Buildings must be built to the requirements of the building code, a set of laws that governs how a building must be built. Because building codes ultimately govern the form the built environment takes, it's useful to understand how they work, and how they evolve.

Authorities Having Jurisdiction (AHJs)

Like most laws in the US, the authority for regulating building standards exists at a variety of levels, from federal all the way down to county or city requirements. The government or organization that’s responsible for the building code in a given area is called the Authority Having Jurisdiction (AHJ.)

Most building code adoption in the US is handled at the state level, where each state will have its own building code that applies statewide. Some states, however, have no statewide building code (or only have statewide codes for certain portions of the building, such as septic requirements), and building code adoption is left entirely to local jurisdictions (cities, counties, townships, etc.) In some cases a jurisdiction may not decide to adopt a building code at all, (though this is uncommon, and is typically limited to rural, unincorporated areas.)

For states that do have a statewide building code, in many cases local jurisdictions will add their own code requirements. The City of Decatur, for instance, has its own set of high performance building requirements that goes beyond the state building code. In most cases, the state code is a minimum requirement, but a few states allow local jurisdictions to reduce state level requirements.

Many states have a variety of carve-outs specifically for single family homes. Kentucky and Vermont have state building codes, but not for 1 or 2 family dwellings. Montana doesn’t require jurisdictions to adopt single family home building codes. Maine allows towns of 4000 or less people to opt out of having a 1 or 2 family dwelling code.

Because there are thousands of local governments across the US, there are thousands (an estimated 20,000+) of permitting jurisdictions, and thousands of sets of building requirements. And beyond states and local governments, there are a variety of other jurisdictions that might oversee building requirements. A few examples:

Manufactured homes are built to HUD safety standards, not any local code requirements (though they must still typically meet local zoning requirements.)
The Department of Defense has its own set of building standards.
Indian reservations may have their own building codes.
Disney’s Reedy Creek Improvement District (RIP) has its own set of building codes.

AHJs today typically don’t do the work of creating their entire building code themselves (though this is less true historically - New York City authored its own building code up until 2014, for instance.) They will instead typically adopt a model code (or a series of model codes) written by some code authoring organization, and make changes (sometimes major, sometimes minor) as they see fit. For instance, I live in Georgia, which has adopted a variety of model codes published by the ICC and the NFPA, and made some state-specific changes.

Like any set of laws, building codes are subject to lobbying from groups who think a given provision will help or harm their interests. The National Association of Homebuilders (NAHB) for instance, has lobbied extensively (and successfully) to have states remove fire sprinkler requirements for their codes for single family homes. Plumbers unions have often advocated for the removal of code provisions that would allow for the use of PVC pipe instead of copper (ostensibly for safety reasons, though most folks think this is a protectionist measure, since the use of PVC, which is easier and cheaper to install, would mean less work for plumbers.)

Model codes

Most building codes in the US are originally authored by some model code organization. These are private organizations (though they often have government code officials as members) that convene committees of experts to author building codes and standards designed to be adopted into law by AHJs. Some major model code organizations in the US are:

International Code Council (ICC), which creates probably the most widely used model codes in the US - the ICCs International Building Code (IBC), which covers general requirements for most buildings outside of single family homes, has been adopted by nearly every jurisdiction in the US, as has their International Residential Code (IRC) for single family home requirements. The ICC also has model codes for a variety of other building requirements, such as mechanical systems, plumbing, and energy conservation. The ICC was formed in the 1990s when 3 previous model code organizations merged.
International Association of Plumbing and Mechanical Officials (IAPMO), which creates the Uniform Mechanical Code and Uniform Plumbing Code model codes (which compete with the ICC’s mechanical and plumbing code.)
National Fire Protection Association (NFPA), which creates codes relating to fire protection, as well as the National Electrical Code (NFPA 70) which nearly every jurisdiction has adopted for electrical requirements.
Facilities Guidelines Institute (FGI), which creates code requirements for healthcare facilities. FGI guidelines are widely adopted by most jurisdictions.

Trade organizations and technical societies will also often create model codes for some particular aspect of the building. The American Society of Mechanical Engineers (ASME) has a code for elevator and escalator safety, and the American Concrete Institute (ACI) has a model code for structural concrete. These codes may be adopted directly by jurisdictions, or referenced in a larger “umbrella” model code. The code for structural concrete, for instance, is referenced by the IBC and IRC.

There are a variety of mechanisms in place to try to ensure model codes reflect actual expert opinion about how to best balance building safety, performance, and cost, and don’t get captured by special interests. The development process is often ANSI-certified, which requires adhering to a certain set of best practices for due process in code and standard development. ANSI-certified codes and standards must be (among other things) developed openly, include a variety of interests without being dominated by a single one, be decided by consensus vote, and have an appeals procedure.

Most model code development follows this sort of process. For instance, here’s the development process for ICC Code changes:

Proposed code changes are submitted. Anyone can submit a code change.
A public hearing for proposed code changes is scheduled, and all proposed changes are posted publicly 30 days before.
A hearing discussing the code changes takes place. Hearings are public, and anyone can attend and testify. At the end of the hearing the code development committee make recommendations for the code changes. The results of the hearing are posted publicly.
Public comments are solicited on the results of the hearing and the proposed changes.
A final hearing takes place where eligible members vote on the proposed changes. Though ICC development committees consist of a variety of experts (code officials, architects and engineers, contractors, building product manufacturers), only code enforcement officials are allowed to vote on proposed changes.
Approved code changes are incorporated into the next version of the model code.

ICC also has an appeals procedure, and specific rules for what sort of standards they will incorporate into the model codes (for instance, standards that require a proprietary material or method are prohibited.) IAPMO and NFPA have similar code development processes.

But this doesn’t mean that the special interests don’t try. Model code changes tend to be extensively lobbied for (and against) by relevant industry interests. Prior to its advocacy at the state level, for instance, the NAHB argued against the inclusion of fire sprinklers in the International Residential Code. And the recent changes to the IBC that allow the construction of tall wood buildings were lobbied against by steel suppliers and concrete trade organizations (and lobbied for by wood trade associations and industry members.) Here, for instance, is a public comment from a concrete trade association arguing against allowing taller wood buildings (bold/caps are original):

At the recent ICC Committee hearings in Columbus, OH, your committee Failed you. The general committee charged with looking at proposals and weighing justification FAILED to do their job when it came to Tall Wood Buildings. Despite overwhelming testimony that fire tests were inadequate, the committee simply ignored the fact that the TWB ADHOC committee only considered a two story residential structure during testing, and then used 'Engineering Judgment" to determine that those results will be sufficient for 18 stories.
WHERE is the testing for all the other occupancy groups? 100% increases in story height are proposed for other use groups without any justification.
The ICC TWB ADHOC Committee has taken it upon themselves to develop a prescriptive TWB approach that exceeds the allowable heights of every country in the world. The United States just recently began looking at Mass Timber for taller buildings and yet, if this proposal goes through, we will allow mass timber 6 stories higher than any other country.
Not only will the U.S. allow the tallest buildings, we will also allow 12 story Mercantile, Storage and Factory to be built without gypsum covering on 40% of the CLT surface.
While mass timber may be an acceptable building material, it has not gone through the rigors of that are needed for high rise buildings. Do not let the U.S. be the testing ground for these Tall Wood Buildings.

Because they're private organizations, there's sometimes competition between different code organizations as they try to advocate for their particular model code. For instance, both ICC and IAPMO have their own particular plumbing code (the International Plumbing Code, and the Uniform Plumbing Code, respectively) they try to encourage jurisdictions to adopt. And in the early 2000s, as a response to the ICC developing a life safety code that would compete with their NFPA 101 code, the NFPA developed their own model building code (NFPA 5000) to try to compete against the International Building Code (almost no jurisdictions adopted NFPA 5000.)

But there’s less competition than you might think - model codes often have a “winner take all” dynamic, probably due to returns to scale (since there are large fixed costs spent on developing a code), network effects (since each jurisdiction that adopts a code makes it marginally easier for the next jurisdiction to also adopt), and high switching costs. Outside of mechanical and plumbing, most model codes today have little to no real competition, and it’s hard to see any new model code organization springing up and being able to compete.

This in fact was a result the industry was aiming for - in the 1970s and 1980s there was a great deal of frustration over the duplicated effort required at maintaining many different model codes, and desire to replace them with a single model code:

As the quality and breadth of the three model codes grew through the 70’s and 80’s the duplication became more and more apparent. Individuals wishing to introduce new ideas, products or standards were having to attend 6 or more meetings each year essentially bringing forward the exact same arguments and rational in front of three different audiences. Designers were constantly attempting to discern which provisions applied in which locality. Building owners operating nationally were constantly attempting to understand which provisions applied in which areas. Over time many voices were calling for the creation of a single model building code or at the very least consistency between their provisions as expressed in ASCE’s Policy 340- Uniformity of Building Codes…

The creation of the ICC grew out of these concerns.

Model codes are typically updated on a regular cycle. The ICC, for instance, updates their codes every 3 years. NFPA updates their codes every 3 to 5 years. The FGI updates their guidelines every 4 years. As new versions of model codes are released, jurisdictions update their codes to use newer versions (which typically follow their own multi-year update cycle), which means any given change can take a long time to propagate. For instance, the most recent version of the International Building Code is the 2021 version. This has been adopted by several states and jurisdictions, but other states continue to use the 2018, 2015, and 2012 versions of the IBC. For healthcare facilities, the most recent set of FGI guidelines was released in 2018, but some states continue to refer to the 1996(!) FGI guidelines.

One recurring issue with model codes is the notion of copyright. The law can’t be copyrighted, which means once a jurisdiction adopts a code anyone can freely copy it, put it up on the internet, etc. This potentially impacts a key part of the model code organizations’ business, that of selling copies of their codes, and they often try to stop this practice. The ICC, for instance, tends to sue anyone who puts building codes up online (most recently the building code startup Upcodes), despite the legal precedent being well established. Other standards development groups have done the same.

Standards

Related to codes are standards. These are created by trade organizations, technical societies, standards development groups (such as ASTM and ANSI) and government agencies (such as NIST and FEMA) that define performance criteria or provide design guidelines and information for some particular aspect of the building. The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE), for instance, has developed standards for indoor air quality, thermal conditions for human occupancy, building energy efficiency, and many other aspects of HVAC systems. ACI has several hundred standards and guidelines covering all aspects of concrete design and placement. Like codes, standards are often developed using some sort of procedure to try to ensure fairness and impartiality (such as ANSIs.)

Model codes typically reference various standards as a way to provide requirements without having to detail every aspect of some particular system. In this section on glass requirements, for instance, the IBC directs the reader to a section in the ASCE 7 standard for seismic requirements, and to ASTM E1300 for determining glass load resistance:

Model codes typically heavily rely various performance standards. The International Residential Code, for instance, references several hundred different standards, test methods, and design criteria. And standards, in turn, may reference other standards and guidelines. The IBC, for instance, references ASCE standard 7 “Minimum Design Loads for Buildings and Other Structures”, which in turn references ASCE 24 “Flood Resistant Design and Construction” (along with many other standards and guidelines.)

Like codes, standards are occasionally updated, depending on the update cycle of the development organization. Some are updated at a regular cadence, while others are updated much less frequently. Depending on how this update cycle aligns with model code update cycles, this can mean additional time before a change propagates into an actual building code (the ASCE 7 standard, for instance, was updated in 2016, but wasn’t incorporated into the IBC until 2018.)

If a standard is written in enforceable language, it may be adopted as a code directly by a particular jurisdiction. For instance, many jurisdictions have directly adopted NFPA 13 Standard for the Installation of Sprinkler Systems. ASHRAE 90.1 is a standard governing energy efficiency in buildings. It gets referenced by the ICC’s International Energy Conservation Code as a way to meet code requirements, but some jurisdictions have adopted ASHRAE 90.1 directly. Standards written in unenforceable language, on the other hand, often struggle to get adopted or referenced by model codes. For instance, ASHRAE 62.1 standard for indoor air quality saw limited adoption prior to it being rewritten in “mandatory” language”. More recently, ASHRAE’s standard 188 for legionella prevention is facing difficulty being adopted for similar reasons.

A model code may also base its code provisions off of a standard without referencing that standard directly. The International Mechanical Code and Uniform Mechanical Code ventilation rates are based off ASHRAEs 62.1 standard for indoor air quality, for instance. And the seismic provisions of ASCE are based off of the National Earthquake Hazard Reduction Program’s (NEHRP) Recommended Seismic Provisions.

Testing labs and certifications

For many aspects of the building, (such as an off-the-shelf product), it’s not necessarily obvious whether it will meet a given set of requirements or not. For instance, various parts of the building are required to have a certain level of performance in a fire (resisting combustion for a certain period of time), which can’t be easily verified by inspection. For these cases, codes tend to rely on various certifications to ensure that a product or assembly will meet a certain set of requirements. Jurisdictions will often require fire sprinkler equipment, for instance, to be certified by a third party testing organization, such as Underwriters Labs. The International Residential Code requires lumber to be inspected and certified by a testing organization such as the Southern Pine Inspection Bureau (SPIB.)

Third party testing labs are also used by building product suppliers to demonstrate that their product meets code requirements, or that it has the properties they claim it has. Here, for instance, is a testing report ordered by Simpson for their concrete anchors.

To avoid just moving the problem of trust from the product or assembly to the testing lab (ie: maybe I just hire my brother to “certify” my product), testing labs in turn must be certified. IAPMO, for instance, has had their testing service certified by ANSI to meet ISO standards for testing, and OSHA maintains a list of nationally recognized testing labs (NRTLs) that can test products to meet OSHA requirements.

Sliding scale of requirements

In practice, the way codes and standards influence the built environment is something of a spectrum. A criterion may be in the code adopted by a jurisdiction and thus mandatory, but there are a variety of other ways it can influence a building:

Some states will make a distinction between “mandatory” codes (which a local jurisdiction must adopt) and “permissive” codes (which a local jurisdiction may choose to adopt.) Georgia, for instance, has several mandatory codes and several permissive codes.
Similarly, a given criteria might be in an appendix to the code, which are opt-in rather than opt-out. Whether fire sprinkler requirements should be in the main body of the International Residential Code, or an appendix, has been a significant source of debate.
A standard may help define the “standard of care”. Design professionals like engineers and architects are held to the “standard of care”, which requires them to do “what a reasonable design professional” would do in a given situation. Creating a performance standard for some particular aspect of the building, even if it doesn’t get adopted into code, can help define what is “reasonable,” and thus be a defacto requirement.
Similarly, creating a standard or design method lowers the barriers to owners and designers who want to build to a certain level of performance (or use a particular system.) If you want to build using precast concrete, for instance, this is made much easier by the number of design guides and manuals published by the Precast Concrete Institute. (Building product suppliers often invest a great deal in the design of their catalogs for this reason.)
A jurisdiction may incentivize the use of a particular standard without necessarily requiring it. For instance, there are a variety of “green building” certifications and standards, such as LEED, which are generally not adopted as actual code requirements. However, many jurisdictions, such as the City of Seattle, offer incentives to developers if they achieve some green certification, such as allowing building at higher density.
Organizations that operate and construct many buildings (state and local governments, large organizations such as universities, large companies like Walmart or Amazon) might have their own list of building requirements and standards they enforce.

Evolution of building codes

Building code changes are the result of some perceived insufficiency in existing codes. This often stems from some outside inciting event (such as a high-profile building failure.) Severe fires, for instance, have often resulted in changes to the fire code. Code requirements that stairways and elevator shafts be closed and protected, for example, followed from fires where open shafts greatly accelerated the fire’s spread:

“A closed door prevents a draft of hot air from carrying flames and suffocating gases into the closed-off portion of a building. Fire engineers still bear in mind a tragic memory of the fatal part played by open stairways in 1946 in speeding the spread of fire through the Winecoff Hotel in Atlanta, where 119 people lost their lives. That disaster was just one example in a long list of serious building fires in which open stairways proved a deadly factor.” - “A Spark Lighted in Portland”, history of the National Board of Fire Underwriters

More recently, the Surfside condo collapse in Florida resulted in the creation of a statewide inspection program for condos, and requires condo associations to maintain reserves to fund repairs.

Another example is hurricanes, which have shaped the evolution of building codes in Florida. Hurricanes in the 1950s resulted in the development of the South Florida Building Code in 1957 (which included developing a mathematical model for predicting wind force, ~30 years before a similar model would appear in the more widely adopted ASCE 7-88), and the damage caused by Hurricane Andrew in 1992 resulted in the adoption of a statewide building code.

Earthquake code requirements also follow this pattern. California seismic codes, for instance, were increased following the San Fernando earthquake in 1971, and the Mexico City earthquake in 1985. (Because strong earthquakes occur relatively rarely, can cause severe damage, and building performance in them is not often well understood, a strong earthquake anywhere often results in building code changes.)

The inciting event isn’t always a disaster - further fire code strengthening followed the publication of the America Burning report in 1973, which drew attention to the US’s comparatively poor fire safety performance. And the creation of the ASHRAE 90.1 standard for energy efficiency was the result of the 1973 energy crisis.

In other cases, the changes are the result of a more gradual shift rather than a particular event. Increasingly strict building code requirements for energy performance are the result of concern about climate change and building greenhouse gas emissions. And the recent mass timber code changes in the IBC were the result of increasing interest in using heavy timber for buildings from a variety of sources (architects, engineers, timber manufacturers), which resulted in the ICC creating a committee to investigate the issue and propose code changes.

Code changes can also result from advocacy work. Sometimes this advocacy is on behalf of industry members. For instance, in 2007 Structural Insulated Panel (SIP) Association successfully lobbied to have SIPs added to the International Residential Code. Other times it might be advocacy from experts - code changes requiring the use of fire sprinklers have been driven by a grassroots advocacy movement from fire safety professionals (aided by concerned members of the public.)

New vs existing buildings

Building codes typically only apply to new construction - existing buildings are generally “grandfathered in”, with changes to them requiring complying with a separate code specifically for existing buildings. (These codes typically have a cutoff, where if the renovation is extensive enough it’s required to bring the entire building up to modern code requirements. Renovation projects typically try to massage the design and scope to avoid triggering these requirements.)

However, sometimes a change is deemed to be important enough to require changes to existing buildings. Following the 1985 Mexico City earthquake, for instance, California passed a law requiring the reinforcing of existing unreinforced masonry buildings (which are particularly susceptible to seismic damage.) Jurisdictions have often passed laws requiring the installation of fire sprinklers (or equivalent safety measures) in existing high-rise buildings that lack them. New York City recently passed a law requiring the gradual elimination of carbon emissions from its building stock, which will likely be achieved via extensive retrofits of existing buildings.

Other requirements

There are many other parts of the building requirements landscape - inspections, permitting, zoning, licensing requirements, financing, insurance, union laws, environmental laws (CEQA, NEPA) etc, that this largely doesn’t touch on.

Construction Physics

How building codes work in the US