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Construction costs change because an estimate is a dated forecast, not a context-free promise. It is based on a particular design version, quantities, market prices, schedule, site information, assumptions and exclusions. As those inputs become clearer—or as the owner changes the work, hidden conditions appear, prices move, or access and timing change—the forecast can change and the contract price may change through an authorized adjustment. A revised estimate, an approved change order and a contractor’s request for more money are different events. Your signed agreement and applicable local law determine which adjustments are allowed.
Start with the estimate’s basis, not just its total
Two estimates can show different totals without either being mathematically wrong if they cover different scopes, design versions or dates. The U.S. Government Accountability Office’s Cost Estimating and Assessment Guide treats scope, schedule, technical baseline, work breakdown, assumptions, source data, methodology, risk analysis, documentation and updates as connected parts of a reliable estimate. You can review that framework in the GAO Cost Estimating and Assessment Guide.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Ask when prices were collected, which quantities were measured and which were assumed, what drawings and specifications were used, and what the estimate excludes. An early conceptual estimate may carry broad allowances for systems that have not been designed. A later estimate may rise because it finally includes structural details, fire protection, utility work, finishes or site work that were undefined earlier. It can also fall when the design is simplified or an item is deleted. Compare like with like: the same scope, design maturity, assumptions and market date.
Six common reasons the number changes
1. Design development exposes previously undefined work
Drawings evolve from a concept into coordinated construction documents. Dimensions become fixed, engineering is completed, specifications identify exact products, and quantities are measured instead of guessed. Each refinement can reveal labor, equipment, procurement or coordination work that an early estimate could not price accurately. Conversely, value engineering or a reduced requirement can remove cost. Treat the difference as a change in the estimate’s basis until you identify a specific scope change; calling every later total an “overrun” hides the real cause.
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2. Owner decisions and design changes add dependent work
An upgraded finish, moved wall, revised layout, additional outlet, changed equipment or new performance requirement can affect more than the visible item. It may require redesign, demolition, permitting, long-lead purchasing, new supports, electrical capacity, fire-stopping, testing or resequencing. A deletion may create a credit, but only if the contract allows the associated labor, material and markup to be removed. Put the requested revision and its effect on completion time in writing before work proceeds.
In a 2019 review of federal construction, the GAO reported that complex change estimates can take time to develop and that miscommunication can result in unauthorized work and additional review. That process evidence comes from federal projects; it does not establish a standard private change-order markup or decide a homeowner’s legal rights. See GAO’s report on federal contract changes.
3. Existing conditions differ from the information available before work
Soil, rock, groundwater, buried utilities, framing, wiring, plumbing, moisture, hazardous materials or previous alterations may not match surveys and records. Opening a wall or excavating can reveal work that was impossible to quantify safely at tender. Incomplete drawings and missing building information are recurring contributors to this risk.
The GAO’s 2016 review examined 18 selected U.S. General Services Administration repair and alteration projects valued at $2 million or more. Eleven experienced unforeseen conditions; nine had a cost increase associated with those conditions and four had schedule delays. The sample was non-generalizable federal work, not a representative statistic for residential construction. The findings are useful for understanding the mechanism, not for setting a homeowner percentage. The report is available at GAO-16-273.
4. Labor and material prices move after the estimate date
A long gap between pricing and procurement exposes the project to wage changes, supplier increases, tariffs, shortages and shifts in demand. This is market escalation, not a scope change. The agreement may keep that risk with the contractor, assign it to the owner, or provide a defined adjustment formula. A request for more money is not automatically valid or invalid: check the price-adjustment clause, notice deadlines, covered commodities and required evidence.
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Pennsylvania Department of General Services defines the concept this way: “Cost Escalation is a markup to address future costs due to market inflation, cost increases, and changes in cost due to code or practice.” Its 2025 standard says escalation should be based on current market trends and published construction economic predictions, dated to the proposed midpoint of construction and documented in the basis of estimate. That agency guidance is not a universal private-contract rule; read the full Pennsylvania DGS Cost Estimating Standards.
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5. Schedule, access, occupancy and phasing reduce productivity
A delayed start can move purchasing into a more expensive market or cause a crew to remobilize. A remote site may require longer deliveries and travel. Security checks, restricted delivery windows, an occupied building, dust controls, protection of residents, night work, limited storage and phased handovers can all add supervision, temporary works and labor hours even when the physical design is unchanged. Pennsylvania DGS lists local market or remoteness, security access, occupied facilities, productivity constraints and phased work among project-specific estimating considerations.
Do not assume that any delay creates a right to increase the contract price. Responsibility for the delay, the agreement’s notice and extension provisions, proof of added cost and local law all matter. A contractor who failed to order materials on time may face a different result from an owner-directed suspension or an undisclosed site restriction.
6. Contingency names do not mean the same thing
Ask what each allowance covers, who controls it, whether it is already included in the quoted total and who must approve its use. Design contingency addresses uncertainty while the design is developing. Owner contingency is a reserve controlled by the owner for selected changes or unknowns. Construction contingency may be held for field changes. Escalation is a separate allowance for future market movement. None is automatically a general-purpose fund, and using two allowances for one event can double count.
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Pennsylvania DGS’s 2025 standards illustrate how specific a system can be: they list design-contingency ranges of 15–20% at programming or concept, 10–15% at schematic design, 5–10% at design development and 0% at interim or construction documents. The same standards say their construction contingency is held for construction change orders and is not included in that agency’s cost estimate. These are Pennsylvania DGS practices, not recommendations for every project.
How to tell a revised estimate from a contract change
An estimate revision updates a forecast before or outside the binding price. A change order modifies the contracted scope, price or time through the procedure in the agreement. A payment application or informal request is not necessarily either one. Read the definitions, allowances, unit prices, notice rules, approval signatures and documentation requirements in your contract. A fixed-price label does not erase written change provisions, differing-site-condition clauses, owner-directed changes or statutory requirements; nor does it guarantee that every later cost can be passed through.
For a disputed or substantial amount, obtain advice from a qualified construction professional or lawyer licensed where the project is located. Entitlements, taxes, licensing, consumer protections and permissible escalation clauses vary by jurisdiction and contract.
What to request when the price changes
Ask for a written change proposal before authorizing extra work. Keep the original estimate, every drawing and specification revision, the signed agreement, meeting notes, photographs and schedule updates together. The proposal should answer each of these questions:
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- What changed? Identify the exact work added, removed or revised and cite the drawing, specification, instruction or field condition that prompted it.
- Why did it change? Classify the cause as an owner decision, design development, differing condition, code requirement, escalation, delay or correction of an omission.
- How was the price calculated? Show quantities, labor hours or rates, materials, subcontractor and equipment costs, applicable overhead or profit markups, taxes and credits for deleted work.
- What happens to the schedule? State procurement effects, lead times, resequencing, added days and any required access or temporary protection.
- What contract authority applies? Identify the clause, allowance, unit price or written directive that permits the adjustment, plus notice and approval deadlines.
- How are allowances treated? State whether contingency or escalation was already included, who controls it and how the proposal avoids charging twice for the same risk.
Do not let urgency turn an undocumented instruction into an assumed approval. If emergency work is necessary to protect people or property, document the condition, instructions, photographs, time and materials as soon as practical and follow the agreement’s emergency procedure.
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A practical comparison checklist for two estimates
Place the documents side by side and mark differences under these headings:
- Date and market basis: pricing date, proposed construction midpoint, wage assumptions and supplier quotations.
- Scope and design maturity: drawing and specification revision, engineering completion and owner selections.
- Quantities and exclusions: measured quantities, allowances, unit rates, alternates, temporary works, permits, utility fees and site restoration.
- Contract risk allocation: fixed price, cost-plus or unit-price structure; change-order process; differing-condition and escalation language; notice requirements.
- Contingency ownership: design, owner and construction reserves, their percentages or amounts, and whether they are inside or outside the total.
- Site and existing conditions: survey quality, geotechnical information, concealed components, utilities, hazardous-material assumptions and access limits.
- Execution constraints: occupancy, security, work hours, phasing, storage, remoteness, delivery routes and the schedule used for labor and material ordering.
This comparison often shows that the apparent increase combines several categories—for example, a fuller design, a deleted allowance, a documented owner upgrade and a market adjustment. Separating them makes negotiation and budgeting more rational than arguing over one unexplained total.
What the available figures do—and do not—tell you
| Figure | What it measures | Important limitation |
|---|---|---|
| 11 of 18 projects | Selected GSA repair and alteration projects with unforeseen conditions in the GAO’s 2016 review | Non-generalizable federal sample; not a residential overrun rate |
| 9 of 18 projects | Projects in that same review with a cost increase associated with unforeseen conditions | Same selected sample and federal context |
| 4 of 18 projects | Projects in that same review with schedule delays associated with unforeseen conditions | Same selected sample and federal context |
| Most changes finalized within 60 days | U.S. Army Corps of Engineers contract-change data reported by GAO for 2013–2018 | One federal agency’s data; not a private-project deadline |
| 15–20%, 10–15%, 5–10%, 0% | Pennsylvania DGS 2025 design-contingency ranges at concept, schematic, design-development, and interim/construction-document stages | Agency-specific standards, not universal homeowner allowances |
No reliable nationwide residential percentage is established here. A project-specific risk register, current quantities, documented allowances and a realistic schedule are more useful than applying a single rule-of-thumb overrun figure.
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- Require the estimate to identify its drawing revision, pricing date, inclusions, exclusions and assumptions.
- Obtain the best available survey, geotechnical information, utility records and existing-building documentation before committing to a final price.
- Make owner selections and performance requirements explicit, including brands only where necessary, acceptable substitutions and lead-time-sensitive items.
- Define contingency categories, dollar amounts, control and approval in the contract rather than using one unexplained percentage.
- Set a written change-order workflow with response times, backup, schedule effects and authority to proceed.
- State who bears escalation and what evidence or formula applies if an adjustment is permitted.
- Plan access, occupancy protection, storage, delivery windows, security and phasing with the builder; price those constraints before construction.
- Update the estimate at agreed design milestones and when market or schedule assumptions materially change.
The central question is not simply “Why is the final number higher?” It is “Which input changed, who bears that risk under the agreement, and was the required approval obtained?” Answering those three questions turns a frustrating surprise into a traceable cost decision.
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