Where Should the HVAC Go in an ADU Built Over a Garage? (Attic Hot Roof vs. Garage Ceiling)

By Buz Artiano, BuildX | November 2026

Watch: Jessica Way ADU: Mid-Construction Walkthrough | BuildX Update

If your ADU plan has a two car garage underneath it, the drawings show you the bedroom, the kitchen, and the windows. But usually they're not going to show you the plan for where all the HVAC stuff has to go, but when you're building an ADU with the underneath garage, the HVAC routing is important for you to know. That's because there's no crawl space and no basement under the home the way there usually is, so none of the normal plans for that will work. On the project we're going to talk about, we had two options for where to put the equipment. It could either go up in the attic or down in the garage up against the ceiling. The second option sounds simpler and cheaper. What it costs you, and what our attic approach changed in the roof, tends to surface only after the plan is priced.

My name is Buz Artiano, founder of BuildX. We have built dozens of ADUs across Massachusetts, concentrated on the South Shore and Plymouth County, and we do all of the design, permitting, and construction. That means septic evaluation, lot coverage limits, and the state's by-right ADU rules get worked out before framing starts, and so does the placement of all your HVAC stuff. We build a lot of these units with the garage underneath. Our X7 model, the Gables, is just our X3 Haven plan that's been modified to have a 2 car garage under it.

On the ADU we built over on Jessica Way, we did a 900-square-foot unit with nine-foot ceilings over a two-car garage, and for that one we put the HVAC stuff in a conditioned attic. We did that for several reasons that are specific to that project. If you are reviewing a garage-under plan right now, you should know the pros and cons of a garage-underneath unit before you decide on anything.

Where Did the HVAC Go on Our ADU Over a Garage?

On Jessica Way, it went in a conditioned attic. The HVAC equipment went upstairs, into an attic we modified to be part of the temperature controlled part of the house. When we walked that job part way through, the two reasons for the equipment going in the attic were obvious. Putting the equipment in the garage, against the ceiling, made less sense than the attic because, as we said on site, "A, we'll lose ceiling height. B, you need to be fireproof."

Those were the two options for this particular project, but you might have other options depending on your property and your plan. A mechanical closet inside the living space, a different interior layout, or a ductless system all mean you have different options for where to put your HVAC stuff. On a detached ADU over a crawl space, the crawl space adds a whole extra set of options as well.

On this build, the garage ceiling placement meant there would be less head room and we'd have to fireproof walls. But putting it in the attic meant making changes to the roof, so everything has pros and cons. We decided to change the roof and put equipment in the attic. The rest of this article explains why and how it applies to your plan.

What Do I Give Up If the Equipment Hangs From the Garage Ceiling?

The first thing is the ceiling height. Equipment and the ductwork hang below the ceiling framing, and every bit of space they take up takes a chunk out of your usable area. That space is shared by vehicles, the garage door motor, and any overhead storage. Once the ducts are hung, you can never use that space again.

The second cost is the garage separation. The Massachusetts residential code follows the 2021 International Residential Code, except they made a few changes, which you can see at mass.gov. Under that code, a garage ceiling with habitable rooms above it needs at least 5/8-inch of this stuff called Type X gypsum board or something similar, and any duct in the garage or passing through the wall or ceiling has to have at least No. 26 gauge sheet steel with no openings into the garage.

That requirement is very specific: a specified separation layer and duct rule, which is a totally different thing than a fire-rated assembly. That separation is required on a garage-under ADU regardless of where you decide to put the equipment, because people live in the area above where the cars will be. With garage ceiling placement, the equipment and its ducts sit on the garage side of the separation, and the ducts to the rooms above have to pass up through it. Each run and penetration has to be built so the separation stays intact. So when someone says "you need to fireproof", that's all the work that goes into that.

What Is a Hot Roof, and Why Did We Pair One With Attic Equipment?

Most attics are vented. The insulation is on the attic floor, above the ceiling in the main part of the house, and the whole attic vents to the outside right up under the eaves. The ridge also lets fresh outside air come into the attic. That's what keeps the roof deck from getting too hot and trapping moisture, but it also means a vented attic stays about the same temperature as the outside. Equipment and ducts can go in a vented attic as long as everything is properly sealed and insulated. We did things a little differently on this job. We made the attic into a temperature controlled space.

A "hot roof" is how you make that happen. The insulation moves up to the underside of the roof, and there are no attic vents anymore. Builders call this an unvented roof assembly: the roof is the part that gets insulated and protects from the outside temperature changes. Which means the attic becomes part of the temperature controlled inside of the house. On site this is what we say: "So this will be a hot roof. So this will be an unvented assembly on the roof." And for the equipment: "The equipment will be upstairs. That'll be a conditioned area."

Here's the downside that nobody really talks about. A conditioned attic means making big changes to the roof as well as where we place the equipment. It changes where we put insulation and how the roof deals with moisture. When a roof deck has no airflow we have to design it specially so that warm, humid interior air doesn't create a condensation issue. If this is the kind of setup that your plan needs, make sure you see all that work listed on the quote before you agree to a plan.

We did the roof that way on the Jessica Way project because the benefits were more than the downsides. With the equipment in a conditioned attic, the supply goes directly to the rooms below, and nothing mechanical has to cross the separation ceiling between the garage and the HVAC stuff. The garage doesn't lose any space. The equipment and ductwork are also in the same place that they serve air to. So for this particular project, that combination was a win for them.

Service Access Through a Pull-Down Stair

The first question after attic placement is how will a repair man get up there in order to work on the unit when needed. In this case, we just put in a pull-down stair in the ceiling. When we were walking through the ADU we explained: "So this way here, it goes in the attic, conditioning the attic, and then we have this pull-down for easy accessibility."

The access point needs to be clearly shown on your plan drawings, that should all be settled before the work starts. When you are looking over a plan that has the HVAC stuff in the attic, be sure to find the access point and make a note of which room it opens into. Filter changes, service calls, and even a replacement unit will all come through that opening, so make sure you pick the right location.

Five Checks to Run on Your Garage-Under Plan Before It Is Locked

Here's a list of things you want to check on a builder's drawings for an ADU over a garage. They're all questions that can be answered during the drawing phase, so don't wait until construction starts.

• Find where the HVAC equipment is going to be placed: Will it be in the attic, garage ceiling, a closet, or is it a system that doesn't use duct work.
• If it goes in the attic, is the space going to be conditioned under a hot roof and will it be vented.
• Ask how the ducts are sealed and insulated if they run through the garage or an attic that isn't temperature controlled.
• Ask for the clear height under the lowest duct or unit if the equipment is on the garage ceiling.
• Confirm where the access point will be and which room it opens into.

The red flag is when a plan has the equipment in the attic, but it doesn't say which kind of attic it is (conditioned or vented). If the attic is vented and the drawings don't clearly show you how the ducts are sealed and insulated, ask the builder to explain all that to you before moving forward. If the attic is conditioned, ask to see that special hot roof assembly clearly marked on your quote.

The other thing you'll want to check is the garage ceiling itself. In an ADU where people are living above the garage, the drawings should show the special garage separation regardless of where you decide to put the HVAC equipment. If the equipment is going in the garage, the duct material should be shown on the plan and there should be no openings, so the separation is intentionally designed into the plan, not just figured out as they go.

Is a Conditioned Attic the Right Call for Every Garage-Under ADU?

We put the equipment in a conditioned attic on that project on Jessica Way, but that doesn't mean it's the right choice for every project. This approach has a real cost though, because the hot roof changes the whole roof assembly, and that takes time and money. We can't really give you an exact dollar amount for putting your HVAC in one place vs another, because it depends on the roof and the equipment you're going to get.

The garage ceiling can be built to code. It has all the separation and duct requirements we just mentioned, and whatever installation rules your specific equipment has in addition to that. Using a mechanical closet or a ductless system avoids the question altogether. If keeping the roof simple is important to you and some headroom in the garage really isn't, then the garage ceiling is a great option for your equipment. What matters is that you make the decision having all of the information and knowing all the pros and cons of the different placement options.

If your ADU has no garage underneath, then this doesn't apply to you. A slab or crawl-space unit has all different options.

Want to See Where the HVAC Belongs on Your Plan Before It Is Locked?

On a garage-under ADU, you need to decide where the HVAC equipment will go while you're still in the drawing phase. Settle it with the roof assembly, the garage separation, and the access point all clear on the drawing, and the price you approve will match what gets built.

We're glad to go through that decision with you on your own property and your own plan.