The MEP Clash You Catch in Week 2 vs the One You Catch on Site: What Changes the Cost
It’s the same clash either way: a duct and a sprinkler main trying to occupy the same six inches of ceiling cavity. Caught in week 2 of coordination, it’s a model update — someone reroutes one of the two systems on screen, the fix gets verified, and the project moves on. Caught during rough-in, it’s a field stop: a crew waiting on a decision, a change order being written, and possibly a cut-and-patch repair to finishes that were already installed around the conflict.
Nothing about the clash itself changed. What changed is how much of the project was already built around it by the time someone found it.
The same MEP clash costs less the earlier it’s caught, since less downstream work depends on it. Early, a fix is a model update. Late — during rough-in — it’s a field change, a delay, and sometimes a repair to finished work. This is a widely used construction concept, not a precise formula; the multiplier varies, the direction doesn’t.
Who this is for
GC project executives deciding how early to start MEP coordination relative to the rest of the schedule, and anyone justifying a coordination budget against the cost of skipping or rushing it.
Why does the same clash cost more the later it’s found?
This is a long-standing idea in construction and design, usually illustrated with a curve showing that a project gets harder — and more expensive — to change as it gets more developed. Architect Patrick MacLeamy popularized a version of this in 2004, though a near-identical diagram, “Ability to Influence Construction Cost Over Time,” was published by Boyd C. Paulson Jr. in 1976, and a software engineer named Barry Boehm charted an analogous curve for software defects that same year. The idea has been around, in some form, for decades.
It’s worth being honest about what this curve actually is: a widely cited illustration of a real pattern, not a precisely measured formula. Research into its origins found no underlying empirical study proving a specific multiplier — it should be treated as a diagram representing conventional wisdom, not a statement of exact fact. What it gets right, directionally, is simple: the more work has already been built around a decision, the more that decision costs to change.
What actually drives the cost up over time?
A clash found early touches one thing: the model. A clash found late can touch several, depending on how far construction has progressed:
- The model, same as early — someone still has to decide and draw the fix.
- The schedule, if a crew is standing by waiting on the resolution.
- Already-installed work, if a finish, a ceiling or a wall already closed in around the conflict and now needs to be opened back up.
- Procurement, if the fix requires a different fitting, length or material than what was already ordered or fabricated.
- The paper trail, since a field-discovered clash usually generates an RFI and sometimes a change order, each with its own review and approval time.
None of that exists for a clash caught on screen during coordination. That’s the entire value case for starting coordination early and running it on a real cadence, not a schedule afterthought.
| Caught in coordination | Caught on site | |
|---|---|---|
| What changes | The model | The model, plus schedule, installed work, procurement and paperwork |
| Who’s involved | Coordination team, affected trade | Same, plus field crew, PM, often the GC’s office |
| Typical paper trail | Clash log entry | RFI, often a change order |
| Installed work affected | None | Sometimes — finishes or enclosures already in place |
How much of that cost is actually tied to design and coordination gaps?
Independent compilations of construction rework data point in the same direction without claiming a single precise number. One secondary compilation of rework research (citing underlying academic studies, treated here as reported estimates rather than independently verified figures) put the share of rework tied to design and engineering error above 70%, with a meaningful share of cost deviations traced to design-related causes. The exact percentages vary by study and should not be quoted as settled fact, but the pattern holds across more than one source: a large share of rework traces back to something that could have been caught earlier, on paper or on screen, instead of in the field.
What this looks like on an actual coordination timeline
None of this requires a sophisticated cost model to act on. A coordination schedule that starts once trade models reach a testable level of detail, runs clash tests on a fixed cadence, and resolves findings before trades move toward rough-in is, in practical terms, a plan to keep every clash in the cheap part of the curve. A schedule that treats coordination as something to squeeze in right before construction starts is, by the same logic, a plan to find some of those same clashes in the expensive part instead.
Where this fits into a coordination partner’s scope
If you’re trying to decide how early to bring in coordination relative to your design schedule, Request a Scope Review — send your current project timeline and a note on when trade models are expected to be detailed enough to test. For the coordination process this timing decision supports, see MEP modeling and coordination explained. For how to keep the resolution process itself from slipping once clashes are found, see resolving MEP clashes without blowing the schedule. For the level-of-development question that determines when a model is ready to test, see prefabrication-ready MEP models.
