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Construction projects usually exceed their budgets because the original estimate understates scope, site conditions, uncertainty or the cost of managing change. The overrun appears later as a variation, delay, rework, claim or escalation, but the underlying weakness often existed when the budget was approved. Owners can reduce the risk by defining a firm baseline, testing estimates against comparable completed work, investigating the site and utilities early, assigning contingency to specific risks, requiring approval before changes start, and monitoring cost and schedule against the original approval.
No single contingency percentage or contract type works for every project. The right controls depend on project size, location, design maturity, procurement route, market conditions and the owner’s ability to manage risk.
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What a construction cost overrun really means
A cost overrun is the difference between the approved budget and the forecast or final cost needed to deliver the agreed project. It can arise from a higher price for the original scope, additional work, delay-related costs, financing or inflation effects, or a combination of them. A triggering event may be ordinary and foreseeable in a class of projects even when its exact timing or location is uncertain.
Homes England’s 2024 guidance identifies ground conditions, complexity, scope and design changes, weather, access and permits as commonly cited causes, then makes the more important point: adverse risk is systematically underestimated during development and decision-making. That distinction matters. Improving the estimate and governance before construction is usually more effective than trying to negotiate away costs after work has begun.
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Why construction projects go over budget
Optimistic or incomplete estimates
An early estimate may omit quantities, temporary works, commissioning, testing, site logistics, design development, escalation or owner-supplied items. It may also price the most likely outcome rather than a realistic range. Homes England distinguishes optimism bias—unconscious underestimation of cost, time and risk—from strategic misrepresentation, in which likely costs or durations are deliberately understated or benefits overstated. The two can produce the same budget gap but require different responses: better evidence and challenge for bias, and stronger accountability where incentives encourage deliberate understatement.
Estimate maturity should match the decision being made. A concept-level figure can support option screening, but it should not be presented as a fixed construction price when the scope, design and site information are still developing. Starting a project before scope and benefits are sufficiently defined is identified as a governance risk in UK government analysis of major projects.
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Drawings and records may be incomplete, utilities may be in a different location, soil may be contaminated or weaker than assumed, and access may be more restricted than the concept allowed. Environmental mitigation, hazardous-material handling, permits, weather and third-party approvals can add both direct cost and time. Labor shortages, material disruption and market price movement create additional exposure.
The U.S. Government Accountability Office (GAO) found incomplete drawings and building information among possible causes of unforeseen conditions in General Services Administration repair and alteration work. In its review of 18 projects, 11 encountered unforeseen conditions; nine had cost increases and four had schedule delays. This was a limited review, not a rate for construction generally, but it illustrates how information quality and investigation affect the budget.
Scope and design changes
Design development often changes quantities, specifications and interfaces. Owner decisions made after tender can add rooms, finishes, equipment or performance requirements. Even a small change can require redesign, demolition, resequencing and testing. Costs become harder to control when the baseline does not state exclusions, interfaces or performance requirements, or when a change is approved informally by someone without authority to commit funds.
UK analysis of major projects identifies changes to scope and objectives without a full understanding of their cost or schedule impact as a governance weakness. The practical issue is cumulative: individually modest changes can consume the contingency and still leave the owner exposed to later risks.
Procurement choices and contract interfaces
Selecting the lowest initial bid can obscure exclusions, unrealistic schedules, provisional allowances or the cost of coordinating several parties. FHWA advises estimating the effects of construction sequencing, concurrent contract interfaces, utilities and obstructions, differing site conditions, contaminated soil, environmental mitigation and specialized designs. Each interface creates opportunities for delay, duplicated work or disputed responsibility.
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A Western Australia Auditor General case illustrates the warning. A project moved toward a single-contract approach after separate packages had been considered, and no bids were received within the original budget estimate. All corrected bids remained above that estimate. The lesson is not that one packaging strategy is always better; it is that tender returns are evidence about the credibility of the budget, scope and market assumptions. Repeated bids above budget should trigger a structured review rather than a presumption that the market is simply wrong.
Weak governance and late monitoring
Clear accountability is essential. If no owner-side lead can challenge assumptions, approve changes or escalate a threatened funding envelope, problems remain unresolved until options are expensive. Queensland Audit Office findings included contract variations proceeding without required approvals or being approved after work had begun. Its guidance emphasizes reporting that highlights variances early, explains their causes and informs decisions.
Rebasing an estimate can make a report appear on budget while hiding growth from the original commitment. A 2025 audit by the New York City Comptroller found repeated estimate rebasing and weak project-level tracking reduced visibility into cost changes. The audited sample covered 18 Department of Design and Construction projects with budgets of at least $100 million; budgets rose from $4.14 billion to $4.51 billion, a 9.1% net increase. Many projects were unfinished, the calculation excluded department overhead, and final growth could be greater.
How owners can reduce the risk
1. Define exactly what “done” means
Before approving a budget, create a scope and basis-of-estimate document that a contractor and an auditor could both understand. Include:
- Functional and performance requirements, quantities and quality standards.
- Design documents and the level of design maturity at the approval date.
- Explicit exclusions, allowances, owner-supplied items and temporary works.
- Interfaces with utilities, adjacent properties, tenants, authorities and other contractors.
- Site, access, working-hours, environmental and permitting assumptions.
- Required completion date, commissioning, testing, training and handover deliverables.
Record which assumptions are verified and which remain provisional. If the project is not ready for a reliable commitment, approve the next investigation or design stage rather than disguising uncertainty as precision.
2. Use an outside view of cost and duration
Compare the proposal with completed projects of similar type, size, location, complexity and delivery context. Reference-class forecasting uses the distribution of actual outcomes to counter optimism bias. The comparison must be adjusted for inflation, geography, scope, market conditions and project maturity; a large public facility is not a valid direct benchmark for a small private renovation.
Use the comparison to establish a realistic range and to challenge assumptions such as productivity, escalation, design duration and contingency. Keep the original benchmark visible when the estimate is updated so that improvements in apparent accuracy do not erase the history of the decision.
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3. Investigate the site and existing information early
Investigation should be proportionate to risk, but it should occur before the budget becomes politically or contractually difficult to change. Validate existing drawings and records, survey the site, locate utilities and assess geotechnical, environmental, hazardous-material, access and third-party conditions. For refurbishment, inspect concealed areas and confirm what can actually be opened or isolated.
GAO’s review noted that facility assessments and site surveys were part of agency guidance even though risks that later materialized were sometimes not identified. The control is not simply commissioning a survey; it is making sure findings are incorporated into quantities, methods, schedule and contingency.
4. Build a risk-linked estimate and contingency
Separate the estimate into a defined base scope, assumptions, estimate range, risk register, mitigation actions and contingency allocations. For each significant risk, record its probability, potential severity, owner, response and expected dollar value. FHWA describes construction, design, management and other contingencies; the amount should reflect the risk remaining after mitigation, not an unexplained percentage.
Contingency is not free money. Establish who may draw it, what evidence is required and how the balance is reported. Reassess it when design advances, bids arrive, unknown conditions are resolved or the schedule changes. A project with immature design and uncertain ground may require more risk allowance than a repeatable, well-investigated project, but the exact percentage cannot be set responsibly without those facts.
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When bids arrive, compare each return with the estimate at the level of scope, quantities, labor, materials, preliminaries, schedule and exclusions. Investigate qualifications, provisional sums, alternative methods and unusually short or long durations. Distinguish a genuine market movement from an estimating omission.
If several credible bidders exceed the budget, pause before awarding. Recheck scope, design, site information, packaging, escalation and funding. The Western Australia example shows that correcting the estimate after an initial failed tender still left every bid above the earlier allowance; the early warning should have prompted a broader review.
6. Set change-control rules before work starts
Write a change procedure into the contract and the owner’s governance manual. Every proposed change should state:
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- The revised scope, drawings and reason for the change.
- Direct cost, indirect cost, schedule effect and risk impact.
- Funding source and contingency drawdown.
- Alternatives considered, including doing nothing.
- Effects on other contracts, permits, operations and completion testing.
- The named person or committee authorized to decide.
Prohibit work on an informal instruction except under a narrowly defined emergency process that requires prompt written confirmation. Approval must precede implementation wherever practicable. Maintain a log of approved, pending, rejected and emergency changes so the forecast includes commitments that have not yet become invoices.
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At each reporting period, show the original approved budget beside the current forecast, commitments, actual cost, approved changes, pending changes, contingency drawdown and forecast at completion. Report schedule milestones with the same discipline: baseline date, current forecast, slippage, cause and recovery action.
Explain variances early, assign an accountable owner and record the decision. Do not replace the baseline with a rebased estimate without retaining the original approval and every intervening change. This preserves visibility into growth and gives decision-makers time to reduce scope, change sequence, add resources or reset the funding envelope before options disappear.
8. Match oversight to project risk
Appoint a clearly accountable owner-side lead and a governance group with enough technical, commercial and scheduling expertise to challenge the estimate. Define escalation thresholds for contingency use, unresolved risks, forecast cost, schedule delay and changes to benefits. The governance group should be able to stop work on an unauthorized change, require additional investigation and recommend a reset when costs threaten a fixed capital envelope or a significant schedule overrun occurs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare delivery and contract approaches
There is no universally superior contract form. Compare alternatives against the conditions that drive exposure:
| Decision factor | Questions for the owner |
|---|---|
| Scope and design maturity | Is the design sufficiently complete to price risk, or will the contractor be developing major elements? |
| Historical cost evidence | Are there genuinely comparable completed projects, with reliable location, size and escalation adjustments? |
| Risk allocation | Which party can actually control ground, design, interface and market risks, and are the change provisions workable? |
| Duration and season | Will weather, permits, procurement lead times or financing make a longer programme materially more expensive? |
| Contract interfaces | How many packages and third parties must be coordinated, and who owns the gaps between them? |
| Tender credibility | Do bids include realistic preliminaries, exclusions, qualifications and schedule assumptions? |
| Reporting and approvals | Can the owner see commitments, changes, risks and forecast completion cost in time to act? |
FHWA notes that design-build may reduce some construction claims arising from design errors in its major-project experience. That observation does not make design-build best for every project; capability, scope maturity, market capacity and the owner’s desired control still matter. Whichever route is selected, preserve a clear baseline and enforce the same change and reporting discipline.
Warning signs that the budget is becoming unreliable
- The estimate has no written basis, exclusions or record of assumptions.
- Design, benefits or operational requirements are still being defined after funding approval.
- Site surveys, utility records or hazardous-material investigations are incomplete.
- Contingency is a single percentage with no risk register or drawdown rules.
- Several bids exceed the estimate, or bids rely on major qualifications and provisional sums.
- Work begins on verbal instructions or variations are approved after commencement.
- Reports show only the latest forecast and omit the original baseline.
- Cost and schedule are reported separately, so delay effects are not visible.
- No one has authority to challenge assumptions or escalate a threatened funding limit.
Each sign calls for a specific response: document the scope, complete the investigation, rebuild the risk allowance, analyze tender differences, stop unauthorized work or restore baseline reporting. Continuing without correction converts uncertainty into a likely claim or funding decision later.
What the available figures do—and do not—tell you
| Figure | Context and limitation |
|---|---|
| 62% average cost overrun; 7 in 10 building projects over budget | Historical reference-class data reproduced in Homes England’s 2024 guidance. It describes a category-level sample, not a forecast for an individual building or all construction. |
| $4.14 billion to $4.51 billion, a 9.1% net increase | New York City Comptroller, 2025, covering 18 Department of Design and Construction construction-phase projects with budgets of at least $100 million. Many were unfinished; overhead was excluded and final growth may be higher. |
| 11 of 18 GSA projects with unforeseen conditions | GAO’s 2016 review of selected repair and alteration projects; nine had cost increases and four had schedule delays. The limited sample should not be generalized to all projects. |
These figures come from different jurisdictions, project types, dates and methods. They are not directly comparable and do not establish a universal overrun rate or contingency percentage. Small private projects may face different procurement, reporting and governance constraints than public megaprojects.
Quick Recap
Owner checklist before approving construction funding
- Confirm the scope, performance requirements, exclusions, interfaces and definition of completion.
- Record design maturity and every material estimate assumption.
- Compare cost and duration with genuinely similar completed projects.
- Complete proportionate surveys of ground, utilities, existing structures, environmental hazards and access.
- Build a risk register with probability, severity, mitigation, owner and dollar allowance.
- Set change thresholds, approval authority and an emergency instruction process.
- Define reporting that retains the original baseline and shows forecast completion cost, schedule and contingency.
- Set escalation triggers and ensure the owner-side team can challenge and reset the project when necessary.
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