Which Direction Should My ADU Face for Solar Panels, South or East-west?

By Buz Artiano, BuildX | September 2026

Watch: How Solar Really Works for Builders, Homeowners, and ADUs | BuildX Podcast #35

You have picked a spot in the backyard, you have a rough idea of how big you want the ADU, but during the conversation someone asks about solar. Now you're wondering if the building has to face a certain way for solar panels to be worth installing, and whether the shape of your lot gives you any choice in the matter. Most homeowners just assume they have no choice, the house goes where the house goes, and solar either works there or it doesn't.

My name is Buz Artiano, founder of BuildX. We've built dozens of ADUs across Massachusetts, but most of our jobs have been on the South Shore and in Plymouth County. We handle everything for you, that means design, permitting, and construction. So we're the ones setting the building on the lot and making sure it works with septic placement, lot coverage, and the setback lines. Roof direction isn't random, it's something we specifically draw in on a site plan.

Here is what we want you to understand after reading this article. On an existing house, roof orientation is already settled and you have to work with what's already there. On an ADU that hasn't been built yet, roof direction and roof pitch can be whatever you want it to be, you just have to decide that before the foundation goes in. That's kind of a short window, and most families end up talking about things like countertops instead.

Does My ADU Have to Face South for Solar to Work?

No. South-facing is the default answer because it's technically where you get the most sun, and the reason for that is that in the northern hemisphere the sun's daily path runs across the southern sky, so a south-facing roof collects the most sunlight through the middle of the day. We're not arguing that fact.

The part that people don't think about is that a south-facing roof can only have panels put on that one plane. A plane is a single face of a roof, so a basic gable gives you two of them and they point in opposite directions. Every panel on a south-facing design has to fit on that one face of the roof. If the ADU sits east-west instead, both sides of the roof get plenty of sun, one in the morning and the other side in the afternoon. Neither one of them gets as much exposure as they would if they were facing south. So you're basically getting less sun, but twice the exposure with east and west facing panels. With the southern exposure, it's more intense sun in a smaller area.

For a family who's trying to decide which way to face a roof that doesn't exist yet, that's the entire question. The answer isn't necessarily to point it south or just go without solar. You have two workable options, and the right one depends on what else your lot is asking the building to do.

South-facing Versus East-west: What Each Roof Actually Harvests

Chris Johnson, Director of Business Development at Isaacson Solar, joined us on the BuildX Podcast and explained things to us in an easy way to understand. He said that you get about 80 percent sun capture on each of the two east-west faces of the roof, and about 99 percent on just one south face. Those are the numbers he got from a working solar professional, they're not measured from an official study, and the numbers for your property can vary a little depending on the design. What these numbers are useful for is demonstrating that two faces at about 80% collect more sun than one face at 99%.

The South-facing Roof

Doing it this way makes one set of panels on the south face of the roof do all the work. It collects sun through the strongest hours of the day, and because all the panels are in one area, the layout is simple and the wiring runs are short and easy.

It's the right choice when the lot gives you a good southern exposure with no tall trees on that side, and when the ADU can sit in full sun without getting in the way of setbacks or your septic. On those properties, trying to add panels to another side of the roof would be pointless.

The East-west Roof

The panels are divided up across two sides of the roof and each side captures less than a southern face would. But the building is collecting sun from sunrise through sunset rather than concentrating everything around midday, and the total surface area where you can put panels is doubled.

It's the right decision when the ADU has no choice but face east and west, which on narrow lots is pretty common. It's also the right choice when a shallower pitch is acceptable. On those properties the second side of the roof is the only reason that solar will work at all.

Here's a chart to make it easier. The capture percentages are attributed to Isaacson Solar and describe the general relationship, not a measurement taken on your roof.

What Roof Pitch Should I Ask For?

Pitch is the slope of a roof, it's measured in inches of vertical rise for every 12 inches of roof. An eight pitch rises 8 inches over 12 inches of roof. Builders talk in those terms and solar designers talk about the same thing in degrees, and it all has to be translated in order for you to have a helpful conversation about it.

Chris said to be the most productive the roof should have a 30 to 36 degree pitch on average. In builder terms that's called an eight pitch. An 8 over 12 roof is 33.7 degrees, which is right in the middle of that ideal range. So when a solar designer asks for something in the low thirties, they are asking for a roof that builders in Massachusetts build all the time.

Pitch also tells you whether the second side of the roof is worth putting panels on. The shallower the roof, the more sun reaches across the ridge and onto the far side, which is what makes the east-west setup work in the first place. If you make the pitch steeper, that far side spends more of the day in its own shadow. Make it flatter and direction of the roof stops being as important, because a very low-pitch roof can be turned nearly any direction and get almost the same amount of sun. The panels are almost laying flat at that point, and the direction of the roof doesn't matter so much anymore.

How Far Can I Rotate the ADU on My Lot?

Rotation is the thing most families don't realize they have control over. If turning the ADU 15 or 20 degrees increases the solar coverage, we need to talk about that early in the process. Not after construction is underway.

We decide where the ADU will sit on the lot before anything else gets decided, because rotation is the one thing that affects setbacks and everything else. Turning the ADU 15 or 20 degrees brings the corners closer to the setback lines, changes where the septic tie-in runs and how long that run has to be, shifts the roof relative to the main house and the neighbors, and moves the front door. On one lot, rotating it could be free. On another lot it can require a variance, which costs time and money. That is why the direction the home faces needs to be decided on the site plan, not after the foundation is poured. That's why we ask about panels during design even when a family has not decided whether they want them yet.

The law is the underlying thing here though. Massachusetts permits one ADU by right on single-family lots under the Affordable Homes Act, but that doesn't mean there are no restrictions. Setbacks, lot coverage, height limits, and building code still apply, and those are what you have to adhere to when you try and rotate the position on your ADU. You can review the state requirements at mass.gov.

Architecture is the other part of all this. A building rotated just to accommodate the best solar yield can end up with a blank gable facing the street or a bedroom window that faces the neighbor's kitchen. We evaluate all of these things together, the city rules, the solar geometry, and how the finished ADU looks from the driveway, before a single drawing gets finalized.

What Do the Trees on My Lot Do to the Math?

Orientation is only half of it. The other half is whatever is between your roof and the sun. A solar designer runs your plot plan and your design through shading software that produces a heat map. A heat map is a color-coded map that shows you where sunlight actually lands on the roof once you figure in the trees. That map is what tells you how many panels fit and what offset the setup gives you. Offset is how much of the electric bill it covers.

The way we get the image for the map matters a lot here. A lot of times shading software is just satellite imagery that could be four years old. The data Isaacson uses is called flyover data, and it's been as recent as three days prior to the evaluation. On an active construction site, you have to have recent data that's been used to create those maps.

Trees are where it can start to cost money to get the most out of your solar panels. Getting better sun exposure can mean taking down a lot of trees, and clearing trees is something we price out during site evaluation. Chris makes a point we agree with: when trees come down, put trees back, and choose a type of tree that won't grow tall enough to cast shade on your panels in fifteen years. You shouldn't have to clear trees for your roof twice.

When Rotating for Solar Is the Wrong Move

We design and build ADUs with solar-ready roof setups, so maybe you think that the most important thing is the orientation of the roof. That's not actually the case, and there are three situations where we say so before a family spends money chasing it.

Your lot allows exactly one workable layout. Between setbacks, septic, driveway access, and the main house, some properties only have room for the home to face a very specific direction. On those lots the ADU has to go where it fits and the solar panel setup has to use whatever the roof allows.

Your ADU is an addition or a conversion. When the roof already exists, you have no play in the direction or pitch. As Chris puts it, with an existing house you get what you get. For these, you just have to get creative with the design, there's nothing you can do about the placement of the building.

The rotation costs more than the production returns. If you rotate the ADU twenty degrees, and that forces you to need a variance, or the septic lines redone, or a foundation on worse grade, that can cost more than the solar panels will produce.

Solar panel layout is one thing that depends on where a building sits, but it's not the deciding factor on whether you should install a system or not. If the only way to get anything out of the solar panels is to fight the conditions of your own lot, the honest answer is to place the ADU where it works the best and if you decide to install solar panels, just work with what that placement allows.

What Should You Settle Before the Foundation Goes In?

Roof direction and roof pitch are decisions that have to be made by a certain point in the process. While the ADU is still just a drawing, both are free to change. Once the foundation is in the ground, the roof direction is set, and the only question left is how many panels fit on it.

So if solar is on your list at all, even as something you are considering three years out, you need to bring it up during design. The building can be rotated, the pitch can be changed, and the shading can be mapped while every one of those things are still free to change.