Title 24 for ADUs on a multifamily lot.
Add detached units to an apartment property and the energy report changes shape. A building of three or more stacked units is low-rise multifamily under the 2025 Energy Code — its own certificate, its own solar rule, and a ventilation requirement that is bouncing a lot of these sets at plan check. I model them in CBECC 2025 and the certificates come back registered, one per building, with you named as responsible designer.
Same parcel, a different report per building
Since January 2025, state ADU law lets a multifamily lot take up to eight detached ADUs (never more than the number of existing units on the property) plus conversion ADUs carved out of non-livable space inside the existing building — storage rooms, basements, garages — at least one, and up to a quarter of the existing unit count. The sets that come out of that rule look nothing like a backyard ADU: two, three or four new buildings on one parcel, four-plexes and triplexes mostly, sometimes with a conversion unit carved out of the existing building. Each building gets its own energy compliance document, and which document depends on the building, not the parcel.
A building of three or more dwelling units stacked or side by side under one roof is Occupancy Group R-2, which makes it low-rise multifamily under Part 6. It is modeled on the multifamily side of CBECC 2025 and the certificate is the LMCC-PRF, not the CF1R — registered with an ECC provider (CHEERS) one per building, with you as responsible designer and me as documentation author. The only three-plus-unit buildings that stay single-family are townhouse rows, where each unit runs foundation to roof with open space on two sides. A conversion unit inside the existing building is an addition or alteration to that building and gets its own report rather than a from-scratch new-building model. Smaller buildings on their own lot are covered under multifamily and small residential.
Three things that bounce these sets at plan check
Each of these comes straight out of the 2025 multifamily rules. Each is cheap to get right in the model and expensive to discover in a correction letter.
- Exhaust-only ventilation is out. The exhaust-only bath fan that satisfies a single-family ADU is no longer accepted for a multifamily dwelling unit under the 2025 code. Each unit needs a supply-only or balanced (ERV/HRV) whole-unit ventilation system, every unit in the building the same type, plus a compartmentalization leakage test by an ECC rater before sign-off. It is a mandatory measure, so there is nothing to trade it against, and the plans have to show it. In climate zones 1, 2, 4, 11–14 and 16 the prescriptive baseline is an ERV/HRV, so going supply-only there costs margin in the model.
- Every new building needs its own PV, and the ways out are narrow. A newly constructed low-rise multifamily building has to carry a PV system sized to the smaller of the code equation or the solar-access roof area, and that roof area counts carports and covered parking too. Extra panels cannot make up for an envelope or equipment shortfall — PV never counts toward the efficiency budget — so oversizing buys nothing at plan check. The exemptions are specific: the usable roof comes out under 80 contiguous square feet once north-facing steep roof and anything below 70 percent annual solar access (shading from trees, hills and adjacent buildings) is taken out; the required array works out under 4 kWdc; or the unit is an addition to the existing building, which carries no PV requirement at all. A battery cuts the required size by a quarter. On a recent ten-unit, four-building job in Los Angeles the four-plex needed fourteen panels to pass. Knowing which buildings need a roof array, and how big, before the roof plan is drawn is the whole point of running the model early. The single-family version of this question is at does my ADU need solar.
- The water heater baseline is a heat pump per unit. The 500-square-foot allowance for a point-of-use electric tankless is a single-family exception; the multifamily model has no equivalent. An all-electric multifamily building is measured against a heat-pump water heater in each unit (the standard design matches the proposed fuel), and a resistance tankless in a 400-square-foot unit loses to that baseline by a wide margin. The code lets a 120V heat pump stand in for the 240V unit in a one-bedroom-or-smaller dwelling, which is what these units usually are, and that is what passes.
Everything else is the same discipline as any other job: federal-minimum mini-splits, prescriptive windows, the roof and wall assemblies the plans already show, and the climate zone doing most of the work. If a building does not clear as drawn, the first lever is the real efficiency rating of the mini-split that is going in anyway, the second is the water heater tier — not thicker walls. Plan check comments come to me, and the first two revisions are free.
Pricing is per building, flat, quoted from the plans the same business day — a triplex and a three-story eight-plex are not the same job, so these are not priced sight-unseen. The number on the quote is the number on the invoice, and it is the same number on the next parcel.
Prices
Flat fees, in writing before I start. No hourly, no bid package, no change-order games — the number on the quote is the number on the invoice.
One honest note
I’m not a licensed engineer or architect, and I don’t stamp drawings. On the CF1R I’m the documentation author — you stay the responsible designer. Nothing about your role on the job changes.
That’s on purpose. I’m not after your scope or your clients. I do one piece of the permit set — the piece that usually holds it up — and I do it fast.
Multifamily ADU questions
If yours isn’t here, see the full FAQ or call and ask.
Is an ADU on an apartment lot single-family or multifamily under Title 24?
It goes by the building, not the zoning. A building with three or more dwelling units under one roof is Occupancy Group R-2 and gets a low-rise multifamily LMCC certificate; only townhouse-style rows stay single-family. A conversion unit inside the existing building is an addition or alteration to that building and gets its own report. One parcel can carry both certificate types.
Does every ADU building need its own solar?
Every newly constructed building does, sized on its own. The exemptions are specific: the solar-access roof area comes out under 80 contiguous square feet after shading and north-facing steep roof are excluded, the required array works out under 4 kWdc, or the unit is an addition to the existing building. Extra panels cannot cover an efficiency shortfall, and a battery cuts the required size by a quarter. The model tells you which buildings need a roof array and how large before the roof plan is final.
What is an LMCC, and how is it different from a CF1R?
LMCC-PRF is the Low-rise Multifamily Certificate of Compliance on the performance path — the multifamily counterpart to the CF1R. Same energy model, same CHEERS registration, same place in the plan set; one certificate per building, and you stay the responsible designer on each.
Can a bathroom exhaust fan cover ventilation in a multifamily ADU?
Not under the 2025 code. Multifamily dwelling units need a supply-only or balanced (ERV/HRV) whole-unit ventilation system, the same type in every unit, plus a compartmentalization leakage test by an ECC rater. It is a mandatory measure, so the fix is on the plans, not in the model.
How do you price a multi-building ADU project?
Per building, flat, from the plans, with the quote back the same business day. Each building is its own model and its own registration. The first two rounds of plan-check revisions are included on every one, and the rate does not move on the next project.
Does the set have to come from the architect, or can the owner or builder send it?
Either. The responsible designer named on the certificate has to be whoever is legally eligible to take responsibility for the design — on a multi-building parcel that is usually a licensed architect or engineer — and I register it under that name. I am not a licensed engineer or architect and I do not stamp drawings; I produce the energy compliance package only, which is why the designer stays in charge of the design.
Send me the whole parcel.
Drop the full set in the form — every building, the site plan with its north arrow, and whatever mechanical is called out. A flat per-building quote comes back the same business day, and each certificate comes back registered.