In Plain English
Quick Answer
A document explaining why design decisions were made and what criteria the building systems must meet.
Definition
Definition
The Basis of Design is a document that describes the design intent, criteria, assumptions, and system selections made by the design team to meet the owner's project requirements. It explains why specific systems, equipment, or materials were chosen and establishes the design parameters against which the completed project will be evaluated during commissioning. The BOD is a core component of the commissioning process and is typically developed by the engineer of record.
Context
Why It Matters in Bidding
The Basis of Design tells estimators why systems were selected and the performance criteria they must meet, which clarifies scope ambiguities that drawings alone leave open and helps avoid pricing the wrong equipment. On design-build and commissioned projects, the BOD also frames the contractor's responsibility, so misreading it can lead to substituted equipment that fails commissioning and triggers costly rework.
Example
Example
While bidding a lab fit-out, the mechanical estimator reads the BOD's redundancy and air-change criteria and realizes the project needs a backup air handler that the floor plans did not obviously show, adjusting the equipment quote accordingly.
See Also
Related Terms
FAQ
Questions Contractors Ask
How does the Basis of Design help an estimator price a bid?
It reveals the performance targets and system selections behind the drawings, exposing requirements like redundancy, efficiency, or capacity that drive equipment cost. Estimators use it to confirm scope, justify substitutions, and avoid pricing undersized systems. Reading the BOD early reduces the risk of bidding equipment that later fails to meet the owner's documented criteria.
What is the difference between the Basis of Design and the owner's project requirements?
The owner's project requirements state the owner's goals and expectations in their terms, while the Basis of Design is the design team's technical response explaining how those goals will be met. The OPR sets the targets; the BOD documents the engineering decisions and criteria used to hit them, and commissioning verifies the result against both.
Is the Basis of Design a contract document for bidding?
Its status varies by project. The BOD often accompanies the construction documents to convey design intent and commissioning criteria, but it may not carry the same contractual weight as the drawings and specifications. Estimators should check the bid documents to see whether the BOD governs scope or serves only as informational background.
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