Short answer: almost certainly, if you're pulling a residential building or mechanical permit. Here's when and why.
If you're pulling a permit and someone mentioned "Manual J," the short answer is: if the project involves installing, replacing, or sizing HVAC equipment, you almost certainly need one. The code requirement is specific — IRC Section M1401.3 requires heating and cooling equipment to be sized in accordance with ACCA Manual S, based on loads calculated in accordance with ACCA Manual J or another approved methodology. IECC Section R403.7 repeats the same requirement as a mandatory provision under the energy code. Below is how that plays out project type by project type, because "do I need one" depends heavily on what you're actually building.
Yes — new single-family and two-family home construction requires a Manual J load calculation to size the HVAC system, essentially without exception. This is the clearest-cut case: IRC M1401.3 requires equipment sizing based on Manual J (or an equivalent approved methodology) with no carve-out for new construction. Every jurisdiction that has adopted the IRC or IECC mechanical/energy provisions will expect a Manual J (and typically a Manual S equipment selection) as part of the mechanical permit package for a new house. Building departments increasingly want this at time of permit application, not after equipment is already purchased — rule-of-thumb sizing (the old "1 ton per 400-600 square feet" approach) is not a code-recognized methodology and gets bounced on review.
In most jurisdictions, yes, if the addition adds to or modifies the HVAC system serving that space — either a new load calculation for the added square footage or a revised whole-house calculation, depending on how the addition is served. If you're extending the existing ductwork and equipment into new conditioned space, the building department typically wants to confirm the existing equipment still has adequate capacity, or that a right-sized load calc was done for the addition specifically. If the addition gets its own dedicated system (a mini-split for a new sunroom, for example), that new equipment needs its own Manual J-based sizing just like new construction would. Note this is separate from the REScheck envelope question — an addition often needs both a Manual J for the mechanical side and a REScheck for the new envelope area; see our REScheck for additions guide for how the envelope side works, and our REScheck vs. Manual J vs. COMcheck guide for the difference between the two documents generally.
Yes — an ADU with its own HVAC system is functionally new construction from a mechanical-permitting standpoint and needs its own Manual J. Because an ADU typically has its own conditioned envelope, its own equipment, and its own permit, jurisdictions generally treat it the same as a small new home: equipment sizing has to be justified with a load calculation under the same M1401.3 / R403.7 requirements, not borrowed or estimated from the main house's system. This applies whether the ADU is a detached structure, a garage conversion, or an addition-style unit attached to the primary residence.
Yes, if the converted space will be heated or cooled — a garage conversion to living space is being brought into the conditioned envelope for the first time, and any new or extended HVAC serving it needs sizing under the same code provisions as new construction. This is also one of the project types the DOE explicitly recognizes as commonly requiring a REScheck as well, since you're adding conditioned floor area and, usually, new insulation and window assemblies where garage doors and unfinished walls used to be. Expect both documents to come up in review: REScheck for the new envelope, Manual J for whatever heating/cooling serves the new space.
Yes, in most jurisdictions — replacing a furnace, air conditioner, or heat pump requires equipment sizing per IRC M1401.3, and "the old unit was this size" is not, on its own, an accepted sizing methodology, though some jurisdictions apply this less strictly on direct replacements than on new work. The code language doesn't carve out an exception for replacement equipment matching the old tonnage — the equipment still has to be selected per Manual S based on a Manual J load calculation. In practice, enforcement varies: some AHJs will accept a straightforward like-for-like swap with minimal documentation, while others require the full load calc regardless — check with your specific building department, since this is one of the areas where local amendment and enforcement practice diverges most from the base code language. Given that variability, and the fact that houses often get more efficient windows, insulation, or air sealing between the original install and a replacement — which changes the actual load — running a current Manual J on a replacement is the safer and often code-required move rather than assuming the old tonnage is still correct.
Yes — heat pump installations and gas-to-heat-pump conversions require Manual J-based sizing under the same M1401.3 / R403.7 provisions as any other new or replacement HVAC equipment, and there's an added wrinkle: heat pumps have a heating capacity that drops as outdoor temperature drops, so undersizing (or oversizing without checking the balance point) has real comfort consequences that a straight AC or furnace swap doesn't. Manual S includes provisions for evaluating equipment against manufacturer's published capacity curves, including guidance for variable-speed and multistage equipment where a slightly different sizing approach applies. If you're switching fuel types or adding a heat pump as a supplemental or primary system, the load calculation needs to reflect the equipment you're actually installing, not just carry over the number from the old system.
Not automatically — it depends on whether that calculator is on ACCA's approved software list, and many manufacturer-branded tools are not. Some manufacturer sizing tools (see our guide on why free Manual Js get rejected for specifics) are built for equipment selection or marketing convenience rather than code-compliant load calculation, and building departments frequently reject them even though they produce a number that looks like a Manual J output. Ask specifically whether the tool used is ACCA Manual J-approved before assuming the paperwork is already done.
Your permit gets held until you provide one — this is one of the most common reasons a mechanical or building permit sits stalled, and it's entirely avoidable by getting the calculation done up front instead of after a plan review rejection. Skipping it doesn't save time; it just moves the delay to later in the process, usually after equipment has already been quoted or ordered based on a guess.
Travis Smith, ICC-Certified Residential Energy Inspector / Plans Examiner
Travis reviews these exact permit packages for a living. The Plans Examiner produces a $249 Manual J report in ACCA-approved Cool Calc software, matched to your submitted plans — accepted by the building department or we fix it free. See our Manual J page.
ICC-certified plans examiner — private review for your permit package. Travis Smith — ICC-Certified Residential Energy Inspector / Plans Examiner · E&O insured · general contracting background in Washington. The person who reviews your plans at the building department is a plans examiner. I hold the same ICC certification — except I work for you.