Comparing Drawing Revisions: Impact on Quantities, Price and Scope

Most revision mistakes in an estimate are not comparison errors. They come from pricing a sheet that was already superseded, or from comparing against the wrong baseline. Before opening any overlay, establish exactly which issue you priced and exactly what the new issue contains.

The four places that identify a sheet

• Title block: sheet number, title, revision, date, and often the issue purpose (issued for bid, for permit, for construction). In the US, many sets follow the sheet identifier of the National CAD Standard's Uniform Drawing System: a discipline designator, a sheet type and a sequence number, so A-101 is the first architectural plan sheet. The same standard says sequence numbers need not be sequential, to leave room for inserted sheets: a gap in the numbering is not a missing sheet, the index decides.
• Revision block: one line per revision, with its number or letter, date and a short description. The description is the designer's summary, useful to start from and never enough to price from.
• Sheet index or drawing register: the list of every sheet in the set and its current revision. The index on the cover sheet is itself a sheet, and it is reissued when sheets are added or deleted.
• Transmittal: what was sent with this issue, at which revision, and for which purpose. It is the list you reconcile the received files against.

Issue purpose and revision codes

A revision issued for information or for coordination is not a revision issued for bid. On UK projects run under BS EN ISO 19650, the UK National Annex distinguishes a preliminary revision (P01, P02) from a published, contractual one (C01, C02), and gives each issue a status code, for example S1 for coordination or S3 for review and comment, as the UK BIM Framework guidance explains. When a tender pack mixes P and C revisions, note it in the matrix: you are pricing information the design team has not published as contractual.

Reconcile three lists before comparing any content: the index of the set you priced, the transmittal of the new issue, and the files you actually received. Each sheet lands in one of five states:

• Reissued: a new revision of a sheet you priced. Compare it with the revision you priced, not with the one before.
• Added: a sheet number that did not exist. Everything on it is new until shown otherwise.
• Deleted or superseded: a sheet withdrawn, often with its content moved elsewhere. Confirm where the content went before closing it; a deleted sheet whose content went nowhere is a scope deletion.
• Not reissued: still current at its old revision, but possibly contradicted by a reissued sheet that references it.
• Missing: listed on the transmittal or the new index, not in the files. Ask for it the same day; do not price around it.

A comparison tool finds differences. An estimate needs to know which differences cost something. Give every difference one of four types, and only one: if a change is both a quantity and a scope change, it is a scope change, and it takes several matrix rows.

The classic trap is the moved dimension string. On an overlay it shows exactly like a moved wall: two lines in two colours. The only way to tell them apart is to check whether the element itself moved, against the grid or a fixed reference. The reverse trap is a change of wall type tag: a few characters of text on the overlay, and possibly the largest cost change in the issue.

Drawings are only part of the documents. An addendum can change scope with no sheet reissued, and a sheet can be reissued with a change no addendum item mentions. On US federal sealed bids, an amendment may change quantities, specifications or delivery, or correct a defective or ambiguous invitation (FAR 14.208), and every term it does not modify stays unchanged (FAR 52.214-3). Private owners write the same idea into their instructions to bidders.

Comparing plans with specifications inside one issue is a related but different check; the article on detecting plan and specification inconsistencies before a bid covers it. Here, the question is narrower: what did this issue change compared with the one you priced.

Overlay and compare tools are the fastest way to find differences between two revisions of a sheet, and every estimating team should use one. They are mostly of two kinds:

• Overlay: both revisions stacked in different colours. In Bluebeam Revu, Overlay Pages converts each file to a colour and stacks them as layers; where the colours fall on each other they blend darker, so the elements that changed stand out (Bluebeam). In Procore's drawing comparison, removed items appear in red and added items in blue (Procore).
• Compare with markups: Bluebeam's Compare Documents writes a new file in which each difference is circled with a cloud markup that can be listed and reviewed.

What they show well

• Every place where linework or text differs between two pages, including changes nobody clouded
• Small text changes that the eye misses: a wall type tag, a door number, a note's last word
• Where nothing changed, so a reissued sheet with no difference can be closed quickly

What they cannot tell you

• Whether a difference costs anything: a moved dimension and a moved wall look the same
• How much a quantity changed: an overlay shows a longer wall, not how many feet, at what height, of which type
• Which estimate items a change touches, and which sheets that were not reissued it contradicts
• Anything changed in the specifications, the general notes on another sheet, or the addendum narrative
• Changes on a missing sheet, or on a sheet you never received

They also need comparable inputs. Bluebeam offers page, automatic and manual three-point alignment because a sheet whose view was moved or rescaled lights up everywhere; Procore states that drawings of different sizes cannot be compared. A scanned or re-plotted sheet produces noise. When a whole sheet lights up, realign or compare region by region, and do not read the colours until the grid lines sit on each other.

In short: the tool produces the list of differences. The matrix is where each one becomes a type, a set of estimate items, a quantity, a price and a decision.

One row per affected estimate item, not per change: a change that removes one partition type and adds another takes two rows, plus one for each dependent item. The cost delta is always the difference in quantity times the unit price of that line, as direct cost before markups, so the matrix adds up to the change in the direct cost of the estimate.

A change rarely touches one line. Before marking a row validated, walk through what else the changed item drags with it:

Subcontractor quotes deserve their own pass. A trade that priced revision 0 has not priced the rated wall. The guide to subcontractor bid leveling covers how to compare quotes on the same scope; the revision matters because a quote levelled against an old sheet is levelled against the wrong scope. If you build your estimate in a spreadsheet, the construction estimate template keeps quantity and unit price in separate columns, which is what makes a remove and add change traceable.

Every Medium or Low row carries a question, and every question has a deadline: the last date for bidder questions in the instructions to bidders. Write questions the design team can answer in one line, and write down the assumption you will carry if they do not.

The RFI template has the fields of such a question and a log to follow it. If no answer is issued in time, the chief estimator decides between three courses for each open row: carry the assumption silently, price it with a margin, or state it in the bid. The third is the most honest and not always allowed: on US federal sealed bids, a bid that imposes conditions modifying the requirements of the invitation is rejected (FAR 14.404-2(d)). Private owners and UK tenders set their own rules on qualifications; read them before writing one.

The matrix is also the record of who decided what. Keep two signatures apart: the estimator who measured a row, and the chief estimator who accepted its price and its type. A row is not validated until both have looked at it.

On bid day, the last check is that every addendum is acknowledged on the bid form. On US federal sealed bids, bidders acknowledge each amendment (FAR 52.214-3), and a missing acknowledgment is waived only when the bid shows the amendment was received, or when the amendment has no more than a negligible effect on price, quantity, quality or delivery (FAR 14.405(d)). The guide to estimating under pressure puts that check in the final review, done by someone who did not price the bid.

Everything above happens before the bid goes in, while the documents are still moving and the estimate absorbs each change. Once the contract is signed, the signed documents are the baseline, and a revised drawing is a change to the contract. Three things change:

• The procedure. The contract's change clause governs. Under the US federal Changes clause, the contracting officer orders changes in writing, the contractor asserts its right to an adjustment within 30 days, and an equitable adjustment follows (FAR 52.243-4). Under NEC4, a Project Manager instruction that changes the Scope is a compensation event, and a contractor whose notification has not reached the Project Manager within eight weeks of the event is not entitled to more time or money (NEC).
• Who bears it. Before bid, you price the change into your number. After award, whether a change is paid depends on the contract: some revisions are within the scope already priced, others entitle you to time and money.
• Time limits. Notices have deadlines that do not exist during bidding. Missing one can cost the entitlement however good the pricing.

The impact matrix still does the pricing work after award: same columns, with the contract revision as the old quantity. The rest belongs to change management: the change order template for the form and the log, and, on FIDIC contracts, the FIDIC Red Book notices and claims guide.