Why Do Due Diligence Costs Overrun? The Invisible Billing Lag

Ask a deal lead why the last diligence budget overran and the first answer is usually "the lawyers." It is almost never the right answer. The advisor's rate was agreed in the engagement letter and barely moved. What overran was not the price of an hour but the number of hours, and the deal team did not see those hours accumulate because they had not been billed yet. Due diligence costs overrun because work is incurred weeks before it becomes visible, and a budget you reconcile against invoices is always reading a position the deal has already left behind.

That lag is the whole mechanism, and it is more specific than the usual "scope creep" answer. Scope does expand on a live deal; that part is well understood. The under-examined part is when the expansion shows up in the numbers. Advisors do not invoice the day they work. They carry the hours as unbilled time and send a figure later, so the deal team is told what diligence cost roughly a month after the cost was actually run. By then the question is no longer "should we approve this work?" but "do we want a fight over an invoice three weeks before signing?" So the honest answer to why do due diligence costs overrun is less about scope and more about timing: the budget cannot price drift it cannot yet see.

Why Do Due Diligence Costs Overrun Instead of Landing on Estimate?#

Due diligence costs overrun because the cost of a live engagement accrues continuously while it becomes visible only periodically, and the gap between those two clocks is where the budget breaks. Work happens daily. Invoicing happens monthly, sometimes less often. In between sits a growing pile of authorised-but-unbilled effort that nobody has priced into the running total, because the only number most teams trust is the one an invoice confirms.

The size of that pile is not a hunch. In legal services, where billing discipline is studied closely, the median firm carries about 43 days of unbilled work-in-progress at any given moment, per Clio's 2025 Legal Trends Report, and the slowest quartile carries far more. Diligence advisors are not law firms, but they bill the same way: hours are logged, held as work-in-progress, and converted to an invoice on a cycle that suits the advisor's finance function, not the deal's decision rhythm. So at any point in a live engagement, a deal lead asking "what have we spent?" is looking at a number that is structurally six weeks stale, and the unbilled remainder is exactly the part that has grown since anyone last checked the scope.

This is a different question from the one our field guide to due diligence cost management answers. That guide covers how advisor cost is built workstream by workstream and what live visibility looks like in practice; this post is narrower and about timing alone, namely why the overrun is invisible while it is still cheap to fix and obvious only once it is not.

The Billing Lag Is the Overrun, Not a Symptom of It#

The billing lag is not a side effect of the overrun; on a time-and-materials engagement it is the thing that lets the overrun happen unchallenged. A cost you can see the week it is incurred is a cost you can question, cap, or decline. A cost that surfaces a month later, already worked and already owed, is not a decision any more; it is a reconciliation. The lag converts every would-be scope conversation into a fait accompli, and it does so silently, because nothing in a static budget moves until an invoice arrives to move it.

Scope expansion is the fuel, and it is real: diligence finds an extra entity, a pension liability, a contract that needs a specialist read, and the advisor does the work because not doing it would be negligent. The work is usually justified. The problem is purely one of sequence. The expansion arrives as a quiet "can you also look at" rather than a priced change request, the hours accrue against it immediately, and the cost of it does not enter the budget until billing closes the loop weeks later. That is why scope creep feels invisible: not because the work is hidden, but because its cost is held in unbilled limbo until it is too late to argue. The mechanics of how that drift inflates a fee are worth understanding in their own right, which is the subject of our anatomy of due diligence scope creep.

Timeline drift then compounds the lag, because longer is just another word for dearer on anything billed by time. Deals run long far more often than they run short. BCG's 2024 M&A Report found that around 40% of transactions closed later than the timeline set at announcement, and of the deals that slipped, roughly 63% needed at least three additional months to get over the line. That sample skews to larger, US-heavy, regulator-sensitive deals, so a clean bilateral UK carve-out can move faster. But a financial workstream priced for an eight-week window that runs five months does not land on its original number, and every extra week is more unbilled work-in-progress quietly stacking up behind the last invoice.

The chart is a schematic of the trade-off, not a measured statistic, and the shape is the point rather than the heights: the visible line always trails the incurred line by about a billing cycle, so the deal team is permanently reading last month's deal. The gap between the two lines is the unbilled work-in-progress, and it is widest exactly when scope is expanding fastest, which is when you would most want to see it.

Why Doesn't a Spreadsheet Catch the Overrun in Time?#

A spreadsheet does not catch the overrun because it records billed cost, and the overrun lives in the unbilled gap. It holds the estimate you typed and updates only when someone re-keys an advisor's latest invoice figure, which by definition arrives weeks after the work. So the sheet shows a tidy budget while a month of authorised effort sits off-ledger, accruing against a scope nobody formally repriced. The first time the gap is visible is the first time it is also too late to challenge.

The deeper failure is conceptual: a spreadsheet has no native idea of committed cost, only billed cost. The early-warning signal in any diligence engagement is work that has been authorised but not yet invoiced, the change request agreed last Tuesday that will land as a number next month. A static sheet cannot hold that, so it is structurally blind to the one figure that would let a deal lead act before the overrun is locked in. The honest comparison is therefore not "spreadsheet versus dashboard" but "billed versus committed," and teams running serious time-and-materials exposure eventually need a deliberate way to capture committed spend, which is the discipline at the heart of closing the gap between advisor fee estimates and actual invoices.

How Do You Catch a Diligence Overrun Before the Invoice?#

You catch a diligence overrun early by tracking committed cost the moment it is authorised, so the unbilled gap becomes a number you watch rather than a surprise you absorb. The mechanism is mundane and that is the point: capture a fee estimate, fee type, and cap on every workstream line before kickoff, then each week record what has been authorised, not just what has been billed, and watch committed-plus-forecast against the cap. When a change request lands, you price it then, as a decision, instead of letting it accrue silently into a total that only reconciles at the end.

Two habits do most of the work. First, separate fixed-fee lines from time-and-materials lines, because the lag lives almost entirely on the T&M lines and a single blended "advisory fees" total hides exactly the strand that is compounding. Second, treat committed cost as a first-class number alongside billed cost, so the change request agreed this week shows up this week rather than next month. This is the loop AdviLink is built to run: committed and actual advisor spend tracked against agreed scope by workstream, so the unbilled gap is a figure on screen rather than a discovery on the final bill. Where it can be prevented at the engagement-letter stage it should be, and a surprising amount of overrun is preventable there, which is part of why coordinating several advisors on one budget is its own discipline, covered in our guide to managing multiple due diligence advisors.

There is a market reason this matters more right now than the long-run averages suggest. UK deal volume is swinging hard quarter to quarter: the ONS recorded 352 M&A transactions involving a change in majority share ownership in the first quarter of 2026, down from 495 in the final quarter of 2025. Volatility like that is precisely when scope discipline slips: a team racing to close before a window shuts has the least time to chase advisor numbers, and the billing lag is least forgiving when the deal is moving fastest. The teams that get surprised by diligence costs are usually the ones still treating the engagement letter as paperwork rather than as the budget it actually is, and the unbilled gap as someone else's accounting problem rather than their own live exposure.

Frequently Asked Questions#

Why do due diligence costs overrun on M&A deals?#

Due diligence costs overrun mainly because of a billing lag, not an excessive rate. Advisors carry their hours as unbilled work-in-progress and invoice on a monthly or slower cycle, so the deal team learns what diligence actually cost weeks after the work was done and the scope decision was still reversible. Scope expansion supplies the extra work, but it is the delay between incurring that cost and seeing it that turns a manageable change request into an unchallengeable invoice.

What is the billing lag in due diligence, and why does it matter?#

The billing lag is the gap between when an advisor does the work and when that work appears on an invoice the deal team can see. It matters because cost accrues continuously while it becomes visible only periodically; in legal services the median firm carries about 43 days of unbilled work at any moment, per Clio's 2025 Legal Trends Report, and diligence advisors bill the same way. A deal lead reading the latest invoice is therefore always looking at a position the deal left roughly a month ago, which is the part of the budget where overruns hide.

Which due diligence workstream overruns most often?#

Time-and-materials workstreams overrun most often, because they are where the billing lag bites hardest: hours run against an estimate with no ceiling, and the cost is invisible until billed. The financial workstream is especially exposed when the target's books do not reconcile and reconstruction work expands the scope. A quality-of-earnings engagement runs from about $25,000 for a small business to six figures for a larger or more complex one, with the larger end often starting around $60,000, per Eton Venture Services, and data quality is the main reason a quote lands at the top of that band.

How do you catch a diligence cost overrun before the invoice?#

Track committed cost the week it is authorised rather than waiting for billed figures, separate fixed-fee lines from time-and-materials lines, and price every change request against the agreed cap when it lands. That closes the billing lag deliberately: instead of discovering a month of unbilled work on the final invoice, you see it accrue in real time and decide on each expansion while it is still a decision.

Is a bigger contingency the answer to diligence overruns?#

No. A bigger contingency hides the overrun rather than catching it, and it gets absorbed silently because no one is watching the unbilled work it is meant to cover. The answer is visibility of committed cost, not slack: see authorised-but-unbilled spend as it happens, so the budget reflects the deal as it stands rather than as the last invoice described it.

Sources#

1. Legal Trends Report Benchmarks. Clio, 2025. The median law firm carries roughly 43 days of realization lockup (unbilled work-in-progress) at any given time, with the slowest quartile carrying substantially more.
2. The 2024 M&A Report: Deals Are Taking Longer to Close. Boston Consulting Group, 2024. Around 40% of transactions closed later than the announced timeline; among delayed deals, roughly 63% took at least three additional months to close.
3. Mergers and Acquisitions Involving UK Companies. Office for National Statistics, 2026. 352 M&A transactions involving a change in majority share ownership in Q1 (Jan–Mar) 2026, down from 495 in Q4 (Oct–Dec) 2025.
4. How Much Does a Quality of Earnings Report Cost?. Eton Venture Services, 2025. A quality-of-earnings report costs between $25,000 and $35,000 for a small business under $10M of revenue; for larger businesses, prices often start at $60,000 and can reach six figures.