-year workmanship, manufacturer warranties on appliances/roof/windows; some prefab firms offer extended structural coverage.

Can I finance builder payments?

Yes—HELOC, cash-out refi, or construction loans; draw schedules align with milestones.

How do I compare bids fairly?

Normalize specs: foundation type, insulation, windows, HVAC, finishes, crane/freight, and contingency. Ask for a scope matrix.

Modern ADU exterior with landscaped yard

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Table of contents

TL;DR: Prefab ADUs are generally faster with more predictable pricing; custom ADUs offer design flexibility and fit tricky lots. The right choice depends on your timeline, budget, site access, and finish expectations. Answer a few questions below to shortlist builder types and request quotes.

Prefab vs Custom — What’s right for you?

Factor	Prefab (Modular/Panelized)	Custom (Stick-Built/Design-Build)
Typical total cost	$$–$$$ (more predictable)	$$–$$$$ (varies by design/site)
Speed	Fastest (factory + short onsite)	Moderate (permit + onsite build)
Design flexibility	Curated options, limited changes	Maximum flexibility for unique plans
Site disruption	Lower (short onsite window)	Higher (longer onsite work)
Permit complexity	Often streamlined by factory docs	Varies by city & scope
Inspections	Fewer onsite; factory-approved components	Full sequence of onsite inspections
Change-order risk	Lower (fixed package specs)	Higher (custom selections evolve)
Tricky lots	Case-by-case (transport & crane)	Adaptable with custom foundation/framing
Best for	Speed + predictability	Unique design or tight sites

Builder Types Directory

Prefab Modules

What it is: Factory-built room-sized modules craned into place; fast onsite set.

When to choose: Need speed; standard layouts work; good crane access.
Timeline: 5–8 months total.
Price band: $$–$$$ (packages with upgrades).
Includes: Structural, MEP in modules, standard finishes.
Excludes: Foundation, utility runs, site work (often separate).

Pros: fast, predictable. Watch-outs: transport route, crane swing, HOA aesthetics.

Panelized Prefab

What it is: Factory-built wall/roof panels assembled onsite (hybrid).

When to choose: Prefer prefab efficiency but need site flexibility.
Timeline: 6–9 months total.
Price band: $$–$$$.
Includes: Panels, framing kit, shop drawings.
Excludes: Assembly labor; finishes vary by provider.

Pros: faster dry-in, adaptable designs. Watch-outs: provider ↔ installer coordination.

Design-Build Studio

What it is: One team handles design, engineering, permitting, and build.

When to choose: Desire a single accountable partner and curated finishes.
Timeline: 8–12+ months total.
Price band: $$–$$$$.
Includes: Architecture, engineering, permit set, GC build.
Excludes: Site surprises beyond allowances.

Pros: one contract, smoother handoffs. Watch-outs: change orders impact schedule.

General Contractor + Architect

What it is: Separate architect for design; independent GC builds.

When to choose: Maximum design control, complex sites, premium finishes.
Timeline: 10–14+ months total.
Price band: $$–$$$$$ (wide range).
Includes: Bespoke design; competitive bid potential.
Excludes: Coordination is on owner (unless CM/PM engaged).

Pros: tailored design; competitive GC selection. Watch-outs: coordination effort & bid variance.

How to Choose a Builder

Timeline: Need keys in <9 months? Prefer prefab/module/panelized.
Lot access: 12–16 ft wide clear path + crane pad available? Prefab modules OK; else panelized/custom.
Design must-haves: Non-standard or HOA styling → custom/design-build.
Budget style: Prefer fixed packages? Prefab. Comfortable with allowances? Custom/design-build.
Risk tolerance: Minimize change orders? Prefab/design-build.
Utilities & soils: Unknowns call for contingencies; custom handles surprises better.
References: Ask for 2 recent builds within 25 miles; verify permits & passed inspections.
Contracts: Milestones, draw schedule, lien releases, and stated warranty.
Insurance & license: Confirm active license, workers’ comp, liability limits.
Apples-to-apples bids: Normalize spec levels, crane/foundation/utility line items.

Red flags: Unwilling to pull permits, vague allowances, missing schedule, large deposits upfront without plan.

Typical ADU Project Timeline

1) Feasibility (1–3 weeks)
Site review, setbacks/utilities check, rough budget. Deliverable: Feasibility memo + concept plan.

2) Design & Scope (3–8 weeks)
Plans, selections, engineered foundation. Deliverable: 80% permit set + scope matrix.

3) Permitting (4–12+ weeks)
Submittal, plan check, revisions. Deliverable: Approved permit; fee receipts.

4) Pre-construction (1–2 weeks)
Mobilization, purchase orders, schedule baseline, utilities locate. Deliverable: Final budget + construction schedule.

5) Build (12–20+ weeks)
Foundation → structure → MEP rough → insulation → drywall → finishes → final. Deliverable: Passed inspections, punchlist.

6) Closeout (1–2 weeks)
Final inspection, CO, O&M manuals, warranty packet. Deliverable: Lien releases, as-builts, photos.

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750 sf detached ADU with light wood siding — San Diego, CA — 750 sf detached (panelized prefab). Permit-to-keys: 7.5 months. Rent: $2,350/mo.

Converted garage ADU with new windows and entry — Portland, OR — 420 sf garage conversion (design-build). Seismic upgrades. Completed in 5 months.

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