Construction Period Modelling
Executive Summary
Key Takeaways
- ✓ The construction phase has no operating revenue and is governed entirely by the construction cost curve, the drawdown profile, and interest during construction, capitalized into the total funding requirement.
- ✓ The commercial operations date (COD) test should be built as a single controlled switch, typically comparing actual or forecast completion against a long-stop date, rather than duplicated across separate construction and operations logic.
- ✓ Contingency drawdown should be modelled as a distinct, triggered mechanism, not folded into the base construction cost curve, so the model can represent both the base case and cost overrun stress scenarios.
- ✓ A construction period model that does not tie the drawdown schedule to the actual construction cost curve will misstate both the funding requirement in any given period and the resulting interest during construction calculation.
- ✓ Delay risk should be modelled explicitly, since a construction delay extends the period over which interest accrues, increasing total funding requirement even before considering any associated cost overrun.
Institutional Definition¶
Construction period modelling is the discipline of representing a project finance model's pre-revenue construction phase as a distinct, funding-driven module: a construction cost curve, a drawdown and interest during construction calculation, a contingency drawdown mechanism, and a controlled commercial operations date (COD) test governing the transition to the operating phase.
Why the Construction Phase Requires Its Own Modelling Discipline¶
Unlike the operations phase, which is governed by revenue, operating costs, and debt service, the construction phase generates no revenue at all. Every cash flow during this phase is a funding event: a cost is incurred, funding is drawn to pay it, and interest accrues on the funding drawn. Getting this phase's mechanics right directly determines the total funding requirement the transaction must be sized for — an error here does not merely misstate a single period's cash flow, it misstates the size of the entire financing.
Core Mechanics¶
The Construction Cost Curve¶
The construction cost curve is the period-by-period profile of costs actually incurred by the contractor, typically derived from the EPC contract's payment milestones or an S-curve profile reflecting the physical pace of construction (slower at the start and end, faster through the middle of the programme). The drawdown schedule described in Drawdown and Funding Mechanics should track this curve closely, allowing only for a short payment-processing lag.
Interest During Construction (IDC)¶
IDC is calculated on the cumulative drawn debt balance each period, using the opening-balance convention consistent with operating-phase debt sculpting (see Debt Sculpting Mechanics) to avoid an unnecessary second layer of circularity. IDC is capitalized, added to the total funding requirement, rather than paid in cash. See Interest During Construction for the full calculation treatment and its structural circularity with the sources and uses statement.
Contingency Drawdown¶
Construction contingency should be modelled as a distinct mechanism, triggered separately from the base construction cost curve, with its own drawdown logic and, where relevant, its own interest accrual (since a contingency drawdown funded by debt itself increases the drawn balance and therefore IDC). Folding contingency into the base cost curve undifferentiated prevents the model from representing a cost overrun stress scenario cleanly.
The Commercial Operations Date (COD) Test¶
The transition from construction-phase to operations-phase logic should be governed by a single controlled switch cell, typically testing actual or forecast project completion against defined milestone and long-stop dates specified in the project's construction and financing documents. This switch should drive every downstream module consistently, rather than requiring separate, independently maintained construction and operations logic that must be kept manually in sync.
COD Test (illustrative):
IF Actual/Forecast Completion Date ≤ Long-Stop Date → Operations logic active from Completion Date
IF Actual/Forecast Completion Date > Long-Stop Date → Delay/default provisions triggered (per financing documents)
Delay Risk¶
A construction delay has two compounding financial effects that the model must represent: first, the direct effect of extending the period over which interest accrues on drawn debt, increasing total IDC; second, any additional cost directly associated with the delay itself (extended contractor mobilization, liquidated damages receivable from the contractor under a fixed-price EPC contract, or additional owner's costs). These should be modelled as separable effects so a delay scenario shows both components of increased cost distinctly.
Common Errors¶
Error 1 — Straight-Line Drawdown Independent of the Cost Curve¶
Assuming funding is drawn evenly across the construction period rather than tracking the actual construction cost curve materially misstates the funding requirement in individual periods and the resulting IDC.
Error 2 — Contingency Folded Into Base Cost¶
Combining contingency with the base construction cost curve into a single line prevents the model from representing a stress scenario in which contingency is partially or fully drawn, distinct from the base case.
Error 3 — Duplicated Construction and Operations Logic¶
Building the construction phase and operations phase as entirely separate calculation blocks, rather than governed by a single COD switch, risks the two falling out of consistency as the model is revised, and makes testing a delayed COD's knock-on effects across the model unreliable.
Error 4 — Delay Effects Not Separated¶
Modelling a construction delay only as an extension of the interest accrual period, without separately representing any directly associated cost (liquidated damages, extended owner's costs), understates the full financial effect of a delay scenario.
Audit Checks¶
Cost curve alignment check. Confirm the drawdown schedule tracks the actual construction cost curve, not a simplified straight-line assumption.
IDC calculation check. Confirm IDC is calculated on the opening drawn balance each period and feeds back into total uses, not estimated statically.
Contingency separation check. Confirm contingency drawdown is modelled as a distinct, triggered mechanism from base construction cost.
COD switch check. Confirm the construction-to-operations transition is controlled by a single switch cell tested against defined milestone and long-stop dates, and that changing the COD date produces a consistent response across every downstream module.
Delay scenario check. Test the model with an extended construction period and confirm both the increased IDC and any directly associated delay cost are correctly represented.
Best Practices¶
| Best Practice | Why It Matters |
|---|---|
| Tie the drawdown schedule directly to the actual construction cost curve | Ensures the funding requirement and IDC calculation reflect the real construction programme, not a simplified approximation |
| Model contingency as a distinct, triggered mechanism | Allows the model to represent both the base case and cost overrun stress scenarios cleanly |
| Govern the construction-to-operations transition with a single COD switch | Prevents duplicated logic from diverging as the model is revised |
| Separate delay-driven interest accrual from directly associated delay costs | Gives a complete, decomposed view of a delay scenario's total financial effect |
Further Reading¶
- IFC, Project Finance in Developing Countries, International Finance Corporation
- Equator Principles Association, Equator Principles IV
Continue Reading¶
Prerequisites¶
- Project Finance Model Audit — the parent pillar
- Project Finance Model Structure
Related Technical Guides¶
Related Glossary¶
Related Products¶
- Financial Model Audit Engine (FMAE) — deterministic structural auditing referenced throughout this guide
How OXXON tests thisRun a free structural check with FMAE
Frequently Asked Questions
What makes the construction phase different to model than the operations phase?
The construction phase has no operating revenue; all cash flow is funding-driven, governed by the construction cost curve, the drawdown schedule, and interest during construction, rather than by revenue, operating costs, and debt service.
What is a commercial operations date (COD) test?
The mechanism, typically a single controlled switch cell, that determines when the model transitions from construction-phase to operations-phase logic, usually driven by comparing actual or forecast project completion against defined milestone or long-stop dates.
How should contingency drawdown be modelled during construction?
As a distinct, separately triggered mechanism from the base construction cost curve, so the model can represent both the base case, in which contingency is unused, and stress scenarios in which some or all of contingency is drawn.
What happens to interest during construction if the construction period is delayed?
The period over which interest accrues on drawn debt is extended, increasing total interest during construction and therefore the total funding requirement, even before considering any cost overrun directly associated with the delay itself.
Why must the drawdown schedule track the actual construction cost curve?
Because a drawdown profile built independently of the actual cost curve, for example a simple straight-line assumption, will misstate the funding requirement in individual periods and the resulting interest during construction calculation, which depends on the timing of drawn debt.
Related Articles
Interest During Construction (IDC)
Interest during construction (IDC), also called capitalized interest, is the interest that accrues on project finance debt drawn during the construction phase, before the project reaches commercial operations and begins generating revenue to service that debt. Because there is no operating cash flow available to pay this interest as it accrues, IDC is typically capitalized, added to the total funding requirement and financed as part of the debt facility, rather than paid in cash during construction. IDC is calculated on the cumulative drawn balance, which itself depends on the total funding requirement, creating a circular reference that is one of the most common structural features of a project finance construction-phase model.
Drawdown and Funding Mechanics
The drawdown schedule translates the total funding requirement from the sources and uses statement into a period-by-period draw of debt and equity during construction, governed by a funding competition rule that determines the relative proportion of debt versus equity drawn each period. This guide sets out how to build pro-rata, equity-first, and debt-first funding competition mechanics, how to sequence multi-tranche debt drawdowns, and how standby facilities interact with the base drawdown schedule.
Sources and Uses Modelling
The sources and uses statement is typically the first schedule built in a project finance model and the first schedule a lender reviews. Building it as a live, formula-driven reconciliation rather than a static summary requires resolving the circularity between total uses (which includes interest during construction, itself dependent on the debt drawn) and total sources (which includes the debt sized against that same total uses figure). This guide sets out the construction sequence and common errors in building a sources and uses statement that reconciles automatically as assumptions change.
Construction Contingency
Construction contingency is an amount of funding reserved in a project finance sources and uses statement specifically to absorb cost overruns during the construction phase, distinct from and additional to the base construction budget. Because a project finance lender's exposure is fixed at financial close while the construction contract's final cost is not fully certain until completion, contingency sizing and its drawdown mechanics, including who bears responsibility for funding a shortfall once contingency is exhausted, is one of the most heavily negotiated points in project finance structuring.
Project Finance Model Structure
A project finance model differs structurally from a standard corporate model because it spans a construction phase with no revenue, an operations phase with a debt-sculpted repayment profile and a tiered cash waterfall, and, for concession-based assets, a defined end-of-term handback or termination position. This guide sets out the module architecture that makes such a model auditable and maintainable, building on the general workbook design discipline with the specific sequencing project finance mechanics require.