Yes. An ADU is a dwelling unit with its own heating and cooling system, so it needs its own Manual J/Manual S sizing whether it's detached, above a garage, or a garage conversion - and because it's new conditioned space, it usually needs its own energy-code compliance too.
Short answer: yes. An accessory dwelling unit is a dwelling unit with its own heating and cooling equipment, so it needs its own Manual J/Manual S sizing under IRC M1401.3 - detached, above a garage, or converted from a garage, the code does not distinguish. And because an ADU is new conditioned space, it also needs its own energy-code compliance path: a REScheck where the IECC applies, or the state-specific equivalent where it doesn't.
IRC M1401.3 requires that "heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S... based on building loads calculated in accordance with ACCA Manual J." That requirement is not scoped by building type or by how the space is described on a permit application - it attaches to whatever equipment is heating or cooling the space. An ADU has its own thermostat, its own equipment (or its own zone on a shared system), and its own envelope, so the load on that equipment has to be calculated on its own. Borrowing a number from the main house's Manual J, or estimating tonnage from square footage, is not a documented sizing methodology and is one of the more common reasons an ADU mechanical permit stalls at review.
This holds regardless of ADU type. A detached backyard cottage is functionally a small new house from a mechanical-permitting standpoint. A unit built above a garage has its own envelope exposed on most or all sides. A garage conversion brings a previously unconditioned space into the thermal envelope for the first time. In every case, new or extended HVAC equipment is serving a space that did not have a documented load before, and M1401.3 applies the same way it would to new construction.
It depends on whether the ADU is its own building or an addition to the existing one - and either way, the answer is that the ADU's conditioned space gets its own compliance treatment, not a free pass on the main house's paperwork.
A detached ADU is a new building. The IECC's existing-buildings chapter (Chapter 5, which covers "alteration, repair, addition and change of occupancy of existing buildings and structures") does not apply to it, because there is no existing building being altered - the ADU is new construction, full stop. It runs through the same new-construction energy-code path as a small new house, with its own REScheck (or state equivalent) sized to its own floor area.
An attached ADU - one that shares a wall, floor, or ceiling with the main house, or a unit built onto the existing structure - is generally treated as an addition under 2021 IECC R502. Per the U.S. DOE's own REScheck guidance, additions are evaluated the same way as new construction, but only the new floor area gets modeled: "REScheck determines compliance for additions in the same manner as new construction. When entering an addition, only the new portions of the building need to be shown in the software." That means an attached ADU's REScheck covers the ADU's own walls, windows, ceiling, and floor - not the existing house's envelope, and not a re-run of the main house's original report. See our REScheck for additions guide for how that new-portions-only scoping works in more detail, and our Do I need a Manual J? guide for how the requirement applies across project types generally.
What does not happen, under either path: a single REScheck covering "the house plus the ADU" as one combined calculation, or an ADU skipping its own report because the main house already has one on file. The ADU's conditioned floor area needs its own accounting one way or the other.
A garage conversion does not get a discount for starting with four walls and a slab already in place. Under 2021 IECC R502.2, "any unconditioned or low-energy space that is altered to become conditioned space shall be required to be brought into full compliance with this code." That is a stricter standard than the ordinary addition path - it is not a trade-off calculation scoped down to whatever is convenient; it is full compliance, the same as new construction.
In practice that means the new insulation, air sealing, and windows going into a converted garage have to meet the same envelope sections a brand-new room would: R402.1 (thermal envelope requirements), R402.2 (assembly-specific prescriptive requirements for walls, foundations, roofs, and floors), R402.3.1 through R402.3.5 (fenestration), and R402.4 (air sealing) - the same set of sections the code applies to any new addition's envelope. The existing slab, the existing stud bays behind old garage-door tracks, none of that changes the target. If the garage doesn't currently have insulated walls or a rated ceiling (most don't), all of it is new work that has to hit code, not a partial upgrade from whatever was there.
On the mechanical side, the same M1401.3 logic applies: whatever heating and cooling equipment serves the converted space - extended ductwork from the main house's system, or new dedicated equipment - needs its load calculated and its equipment sized on its own, because the space did not have a heating or cooling load documented before the conversion.
California does not use the IECC for residential energy compliance. It runs its own code, Title 24, Part 6 (the Energy Code), and REScheck is not the accepted compliance document there. If your ADU project is in California, this site's REScheck service does not apply to your project's energy-code compliance - say so plainly and check with a party that works in Title 24 compliance for that piece.
The California Energy Commission's ADU FAQ for the current Energy Code cycle draws the same detached-versus-addition distinction the IECC does, using its own terms. A newly constructed, detached ADU is treated as a newly constructed building and has to meet the full new-construction Energy Code package. An attached ADU, and a conversion of unconditioned space (a garage, for instance) to conditioned space - whether attached or detached from the existing house - is treated as an addition and complies under the Energy Code's addition provisions. The same FAQ addresses mini-split and multi-split HVAC directly: those systems still have to meet the applicable mandatory and prescriptive equipment requirements, and in most cases trigger field verification and diagnostic testing (including refrigerant charge) by a certified rater before the unit can be signed off.
The practical takeaway for anyone building an ADU in California: the underlying logic - a detached unit is treated like new construction, an attached unit or a garage conversion is treated like an addition - is the same shape as the IECC approach described above. The forms, the software, and the verification requirements are different, and they are not something a REScheck certificate satisfies.
Ductless equipment does not exempt an ADU from equipment sizing. IRC M1401.3 requires the load calculation and equipment selection regardless of how the conditioned air gets distributed - it names Manual J for the load and Manual S for equipment selection, with no carve-out for mini-splits, multi-splits, or any other ductless system. What a ductless ADU does skip is Manual D, the duct design manual, because there is no duct system to design. A real ADU submittal checklist makes this distinction explicitly: it requires "Manual J, D, S" for HVAC energy compliance, then notes "if there is no ductwork - such as mini-split system - then a Manual D is not required," while Manual J and Manual S are still required. Manual D drops out with the ducts. Manual J and Manual S do not.
This matters because mini-split-only ADUs are common - a single ductless head or a small multi-zone system is often the simplest way to condition a small, well-insulated unit - and it's easy to assume a room that "doesn't have ductwork" also doesn't need a load calculation. It does. The room-by-room and whole-unit loads still drive which head size and how many zones the equipment needs, and an undersized or oversized ductless system has the same comfort and efficiency problems any mis-sized system does. For more on how ductless equipment specifically changes what a load calculation needs to account for, see our mini-split Manual J and permit requirements guide.
Washington runs its own residential energy code, the Washington State Energy Code (WSEC-R), with a credit-based prescriptive path instead of the IECC's UA trade-off - REScheck is not the accepted document for a Washington ADU's envelope compliance. Washington State energy code work runs through the sister product wsec.ai, built specifically for WSEC-R. Manual J is still prepared here for Washington jobs: the HVAC-sizing requirement under IRC M1401.3 is a mechanical-code provision, not an energy-code one, so it applies in Washington the same way it applies everywhere else, independent of which envelope compliance path the state uses.
For an ADU's Manual J, send the floor plan and elevations for the ADU itself, the window and door schedule, and whatever's known about the HVAC equipment (ductless heads, a small split system, or ducted equipment sizing and layout). For a REScheck, send the same plan set plus insulation values and, for a garage conversion, what's already in the walls and ceiling versus what's being added. If the ADU is attached, tell us that up front so the report is scoped to the addition's new floor area rather than modeled as a standalone building. If the project is in California or Washington, say so before ordering - see state-by-state for which compliance document applies where.
Yes. IRC M1401.3 requires equipment sizing from a Manual J load calculation for whatever is heating and cooling the ADU, whether it's detached, above a garage, or a garage conversion.
A detached ADU is a new building and gets its own report. An attached ADU is treated as an addition, and REScheck evaluates only the new ADU floor area - not the existing house, and not a shared report.
No easier requirements. Under 2021 IECC R502.2, converting unconditioned space to conditioned space requires full code compliance, the same envelope sections new construction uses. The existing slab and framing don't earn a credit.
Yes. California ADUs comply with Title 24 Part 6, not the IECC, so REScheck doesn't apply there. Detached ADUs are treated like new construction; attached ADUs and garage conversions are treated as additions under California's own Energy Code.
Yes. Ductless equipment skips Manual D (there's no duct system to design), but Manual J and Manual S still apply under M1401.3.
Washington's WSEC-R runs its own credit system instead of REScheck for the envelope side, handled through wsec.ai. Manual J HVAC sizing is still prepared here for Washington jobs.
Section numbers above are from the 2021 IECC and 2021 IRC; many jurisdictions have adopted other editions, and Washington, Oregon, and California use their own amended codes or entirely separate codes. Look up your state for the edition and compliance path that actually applies to your permit. Final authority rests with your building official.
Travis Smith, ICC-Certified Residential Energy Inspector / Plans Examiner. The same certification your building department's plan reviewer holds - working for you.