Duplex ADU plans. Side-by-side and stacked. Plans and permits at a fixed price.

The legal pathway — California state law

California Government Code §§ 66310–66342 (the renumbered ADU statute, post-SB 477) is the controlling text. Three sections matter most for duplex-style configurations:

• § 66323 — Statewide Exemption ADU. Sets the by-right entitlement for ADUs on every residential lot. Recently expanded by SB 1211 (Jan 1, 2025) to allow up to 8 detached ADUs on multifamily lots.
• § 66314 — Local ordinance standards. Sets the development standards (size, setbacks, height, parking) that apply when a city's ordinance is silent.
• § 66316 — HCD enforcement authority. Allows the California Department of Housing and Community Development to challenge local ordinances that don't comply with state law.

The interaction matters: cities can adopt their own ADU ordinances, but they cannot be more restrictive than state law on the issues state law preempts. If a city's ordinance says two detached ADUs on a multifamily lot, but SB 1211 says eight, the homeowner gets eight. State law governs.

A note on duplex outcomes via 1 ADU + 1 JADU on SF lots

A note on terminology: a true duplex (the focus of this page) is one building with two units sharing a wall or stacked. It is different from a “duplex outcome” achieved on a single-family lot through 1 ADU + 1 JADU — that configuration is two SEPARATE units (the primary and the accessory) plus a junior ADU within the primary’s walls. Three rental-ready units, but not a duplex building.

For homeowners exploring multi-unit options on a single-family lot, the ADU + JADU pathway is by-right under California state law:

• Primary dwelling — the existing single-family residence stays.
• 1 detached or attached ADU — new construction up to 1,200 sqft (state floor), often more under local ordinances. Independent kitchen, bathroom, entrance.
• 1 JADU (Junior ADU) — up to 500 sqft within the primary's existing walls. Often a converted master suite or attached garage interior.

That's three rental-ready units on a single lot, no rezoning required. Some cities allow a fourth unit:

• Beverly Hills: The Incentive ADU (IADU) under BHMC § 10-3-5000.A.1.a allows a third ADU (in addition to the standard ADU + JADU) on lots ≥ 13,000 sqft. Up to 4 units total per single-family lot.
• Other cities: Some have adopted local ordinances allowing up to 4 units on SB 9 urban-lot-split parcels. We confirm this lot-by-lot during the Backyard Review.

For lot owners who don't have multifamily zoning but want multiple rental-ready units, the SF-lot duplex outcome is the fastest legal path. We design the configuration around the primary dwelling's existing footprint and your specific rental-yield targets.

How the duplex plans offer works

The duplex plan offer is structurally different from our Signature Home lineup. With Signature Homes, we sell a complete turnkey package — design, permits, AND construction at one fixed price. With duplex plans, we sell two of those three:

• Duplex plans — fixed price. Pre-engineered architectural drawings, structural engineering, Title 24 energy compliance, and the complete permit-submittal package. Same plan-quality standard as our Signature Homes.
• Permit processing — fixed price. We handle the city submittal, plan check corrections, and ministerial approval coordination through to issued building permit. State law requires the city to approve or deny within 60 days of a complete application (Gov. Code § 66317).
• Construction — quoted separately. You hire your own general contractor (or one we recommend), and they price construction based on your specific lot conditions, finish selections, and site complexity.

The model fits three audiences specifically:

• Builders and developers who already have construction crews and want pre-engineered plans they can permit and build at scale.
• Architects and designers looking for a plan-and-permit foundation they can customize for specific clients.
• Homeowners with existing GC relationships who want California-compliant duplex plans without committing to our construction stack.

For a fully turnkey project (one fixed price including construction), our Signature Home lineup is the right starting point — though Signature Homes are single-unit ADUs, not duplexes.

Permitting a multifamily ADU project

Multifamily ADU applications are ministerial under Gov. Code § 66317 — no public hearing, no design review, no discretionary approval. The same building-permit-only pathway that applies to a single-family ADU applies to multifamily ADU projects.

The state-mandated timeline:

• Completeness review: 15 business days from submittal (post-SB 543, effective Jan 1, 2026).
• Approval or denial: 60 days from the date the application is deemed complete.
• Deemed approved: if the city misses the 60-day clock, the project is approved by operation of law.

What's different at multifamily scale:

• Utility coordination. Multifamily ADU projects often require water, power, and electric utility upgrades. Coordinated separately from the building permit clock.
• Fire sprinklers. Once the total dwelling-unit count on a parcel exceeds the threshold (typically 4+ units depending on local fire code), sprinklers may be required across all units.
• Common Interest Development entitlements. If the project intends to subdivide the units as condominiums (only possible in cities that have opted in to AB 1033), additional Davis-Stirling Act and condominium-map entitlements apply.

The Backyard Review for a multifamily project covers all three additional pathways. We sequence them in parallel with the building permit so the project clock doesn't stack.