What is an addendum in construction?
An addendum is a formal change to the bid documents issued while a project is still bidding, before the contract is awarded. It is numbered, becomes part of the contract documents, and must be acknowledged in the bid. For a steel estimator, each addendum reopens the takeoff, because the count has to carry every change.
Key facts
- An addendum changes the bid documents before award. A change order changes the contract after it. The two get confused constantly, and they are priced completely differently.
- Addenda are issued by the owner or architect to the bidders of record, numbered in sequence, and each one is part of the contract documents. A steel fabricator usually receives them relayed through the GC.
- For a steel estimator, an addendum is not a memo. It is a change to the count, and the takeoff has to be reopened to carry it.
- The estimators we talk to lose roughly a day per addendum on a manual takeoff of a mid-size job. A heavy issue with hundreds of changes runs into days on its own.
- Most steel bids see at least one. On the projects the estimators we talk to bid, a set that closes with no addenda is the exception.
What arrives in an addendum
Anything the design team changed after the ITB went out: revised structural sheets, new details, spec changes, answers to bidder questions that alter scope, and sometimes a full re-issued set. The dangerous ones for steel are quiet: a beam size bumped on one framing plan, a connection note added to a detail nobody re-reads, stud counts that moved.
The sequence matters. Addendum 2 arrives as a change against the set as amended by addendum 1, not against the original. A beam deleted in addendum 1 must not be deleted again when addendum 2 restates the sheet, or the tonnage goes wrong in the direction that loses money quietly.
Is an addendum the same as a revision?
No, though the trade uses the words loosely. An addendum is the formal instrument that issues changes during bidding. A revision is any re-issued version of a drawing, whenever it happens, marked in the title block. Addenda deliver revisions during the bid window. After award, revisions arrive through bulletins, ASIs, or construction change directives, and their cost implications become change orders.
What an estimator does when one lands
Log it, read the narrative, then walk the changed sheets against the takeoff. Every affected line gets found, changed, and marked with which addendum changed it, so the bid can state what it includes. The bid form asks you to acknowledge each addendum by number. On sealed public work, a GC's bid that misses one is nonresponsive. A fabricator's quote to that GC does not get thrown out the same way. It is simply mispriced, which is worse, because nobody tells you.
Example
The bid set shows beam B-12, a W16x26 at grid C between 3 and 4. Addendum 1 deletes it and drops two others a size. Addendum 2 re-issues the whole framing plan for an unrelated column move, and B-12 is still absent from the new sheet. An estimator diffing each addendum against the original set deletes B-12 twice, and the takeoff is now a beam light: 30 feet of W16x26, 780 pounds, gone silently. Carried as a running state, addendum 2 shows what it actually changed, the column, and B-12 stays deleted exactly once.
How Ferra handles this
Ferra compares the amended set against the one already taken off and lists what changed, so the day per addendum becomes a review instead of a re-count. The longer treatment of where those days go is in why steel estimators lose days to addendums.
Sources
- AIA A701, Instructions to Bidders, the document that defines addenda as written or graphic instruments issued prior to execution of the contract that modify or interpret the bidding documents.
- Ranges for time lost per addendum reported by fabricators using Ferra in 2026, stated without customer attribution.