What Happens After the Zoning Code Says "Yes"

Picture two identical buildings on a block. They are both the same height and the same size, they use the same facade materials and have the same roof pitch. You might even make the assumption that these two buildings have the same floor plan, and perhaps they could have. Instead, one of the buildings has two homes and the other one has three. The only difference noticeable from the street is the extra mailbox. Though these two residential buildings look the same, at the jurisdiction’s permitting office, they can be treated under two different building code classifications and follow very different permitting paths.

Many states in the past few years have been legalizing both of these building types and that’s a win for incremental development and missing middle housing. Interestingly enough, though, far fewer three-home projects are being permitted and built than two-home projects. Both of those buildings are legal to be built, so what’s the issue?

In our second housing toolkit, “Who Will Build the Housing-Ready City?”, we talk about how making housing legal is really just step one. Even after a city changes its rules, we’re still not seeing as many of these newly legal homes get built, and it’s usually not because anyone told the builders "no." The process just isn’t as viable or it can get confusing, or the financing doesn’t work out. So this is the other part of the process that needs to be talked about. What happens once zoning says a project can be built, and you actually have to design and permit it under the building code?

When a city legalizes triplexes, it changes its zoning code. Zoning decides what you are allowed to build on a lot, how tall it can be, how far back from the sidewalk it has to sit and what else can happen on the lot. Zoning, however, isn’t the only rulebook a builder goes through. Once a project is allowed, it has to go through the applicable building code, which decides how the building will actually get built. That's everything from how the frame holds up the floors to how people get out if there's a fire, down to details like how big a bedroom window has to be.  A city can rewrite its zoning in a few years of public meetings. The building code mostly comes from a national model code that states adopt, and in most places, the city doesn't get to change it on its own.

So the first building, holding two homes, generally falls under the International Residential Code (IRC), which is the same code used for single-family homes. The second building has three homes, which can put it under a different set of building-code requirements depending on how it’s designed and how the state or jurisdiction has adopted the code. In many cases, that means the International Building Code (IBC) comes into play, along with more detailed construction documents and, where required by the jurisdiction, a registered architect or engineer involved in preparing the plans.  

Before anyone breaks ground, that already changes how the project is approached. The IRC is more like a cookbook. It tells you how far a floor can span before it needs more support, how steep a staircase can be, how a wall needs to be built. A builder can look up the requirement, be able to follow it and keep going. It's why plenty of houses get built without an architect ever being involved. 

The IBC is different. You have to show more of how the building meets the requirements in the drawings. The walls and floors between each home have to be designed to hold back a fire. In some instances, accessibility requirements come into play, too. And in a lot of places, the drawings have to be signed and sealed by a licensed architect or engineer, which means hiring one even if the owner wasn't planning to. The permit can also take a different path, with more review and more time before you get an answer.

The difference may not sound like much on paper, but it can change who can realistically take on the project. A homeowner or small builder who could handle a two-home project under the residential code may now need to bring in additional professionals and spend more time figuring out what the building department is asking for. Again, that doesn't mean the building is unsafe or that these requirements aren't needed. When a small project crosses into a different set of requirements, it can add a lot more to the process even when the building itself hasn't changed that much. And if these are the kinds of small projects that add more housing, making them so much harder to build starts to work against the whole point. 

One quick clarification, because it matters for everything that follows. Not every triplex is reviewed under the IBC. Three homes built as townhouses side by side usually stay under the residential code while a stacked triplex may cross into the IBC. So whether those two buildings from the start of this story get treated the same really comes down to the state. Though the national model codes draw a line between one- and two-family residential buildings and other types of buildings, states can amend and adopt the codes differently. North Carolina is one of the few states that moved it, allowing triplexes and fourplexes can be built under the residential code across the whole state. So the three-unit cutoff isn’t necessarily set in stone. It depends on where the state draws the line. 

In 2020 Minneapolis made triplexes legal on every residential lot. In the two-and-a-half years that followed, 72 duplexes went up and only 37 triplexes were built. Looking at these numbers, we don't quite get the whole picture. Sure, that’s still 37 triplexes that didn’t exist before, though the fact that it’s about half the number of duplexes is telling. Over roughly the same time period, about 11,500 homes were built in larger apartment buildings. So it wasn’t that Minneapolis stopped building housing. The gap between 72 and 37 suggests there’s another part of the story we're missing. It doesn't tell us exactly why fewer triplexes were built, and there are probably a few things going on at the same time.

Part of that comes down to Minneapolis's own size limits, which left very little room for a third home on a typical lot. Even when that third home can physically fit, the numbers still have to work. In Minneapolis, adding that third home also changes the building code that applies. One- and two-family homes fall under the Minnesota Residential Code, based on the IRC. Once you get to three units, the project falls under the Minnesota Building Code, based on the IBC. 

How we measure what happens after legalizing different building types matters, too. If we look at the number of permits, how many of those buildings actually got completed? How many started as triplexes and ended up as duplexes because of the requirements that come with that third unit? How many ended up staying under the IRC threshold instead? That gap is worth paying attention to. It can tell us what else needs to be worked on after we've changed the zoning code.

Some states also give cities more room to make their own rules. Tennessee is one of them. In 2021, Memphis used that flexibility to allow up to six homes under its residential code. The state fire marshal pushed back, requiring sprinklers, and Memphis eventually moved those buildings back under the IBC. It gets a little technical, but it's worth understanding. Triplexes and fourplexes there can skip sprinklers, as long as there's a two-hour fire separation between the homes. In plain terms, that means the walls and floors between each home are built to hold back a fire for about two hours, which gives people time to get out and firefighters time to arrive. It's a different way of handling the same safety concern, and for a small building that difference will matter a lot.

Memphis also shows that there isn’t just one way to handle these requirements. If the walls and floors between homes can hold back a fire for about two hours, do you still need the same sprinkler system required in a larger building? It addressed the same fire-safety concern without requiring a sprinkler system, eliminating a significant cost for a small building. Those are the kinds of questions that get lost when the building code draws a hard line based on the number of homes. 

And to be fair, something real does change when you add that third home. More families, more kitchens, more people who don't know each other sleeping behind the same walls. Those walls should be built to hold. Nobody's arguing otherwise. The problem is that the code doesn't treat it like a step up. It treats it like a whole different kind of building. Three homes can put you into the same rulebook used for a 50-unit building, even though the buildings are nowhere near the same scale. 

We changed the zoning. Now we need to pay attention to what happens when someone actually tries to build what we said was allowed. The number of permits tells us what people tried to build. Finished buildings tell us what actually made it through the process.