Bid day is almost done. The takeoff is checked, the estimate extends cleanly, and the proposal page already has a total the owner can read. Then an addendum lands — an IDF closet on a revised electrical sheet, a wall that moved overnight, a note that changes the hang height of raceway on half the floor. Shops that priced a lump scramble for a new round number. Shops that priced named rates re-extend the lines that moved and send a revised offer that still looks like the same book.
This guide is that before-award moment: what an addendum is, how it differs from an RFI clarification and from a post-award change order, and how a commercial shop keeps takeoff, estimate, and proposal honest while the drawings still move. The suite through-line is the same illustrative vacant 4,200 square foot second-floor Class B office package used in tenant improvement estimating. After award, the same footage becomes a change order. What follows is still in pursuit.
What an addendum is (and what it is not)
An addendum is a formal change to the bidding documents before award. It revises drawings, specifications, quantities, or the bid window, and it binds every bidder who stays in the race. Estimators who treat the email as optional reading can discover, after the award, that the IDF was always on the record set. You measure what moved, you extend the rates that still fit, and you revise the owner-facing sentences so the offer matches the set you will actually build.
An RFI clarification is different. It answers a question about the set without rewriting the set. When the answer only confirms what was already drawn, you may not move a quantity at all. When the answer effectively changes scope, treat that answer the way you would treat an addendum: update the takeoff, the estimate, and the proposal language, and keep a record of which question moved the number.
A change order is different again. It revises a number the owner already accepted. Before award there is no accepted baseline to revise — only a live offer. Folding an addendum into “we will catch it later as a CO” is how a shop wins a package that no longer matches the drawings. The change-order guide covers the after-award path. This page stays on the path while the bid is still open.
Keep three documents while addenda move
Takeoff, estimate, and proposal stay three documents on the same book. The addendum is why that split matters.
The takeoff measures what the current set shows — linear feet of raceway, each of device, each of fixture — including every addendum you will send against. The estimate extends those quantities with named library rates under the same install conditions: vacant suite, standard shift, raceway exposed and ≤10' above finish floor. The proposal is the owner-facing page: scope sentences, inclusions, exclusions, quantities, and the total after overhead and fee.
When Addendum 1 lands, you do not create a fourth document. You update the three you already have. The takeoff grows where the sheet grew. The estimate re-extends those lines. The proposal picks up the addendum in the scope sentence and shows the new total. A shop that merges everything into one lump cell labeled “second-floor electrical” cannot say which assembly moved when the IDF appears.
Re-takeoff what moved, re-extend named rates, revise the sentences
Start with the sheet, not with a percentage. Re-measure only the assemblies the addendum actually changed. If the IDF adds raceway and the device count stays put, the raceway line moves and the device line does not. Keep the unit identical on both sides of the handoff — LF stays LF, EA stays EA — so the next reviewer can see the delta without decoding a new theory of the job.
Pull the same named library rates that priced the base offer, as long as the install condition still matches. Vacant, standard shift, exposed ≤10' AFF stays on the vacant rate. If the addendum pushes work into an occupied suite or after hours, that is a condition change: use the occupied or after-hours rate, or walk the package. Do not hide a silent premium inside the vacant number and hope the owner never asks.
Apply overhead and fee the same way you did on the base offer. On this electrical through-line the shop recovers 12% overhead on direct, then 8% fee on direct plus overhead. The library rates stay direct. Markup sits once on the proposal. Changing the recovery method because an addendum arrived is how a revised offer stops matching the story you told on the first send.
Revise the owner-facing scope so a PM can match the page to the walls. Name the addendum in the scope sentence. Update the inclusion quantities that moved. Leave exclusions that still apply. If stick or wages moved in the library after you sent the first offer, version the rate for the revision and say which version the new total uses. Sent proposals keep the version they used; you do not silently edit yesterday’s $10.36 into today’s number on a live pursuit.
One simple check keeps the arithmetic honest: revised extension equals the new measured quantity times the named library rate that still fits the condition. Everything else on the page — recovery, scope language, version — follows from that line.
Worked story: Bid A, then Addendum 1
Bid A is the electrical package on that illustrative vacant 4,200 square foot second-floor suite — Class B building, standard shift, raceway exposed and ≤10' AFF. Every dollar is an illustrative 2026 U.S. mid-market unit rate, not a real client quote. The same base stack appears in the tenant-improvement guide.
Base takeoff and direct extensions:
| Line | Qty | Unit | Direct rate | Extension |
|---|---|---|---|---|
| 3/4" EMT, exposed ≤10' AFF | 1,850 | LF | $10.36 | $19,166.00 |
| Duplex / data devices | 64 | EA | $88 | $5,632.00 |
| 2x4 LED lay-in | 48 | EA | $215 | $10,320.00 |
| Electrical direct | $35,118.00 |
Recovery on the proposal, matching the other guides:
- Overhead: 12% of $35,118.00 = $4,214.16
- Direct + overhead = $39,332.16
- Fee: 8% of $39,332.16 = $3,146.57
- Base electrical offer: $42,478.73
Late on bid day, Addendum 1 adds an IDF closet and 290 LF of 3/4" homerun, still exposed and ≤10' AFF, still vacant, still standard shift. Devices stay at 64. Fixtures stay at 48. Only raceway moved.
Re-takeoff: 1,850 + 290 = 2,140 LF. Re-extend: 2,140 × $10.36 = $22,170.40 against the old $19,166.00 — a $3,004.40 move on raceway direct. New electrical direct: $35,118.00 + $3,004.40 = $38,122.40.
- Overhead: 12% of $38,122.40 = $4,574.69
- Direct + overhead = $42,697.09
- Fee: 8% of $42,697.09 = $3,415.77
- Revised electrical offer: $46,112.86
That is $3,634.13 above the $42,478.73 base send. The inclusion sentence picks up Addendum 1 and 2,140 LF. The exclusion list — conductors, above-ceiling work, fire alarm, after-hours — stays, because those packages were never in the base offer either.
Now contrast the same 290 LF after award. Once the owner has accepted $42,478.73, that footage is no longer a bid revision. It is a change order against the awarded rates: $3,004.40 direct + $360.53 overhead + $269.19 fee = $3,634.12 (one-cent rounding difference), the sequence in the change-order guide. Before award you rewrite the offer. After award you revise the contract. Mixing those two moments is how a shop either underbids the set or argues about a “CO” that was never awarded.
When an addendum should make you walk
Not every addendum is a re-extend. Some change the job you thought you were pricing.
Walk, or at least pause hard, when the addendum changes the type of work: a power-and-lighting package that suddenly includes a fire-alarm design-build, a vacant suite that becomes an occupied night package, or a specialty electrical invite that grows into a multi-trade interiors package you do not buy. Those are not quantity deltas on the same rates. They are new pursuits wearing the old bid number.
Walk when the condition flips and you do not have the rate. Vacant standard-shift EMT at $10.36 is the wrong unit for occupied after-hours hanging around a working tenant. If the library has no occupied rate and the window is too short to build one honestly, declining is clearer than a silent premium.
Walk when the package shift is larger than the desk can digest before the due time. An addendum that rearranges half the reflected ceiling two hours before bid is not a courtesy to the shops that already finished. Send a decline, or ask for time, rather than a revised lump that guesses.
Staying in is right when the addendum is measurable, the condition still matches the named rates, and you can update all three documents before the window closes. The Bid A IDF add is that case: same suite, same shift, same exposed ≤10' raceway, more length. The shop that already had a named $10.36 rate did not need a new theory of electrical work. It needed a short pass with the sheet and a revised total the GC could still compare to the first send.
Common misses before award
The easy misses are almost always the same ones.
Ignoring a late addendum is the cleanest way to win the wrong job. If the set you priced is not the set that will be awarded, the first week on site becomes an argument about who owned the IDF.
Folding addenda into a silent lump hides which assembly moved. When the owner asks why the total grew $3,634.13, “electrical went up” is not an answer. “290 LF of exposed 3/4" EMT at the same $10.36” is.
Treating an addendum as a change order before award skips the revised offer. There is nothing to change-order until someone accepts a baseline. Price the live set, then send the new total.
Growing a new reason code or a new recovery method for every addendum wrecks the book. Keep the closed list of why a pursuit moved, and keep 12% then 8% (or whatever your shop actually recovers) stable unless the invite truly changed the commercial terms.
Sending the estimate as the proposal is still a miss after an addendum. $38,122.40 is a direct subtotal. The owner accepts $46,112.86 plus the sentences that name Addendum 1. Wage and burden rows stay in the library.
Keep the offer clear, then send it
An addendum is a test of whether your rates were real. If the library names the assembly and the condition, the revision is mostly measurement and arithmetic, and the owner can still see which line moved. If the first offer was a round number with a hopeful exclusion paragraph, every addendum becomes a new guess, and the pursuit stops being a book you can defend.
On the next addendum, re-measure only what moved, re-extend the same named rates, and send the revised total before the window closes.