Multifamily ADU Plans
two to four units, one building.
If your property already has two or more attached units, California law entitles you to add that same number of detached ADUs — and they may share a single structure. Permit-ready plans for duplex, triplex and fourplex buildings, at a fixed plan fee.
Four configurations, two families.
Every plan below is a single detached building containing multiple ADUs — not a cluster of separate cottages. Which one you can build is decided by your height allowance and your buildable width, in that order. Green clears the 16-foot base allowance every qualifying property gets; amber needs 18 feet, which is conditional.
Duplex — two units
Most popular Side-by-Side Duplex
16 ftTwo mirrored units under one roof, each with its own entrance.
Plans $9,490 · est. build from $265,000
Stacked Duplex
18–20 ftHalf the footprint of side-by-side, on a lot that cannot spare the width.
Pricing on request
Multi-unit — three and four units
Triplex
16–20 ftThree units in a bar or stacked, depending on what your height allows.
Pricing on request
Fourplex
16–20 ftThe strongest per-square-foot economics in the line — with a real code caveat.
Pricing on request
Already looking specifically at a duplex? Our duplex ADU page covers both duplex configurations side by side. For the rules behind all of it — unit counts, height tiers, setbacks and fees — see the multifamily ADU guide.
The test is your structure, not your zoning.
State ADU law keys off what is already standing on your lot. If it is one building with two or more attached units, you qualify — even in a single-family zone. Your existing unit count then sets how many detached ADUs you are entitled to, and one more unit can be converted from non-livable space inside the existing building.
Check my property$490 · credited toward design
Detached counts per Gov. Code § 66323(a)(4)(A)(ii), as amended by SB 1211. Conversion per § 66323(a)(3)(B).
Four ADUs does not mean four cottages.
This is the question that decides whether a multifamily ADU project is worth doing. If you are entitled to four detached ADUs, do you have to build four separate freestanding structures scattered across your rear yard? Four foundations, four roofs, four utility runs, each clearing setbacks independently — on a typical urban lot that is often physically impossible and almost always financially unattractive.
You do not. The statute counts units, not structures, and HCD has answered the question directly in its ADU Handbook: detached ADUs on multifamily lots “do not have to be detached from each other or other accessory structures on the lot.” The building has to be detached from your existing apartment building. It does not have to be detached from itself.
That single sentence is what makes every plan on this page legal, and it is why we design this category as multi-unit buildings. The full citation and the plan-check note are on the rules guide — worth reading before you submit, because most cities have published nothing either way and you should expect to hand the plan checker the Handbook page.
Four steps, in this order.
Owners tend to start with unit mix and finishes. The binding constraints come first — and getting the order wrong is the most common way a multifamily project stalls.
Entitlement & height determination
We confirm your building qualifies as a multifamily dwelling, how many detached and conversion units you are entitled to, and your height allowance — 16, 18 or 20 feet — with the transit determination documented. That number decides which plans remain available to you.
Envelope, overlays and utility capacity
Rear-yard dimensions minus four-foot setbacks and building-code fire separation. Overlay screening — Coastal, HPOZ, fire, hillside — plus RSO exposure, and a utility review against the added unit count. On multi-unit work, service capacity is routinely the largest unbudgeted line item.
Plan selection and unit sizing
You choose from the configurations your property actually supports, with per-unit square footage set deliberately against the 750 sqft impact-fee threshold — which applies per ADU, so on a four-unit project it is four separate exemptions to protect or forfeit.
Permit set and submittal
Architectural plans, structural engineering and Title 24, plus entitlement documentation citing § 66323(a)(4) and the HCD Handbook. Then a submittal built to survive the completeness check — the 60-day ministerial clock does not start until the application is genuinely complete.
What the plan fee includes.
- Floor plans and elevations designed in-house by expert ADU designers
- Structural engineering and Title 24 energy compliance coordination
- Your interior and exterior finish selections, specified in the plans
- Entitlement documentation citing § 66323(a)(4) and the HCD Handbook
- Done-for-you permit processing and expediting with your city
- Unlimited plan-check corrections until your permit is issued
- A builder-ready set any licensed contractor can price and build
Multifamily ADU plans are designed in-house by our ADU design team and carry the same curated finishes as our detached Signature Homes. Your plan fee is fixed. Because multi-unit projects vary more than single ADUs — utility capacity, site access, grading, and the condition of your existing building — the cost to build is an estimate that starts at the figure shown and is confirmed for your specific property; it is not a single guaranteed price. Construction is performed by an independent licensed general contractor.
Questions owners ask.
Who can build a multifamily ADU?
Can multiple ADUs share one detached building?
Which plan can I actually build?
Do I need to provide parking?
Why is construction quoted separately from the plans?
How long does permitting take?
Regulatory references reflect California Government Code §§ 66310–66342 as amended through 2026. Local ordinances vary and are frequently behind state law. Entitlement is always confirmed against your specific parcel before design begins — see the multifamily ADU guide for full detail and citations.
Browse the rest of the lineup.
Multifamily plans need an existing building with two or more attached units. If that is not your property, our detached Signature Homes, garage conversions and custom design-build are the routes that fit.
Two units become five. Find out what your lot supports.
A written determination of your entitlement, height allowance, buildable envelope, and which configurations actually fit.
Designed in-house · Transparent pricing · Your choice of builder