Basis of Design (BOD)
The document that records why the design team chose the systems, codes, and criteria behind the drawings.
Quick Answer
A Basis of Design (BOD) is a technical narrative documenting the design criteria, codes, standards, and system choices behind a building's design. It explains why specific systems and approaches were selected, not just what was drawn. Design teams write it alongside the Owner's Project Requirements to keep design intent traceable through construction and operations.
The Full Picture
Drawings show what was designed, not why. An owner, a plan reviewer, or a facility manager years later needs a record of the reasoning: which code edition applied, why a mechanical system was chosen over alternatives, what capacity or performance criteria drove sizing. Without a written basis, that reasoning lives only in an engineer's head and disappears the moment the project team disbands.
A BOD is typically organized by discipline or building system — structural, mechanical, electrical, life safety, envelope — and states the applicable codes and standards, the governing design criteria such as loads and ventilation rates, the major system selections and why they were made, and the key assumptions behind them. It's started early, often at or before schematic design, and updated as decisions firm up through design development and construction documents.
The BOD pairs with the Owner's Project Requirements (OPR): the OPR states what the owner needs, and the BOD states how the design team intends to deliver it and why. Commissioning agents use the two documents side by side to verify that what gets built actually satisfies what the owner asked for, which is why both are standard commissioning-process deliverables on any project that includes formal commissioning.
For preconstruction, the BOD is the fastest way to understand the reasoning behind decisions that show up as cost in an estimate or takeoff — a redundant chiller, an upsized electrical service, a fire-rated assembly. An estimator who reads it can tell code-required scope apart from preference, which changes how a value-engineering conversation goes. A common failure is a BOD that's never updated after early design, so it no longer matches what's actually specified.
Real Examples
Common Misconceptions
People assume: A Basis of Design and a specification section are the same thing.
Actually: Specs tell a contractor what to install and to what standard. A BOD explains why that system was chosen and what criteria it has to meet — it's a design-intent document, not a procurement document, even though well-written specs should trace back to it.
People assume: The BOD only matters to the design team.
Actually: It matters through the life of the project. Preconstruction teams use it to separate code-required scope from preference, contractors reference it when evaluating substitutions, and facility managers rely on it for years after occupancy to understand why systems were sized the way they were.
Does MeltPlan Solve This?
Partially — adjacentPartially — MeltPlan doesn't author or maintain a Basis of Design document; that's a design-team deliverable. But MeltPlan's AI reads the drawings and specifications that are supposed to implement the BOD and flags missing requirements and inconsistencies between them, which is often how a gap between the stated basis and the actual documents gets caught before bid.
Review your drawings and specs for missing requirements →Frequently Asked Questions
What is a Basis of Design (BOD)?
A Basis of Design is a technical narrative, organized by discipline, that documents the codes, standards, design criteria, and system selections behind a building's design and explains the reasoning for each. It's a design-intent record, distinct from the drawings and specs that implement it.
How does a Basis of Design work in practice?
The design team starts it early, often at schematic design, stating the criteria and system choices known at that point. It's updated as design development and construction documents fill in the remaining decisions, so by the time drawings are complete the BOD should match what's actually specified.
Who writes the Basis of Design?
The design team — typically the engineer of record for each discipline, with the architect coordinating — writes and maintains the BOD, often with input from a commissioning agent who will later use it to verify the completed building.
How does a Basis of Design relate to the Owner's Project Requirements?
The OPR states what the owner needs the building to do; the BOD states how the design team intends to achieve that and why specific systems were chosen. Commissioning agents compare the two throughout the project to confirm the design still satisfies the owner's stated requirements.
Why does the Basis of Design matter for preconstruction?
It tells an estimator which costly decisions are code-required and which are preference, which changes how value engineering gets discussed. A BOD that's out of date with the actual drawings is also a warning sign that the design has drifted from its stated criteria without anyone documenting why.