A basis of design (BOD) is a document that records every principle, assumption, calculation, and decision the design team makes during a building or engineering project. It explains not just what was designed, but why each choice was made, what criteria drove it, and which codes and standards apply. Think of it as the written reasoning behind the blueprints.
The BOD sits alongside the technical drawings and specifications, filling in the logic that drawings alone can’t communicate. For anyone managing, reviewing, or eventually commissioning a building project, it’s one of the most important reference documents produced during design.
What a Basis of Design Actually Contains
A BOD covers every major discipline involved in a project: architectural, civil, mechanical, electrical, plumbing, fire suppression, and communications/security. For each discipline, it typically documents the applicable building and design codes, the design conditions and assumptions the team worked from, equipment and material selections (including specific manufacturers, makes, and model numbers), and the rationale behind those selections.
Beyond individual product choices, the document also captures broader project-level information:
- Performance criteria: The measurable targets each system must meet, such as temperature ranges, airflow rates, or electrical load capacities. These are the benchmarks used later during testing and commissioning to verify the building works as intended.
- Life cycle cost analysis: A summary of the options the team considered, analyzed, and ultimately recommended, along with which option the owner selected. This is especially useful when a project gets questioned years later.
- Code compliance: The specific editions of every applicable code and standard, from the International Building Code and energy codes to fire safety standards like NFPA 99 and NFPA 101. Listing exact editions matters because codes change over time, and the project needs to be evaluated against the versions in effect when it was designed.
- Owner preferences and exceptions: Any cases where the owner’s requirements led to choices that differ from standard guidelines, and the reasoning behind those deviations.
The level of detail can be granular. A BOD for a mechanical system, for example, won’t just say “the building will have air conditioning.” It will specify the design temperatures for indoor and outdoor conditions, the type of system selected, the equipment models, and the engineering calculations that sized everything. Highlighted cut sheets and datasheets for selected equipment are often attached as supporting documentation.
How It Relates to the Owner’s Project Requirements
The BOD is often confused with another key document: the Owner’s Project Requirements (OPR). They’re closely related but serve different purposes. The OPR is the owner’s voice. It describes goals, expectations, and how the finished building will be used and operated. The BOD is the design team’s response. It details the concepts, calculations, product selections, and operating conditions chosen to meet those owner requirements while also satisfying applicable codes.
The two documents work as a pair. The University of Michigan’s project procedures describe them together as providing “a complete description of the project’s design intent.” The BOD is formatted to coordinate with and directly respond to the OPR, so any reviewer can trace a design decision back to the owner requirement it satisfies. Information shouldn’t be duplicated between them. Instead, they’re meant to be read side by side.
When the BOD Gets Created
Work on a basis of design typically begins during schematic design, the first formal design phase of a project. This is when the architect consults with the owner to establish project goals and requirements, and the design team starts translating those goals into technical approaches. The BOD takes shape as decisions are made, and it evolves through subsequent phases as the design becomes more detailed. By the end of design development, the document should be comprehensive enough to serve as a clear record of every significant choice.
It’s not a one-and-done document. As the project progresses and conditions change, perhaps the owner adjusts priorities or a specified product becomes unavailable, the BOD gets updated to reflect the new decisions and the reasoning behind them.
Why It Matters for Risk and Accountability
A well-documented BOD protects everyone on a project. During construction, disputes frequently arise over whether a substituted material is truly equivalent to what was specified, or whether a system was designed to meet the correct standards. The BOD provides the baseline for answering those questions because it lists the specification requirements for compliance and defines what qualifies as an acceptable equivalent.
If a project ends up in litigation, design professionals with clear documentation of their decisions and the reasoning behind them are in a far stronger position. Conversely, a vague or incomplete BOD can leave gaps that are expensive to resolve. When the document clearly records that the team considered multiple options, analyzed their costs and performance, and selected one for stated reasons, it becomes much harder for anyone to claim the design was arbitrary or negligent.
The BOD also plays a practical role during commissioning, the process of testing and verifying that building systems perform as intended. Commissioning agents use the performance criteria documented in the BOD as their benchmark. Without those criteria written down, there’s no objective standard to test against.
Specialized Facilities Need More Detail
For standard commercial or residential buildings, a BOD covers the usual disciplines at a reasonable level of detail. For specialized facilities like laboratories, hospitals, pharmaceutical manufacturing plants, or data centers, the document becomes significantly more complex because the design constraints are far more demanding.
A laboratory BOD, for instance, might need to address requirements like completely separating lab spaces from outside areas with four bounded walls, specifying non-porous and seamless bench top surfaces that resist disinfectants, requiring liquid-tight floor construction in chemical storage areas, and documenting security provisions for controlled substances or hazardous biological materials. Each of these requirements traces back to regulatory standards, and the BOD records which standards apply and how the design meets them.
Healthcare facilities bring their own layer of complexity, with specific codes governing everything from emergency power systems to medical gas installations. The BOD for these projects must reference precise code editions, such as the Healthcare Facilities Code for building system categories, and document how each system meets the required safety tier.
What Makes a Good BOD
The difference between a useful BOD and a box-checking exercise comes down to specificity. A strong BOD doesn’t just list what was chosen. It explains why, what alternatives were considered, and what assumptions were in play. If the mechanical engineer sized ductwork based on a specific occupancy load, that number and its source should be in the document. If the architect chose a particular wall assembly to meet both fire rating and acoustic requirements, both rationales should be recorded.
The best BODs are also organized to mirror the structure of the OPR, making it straightforward to cross-reference owner requirements with design responses. They include enough detail that someone unfamiliar with the project, whether a new team member, a commissioning agent, or a facility manager years later, can understand not just what was built but the thinking behind it.