Top 3 Ductless HVAC Design for ADUs (Accessory Dwelling Units) – d-airconditioning
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Top 3 Ductless HVAC Design for ADUs (Accessory Dwelling Units)

Accessory dwelling units – ADUs! They’re the perfect solution for expanding your residential living space on your existing property (usually, the back yard). Whether you call it a ‘multi-generational space,’ a coach house, a casita, a cabana, or even a home office or studio – an ADU is a smart way to get more functional interior square footage out of your current home.

Whether you’re building from-scratch – or upgrading an existing ADU – one thing you’ll certainly want to keep in mind is: comfort! And when it comes to heating and air conditioning, nothing matches the comfort and value of a ductless mini-split system installed by D-Air Conditioning. After all, ductless mini-splits are THE affordable, energy-efficient way to heat and cool your ADU! 

So…what are your options?

Let’s go over two common floorplan designs for ADUs – and take a look at the different options that you have for heating and cooling. Let’s start with two pre-approved ADU plans from San Diego County. (Since most ADUs are around eight-hundred square feet, your ADU is probably very similar to these designs.) We’ll take a look at general layouts, options, and our recommendations – based on years of experience as California’s leading provider and installer of ductless mini-split systems. By the end of this brief video, you’ll be able to use D-Air’s proven methods – for determining the right HVAC system for your ADU!


For most ADUs, we recommend using ductless mini-splits. (D-Air does NOT recommend a ‘traditional central air conditioner’ for an ADU – because ductwork is expensive to run – and it takes up valuable space in the attic.) And think about this: most ADU designs incorporate a floor-to-ceiling height that is as tall as possible – to give the space the illusion of being much bigger than just eight-hundred square feet. Obviously, the more attic space you take up, the more money you’ll need to pay – for materials and space that…well, frankly…you just can’t use! We see it all the time: people building ADUs don’t stop to consider the extra materials and labor needed to build attic spaces for their HVAC system – (which is considerably even more expensive than the HVAC system, itself)!

OK. So…let’s take a look at our FIRST FLOORPLAN. 

Here, we see a two-bedroom, two-bath ADU with a living room and kitchen. Technically, we only need to have a heating source in the living room – so installing just one unit in the living room is the least expensive option. Based on the design and layout, here are three different HVAC options:

  • One eighteen-thousand BTU mini-split in the living room. (Either high-wall unit or ceiling cassette.)
  • A three-zone high wall mini-split system (9+9+12.)
  • A three-zone mini-split system (9+9+12). For the bedrooms we can use high-wall units – and for the living room, we can use a ceiling cassette.

This FIRST OPTION is the least expensive. It typically runs between three-thousand and six-thousand dollars – depending on the exact system that you choose. Here’s how it works: you’ll have just one, larger unit located in the living room – and since the space is quite small – you can open up the doors in the bedrooms and still get cool air – as well as heat – inside the rooms. But…the one downside of this approach? It’s pretty limited when it comes to privacy – so it’s not ideal if the ADU will be used by more than one person.

OPTION TWO is the most common option. Here, we recommend a three-zone mini-split system. Typically, we would use either Daikin, Fujitsu, or Mitsubishi – as they make some of the most reliable products on the market. In 2022, this cost should run between about ten-thousand and thirteen-thousand dollars.

Finally, the THIRD OPTION is a combination of high-wall and ceiling cassette units. Most people will only have guests in their living room area – and they’ll want to make that space as beautiful as possible. As a result, many of our customers choose to have a ceiling cassette in the living room and high-wall units in each of the bedrooms. This option will range between eleven-thousand and fifteen-thousand dollars – depending on the brand that you choose.

OK. That was the FIRST FLOORPLAN. Now, we’ll take a quick look at the SECOND FLOORPLAN.

For this one, you’ll notice that although the layout is different from the first one, you still have the same three options for your mini-splits. (The layout doesn’t really affect your choices that much – since we are mostly concerned about the size of each room – and the zones for each mini-split system.)

So, those are some possible floorplans. Now…let’s talk about your personal, customized ADU project. Here are a few things to consider – ahead of time:

First, consider setbacks! 

What are setbacks? Well, setbacks determine exactly where you can place your unit’s condenser. Every city has its own setback requirements – so be sure to check them before installing your air conditioner. (Of course, at D-Air, we’re experts when it comes to installation. We handle all of the details…in detail – including compliance with your city’s setback rules.)

Next, consider the right phase of when to bring in the HVAC contractor. 

For new construction projects, you should involve the HVAC contractor as soon as framing and roofing is completed. Your major trades: mechanical, electrical, and plumbing should do their work inside the walls and ceiling before you close-up the drywall. (This phase is called rough-in.) You should involve the H-VAC contractor as early in the project as possible – since there may be lead time in acquiring the equipment and materials needed for the installation. 

And remember, your HVAC contractor will work in two phases: 

In the rough-end phase, they’ll install the indoor units, run the copper line-sets, and run the electrical and water lines. And after you have completed the exterior stucco or other surface treatment, your HVAC contractor will then install the outdoor condenser unit – and start up your new system!

A word about communication?

Hey, it’s incredibly important to communicate – on a regular basis – with your mechanical, electrical, and plumbing trades – so that there’s a smooth transition between the work phases – and so that your project can move forward, as fast as possible! 

Alright…almost done…let’s wrap things up. As we’ve seen, most two-bedroom ADU projects generally have three options for mini-splits:

  • One eighteen-thousand BTU mini-split in the living room. (Either high-wall unit – or – ceiling cassette.) This option runs about three-thousand to six-thousand dollars.
  • A three-zone high-wall mini split system (9+9+12.) This solution runs from about eleven-thousand dollars to about fifteen-thousand dollars.
  • Or, a three-zone mini-split system (9+9+12). For the bedrooms, we recommend using high-wall units – and for the living room, we recommend a ceiling cassette. Total cost? From about twelve-thousand dollars to about sixteen-thousand dollars.

Oh…and for those of you considering a garage conversion for your ADU (a popular choice) – here’s our most-common mini-split configuration for that:


Most garage conversions have around four-hundred square feet of space. In these cases, we recommend installing a twelve-thousand BTU high-wall mini-split. The cost of these systems is affordable – ranging from twenty-five hundred dollars to four-thousand dollars, including installation. 

Overall, a garage conversion typically leads to the creation of what most people would call: ‘a large studio.’ Since the space is open, you’ll only need one mini-split unit for the entire space. And if you have an electrical service panel nearby, you can use a two-hundred-twenty-volt system. Otherwise, a one-hundred-fifteen-volt system from Fujitsu or Mitsubishi will also work very well for your garage-conversion space.

And that’s it! You are now a “ductless mini-split ‘expert!’” (Or, at least you’re on your way to becoming one!) If you’re in California and you’re considering building an ADU or converting an existing garage into additional, functional interior space – give us a call at (714) 530-3305 – and let’s talk about a reliable, affordable, energy-saving mini-split installation for your project. 

At D-Air Conditioning, we’re the mini-split experts – offering the top-twelve best brands on the market. Stop by our showrooms in San Jose, California and Orange County. Our team of ductless mini-split pros is ready to help you through any project.

From all of us at D-Air Conditioning: “Thanks for listening!”(Now…go build that perfect ADU!)

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