Have you ever been surprised by unexpected cost increases late in a progressive design-build (PDB) water project? This blog explores the “estimator gap” that can occur after the 60% design milestone in such projects, after cost estimator involvement tapers off. It examines how seemingly neutral, late-stage design changes, made without active cost oversight, can quietly push project costs beyond the original guaranteed maximum price (GMP) estimate. This issue matters to both PDB teams and owners because cost certainty is crucial for planning, and unchecked cost growth or scope creep late in design can undermine budget integrity and erode trust among project stakeholders.
In progressive design-build, owners rely on cost estimates at key milestones (often 30% and 60–70%) to confirm that scope, budget, and risk remain aligned as the team approaches the GMP. But design continues after GMP, and costs can drift if estimating discipline and cost reconciliation do not continue.
Here’s the typical pattern of pricing exercises:
- Baseline: Early collaboration sets the baseline. Estimators are active early to validate big-ticket drivers (major equipment, process systems, civil quantities) and keep design decisions tied to cost reality.
- 30% check: The 30% checkpoint is about “are we still in range?” It’s where assumptions get tested and the team starts managing budget tradeoffs intentionally.
- 60–70%: The cost model is refined and GMP confidence is set, with remaining design development assumed to be manageable within the allocated contingency.
After that point, design development keeps moving—often faster than cost oversight:
- Specification Drift: Late spec changes, or additional specs sneak in.
- Material changes, coatings, access requirements, safety features: Each may be reasonable on their own, but the cumulative impact can be substantial to the budget.
- Stakeholder Supplier Preferences: Sole-source vendors or brand name suppliers.
- Narrowing allowable manufacturers or subcontractors based on owner or designer preference can invalidate the pricing assumptions used to establish the GMP, particularly when others were carried in the estimate.
- Vendor Fatigue: Extended pre-award involvement and repeated quote revisions can lead vendors and subcontractors to disengage.
- Losing supplier and subcontractor pricing support during post-GMP design development increases the risk of unchecked, unexpected cost increases
- Standard details: Standard details get added or upgraded and replicated across the facility.
- A basic hatch can turn into an upgraded standard detail at IFC such as adding H-20 traffic-rated strength, intrusion protection, watertightness, and fall protection—driving a noticeable cost jump.
- Wall penetrations can become more expensive (such as stainless sleeves with link seals instead of plastic sleeves and grout), creating a meaningful cost add when repeated across hundreds of locations.
- Pipe supports sometimes shift into an upgraded standard detail (such as custom structural stainless steel with epoxy anchors or complicated embeds), adding fabrication, coordination, and installation effort that wasn’t in the previous estimates.
- Equipment pads can shift into an upgraded standard detail at IFC (thicker sections, epoxy grout, more reinforcing requirement), creating a noticeable cost add when repeated across multiple equipment bases.
None of these decisions are unusual—what is dangerous is when the decisions happen after pricing exercises have stopped. Individually, they look like normal design development; together, they create scope creep that hits the contingency.
Project stakeholders need cost certainty, and that certainty must last through final design and not become unreliable after the GMP milestone.
Progressive design-build water projects carry risks which require continued estimator engagement through final design. Don’t lose your contingency due to estimator gap. Be the hero of your organization and insist that the estimators stay involved until the end of design development, and your efforts will pay off.