How to Calculate Prolongation Cost in Germany (VOB/B Notice Rules & BGB §650c)

Prolongation costs—the time-related expenses incurred by a contractor when a construction project is delayed through no fault of its own—are among the most commercially significant and frequently disputed elements of construction claims in Germany. These costs typically include extended site overheads, labour, equipment hire, site supervision, insurance, financing, utilities, and other ongoing project expenses that continue to accrue during an extension of time. As Germany’s construction sector continues to face economic uncertainty in 2026, including rising inflation, higher labour costs resulting from new collective wage agreements, fluctuating material prices, and persistent supply-chain disruptions, the financial impact of project delays has become increasingly severe. Even a relatively short delay can substantially increase project costs and significantly reduce or eliminate a contractor’s expected profit.

Recovering prolongation costs, however, requires more than demonstrating that a delay occurred. Contractors must establish that the delay was caused by events beyond their control, demonstrate a direct link between the delaying event and the additional costs incurred, and comply with contractual notice and documentation requirements. The legal basis for such claims may differ depending on whether the project is governed by the German Civil Code (BGB), the Construction Contract Procedures (VOB/B), or bespoke contractual terms. Understanding how to calculate prolongation cost and substantiated within Germany’s legal framework is therefore essential for contractors, employers, quantity surveyors, claims consultants, and construction lawyers seeking to effectively manage risk and resolve construction disputes.

This guide sets out the complete process: choosing between the VOB/B contractual route and the BGB §650c statutory route, meeting notice and evidence requirements, applying accepted calculation methods, and assembling a claim that German courts or arbitral tribunals will take seriously.

Which legal route applies, VOB/B or BGB §650c?

Before any number-crunching begins, the claimant must determine which legal regime governs the prolongation claim. Getting this wrong, or failing to comply with the procedural requirements of the applicable route, can extinguish an otherwise valid entitlement entirely. The two primary routes are the contractual regime under the Vergabe- und Vertragsordnung für Bauleistungen Teil B (VOB/B) and the statutory regime under BGB §650c.

When VOB/B governs, how it modifies statutory claims

Where the parties have incorporated VOB/B into the construction contract, standard practice for all public-sector projects and widespread in private commercial construction, the contractor’s entitlement to additional remuneration for changed or additional works, including the time-related cost consequences of those changes, is primarily governed by VOB/B §2(5) and §2(6). These clauses create a contractual mechanism for price adjustment that, in practical terms, displaces or supplements the statutory rules of the BGB. Crucially, VOB/B imposes its own notice obligations: failure to announce additional cost consequences before executing the changed work can result in a partial or total loss of the claim.

When BGB §650c applies, interim payment and the 80 % rule

BGB §650c, introduced as part of the 2018 construction contract law reform (Bauvertragsrecht), provides a statutory mechanism for adjusting remuneration when the employer orders a modification. Under §650c(1), the contractor is entitled to remuneration calculated on the basis of the actual costs incurred plus reasonable mark-ups for overhead and profit. The provision’s practical power lies in §650c(3): the contractor may demand an interim payment of up to 80 % of the remuneration calculated under §650c(1), even before the final amount is agreed, giving it a rapid cash-flow tool.

Industry observers note that the OLG München has confirmed that §650c can apply even where VOB/B has been incorporated, provided the statutory prerequisites are met, making it a valuable fallback or parallel route for prolongation cost calculation in Germany.

Practical compliance decision checklist

• Step 1. Review the contract: is VOB/B incorporated in full, in part, or not at all?
• Step 2. Identify whether the delay event qualifies as a variation, ordered change, or employer-risk event under the applicable regime.
• Step 3. Serve notices under both VOB/B and BGB §650c where possible, preserving parallel entitlements is standard prudent practice.
• Step 4. Begin contemporaneous evidence collection immediately (see evidence section below).

Notice and timing obligations under VOB/B §2(5)–(6), what to serve and when

VOB/B notice requirements are the single most common reason prolongation claims fail in Germany. The contractor must announce its entitlement to additional remuneration before beginning execution of the changed or additional work. Under VOB/B §2(5), where the employer changes the scope of an already-contracted service, the contractor must announce the resulting price consequences. Under §2(6), additional services not originally contracted must be ordered in writing, and the contractor must again flag any price impact, including prolongation cost, before execution.

Content of a compliant notice

A notice that merely states “we reserve our rights” is insufficient. German case law consistently requires the notice to contain specific factual content:

• Description of the change or additional work. Identify the instruction, drawing revision or site condition that triggers the claim.
• Causal link to delay. Explain, at least in outline, how the change affects the programme, which activities are delayed and by how long.
• Quantification markers. State the categories of cost affected (site staff, plant, temporary works, head-office overhead) and, where possible, provide an initial estimate or the basis on which the claim will be calculated.
• Express reservation of entitlement. Confirm that the contractor will claim additional remuneration under VOB/B §2(5) or §2(6) and reserves the right to submit a fully particularised claim.

Deadlines, extensions and common pitfalls

The critical deadline is before execution: once the contractor proceeds without notice, the claim is at serious risk. There is no fixed calendar deadline (e.g., 28 days), but promptness is expected. Common pitfalls include: serving notice only to the site team rather than to the contractual employer; omitting the causal link between the change and the delay; and failing to update the notice when the scope or duration of the delay evolves. Each of these can be fatal to recovery.

Sample short-form VOB/B notice

A compliant short-form notice should follow this structure: (1) date and project reference; (2) identification of the employer’s instruction or change event; (3) statement that the change will result in additional time and cost; (4) outline of affected programme activities and estimated duration impact; (5) categories of prolongation cost affected; (6) express reservation of rights under VOB/B §2(5) or §2(6). Practitioners should maintain a template and adapt it to each event promptly.

BGB §650c route, interim payment, 80 % rule and calculation triggers

§650c(1)–(3) explained plainly

BGB §650c structures the remuneration adjustment for ordered modifications in three steps. Under §650c(1), the contractor is entitled to remuneration based on actual costs required (tatsächlich erforderliche Kosten) plus appropriate surcharges for overhead and profit. Under §650c(2), where the contractor’s obligation is reduced, the employer may reduce the price, but the contractor retains the right to at least the costs already incurred plus a reasonable profit margin. Under §650c(3), the contractor may demand an interim payment (Abschlagszahlung) of up to 80 % of the amount calculated under §650c(1), even before the parties agree on the final adjusted price. This 80 % rule is the provision’s most powerful feature for cash-flow protection during prolongation.

Interim payment injunctions and timing

Where the employer refuses to pay the interim amount, the contractor can seek a court order. The likely practical effect is that German courts treat the 80 % interim payment claim as a relatively straightforward application, provided the contractor can demonstrate: (a) a valid employer order or deemed order; (b) a prima facie calculation of actual costs; and (c) proper notice. Early indications from appellate practice suggest that courts are willing to grant interim payment even in complex prolongation scenarios, making BGB §650c a commercially attractive route when VOB/B notice compliance is uncertain.

Practical drafting points for claims under §650c

• Base the calculation on actuals. §650c expressly references actual costs, formula-based estimates (Hudson, Emden) are useful cross-checks but may not satisfy the statutory standard on their own.
• Include overhead and profit surcharges. Identify the percentage applied and justify it by reference to the contract or market norms.
• Calculate the 80 % interim figure separately. Present it as a distinct sum to facilitate rapid payment or court enforcement.

How to calculate prolongation cost, methods and chosen approach for Germany

Three principal methods are used internationally, and increasingly in German practice, to calculate prolongation cost. Each has strengths and weaknesses, and the choice often depends on the quality of available records.

Method 1, Hudson formula

The Hudson formula estimates the head-office overhead and profit component of prolongation cost using data from the contract itself. The formula is:

Prolongation cost (H/O element) = (Head-office overhead & profit % from contract ÷ 100) × (Contract sum ÷ Contract period in days) × Delay period in days

Its advantage is simplicity: it requires only three data points readily available from the contract. Its weakness is that it assumes the contract percentage accurately reflects actual overhead absorption, which may overstate or understate the true position.

Method 2, Emden & Ashworth formula

The Emden & Ashworth formula replaces the contract percentage with the contractor’s actual head-office overhead and profit rate, derived from audited accounts:

Prolongation cost (H/O element) = (Actual H/O & profit % from accounts ÷ 100) × (Contract sum ÷ Contract period in days) × Delay period in days

This approach is generally considered more accurate because it reflects the contractor’s real cost structure rather than a bid assumption. It requires disclosure of financial records, which some contractors resist.

Method 3, Actuals (time-related actual cost method)

The actuals method builds the prolongation cost from the ground up: the claimant identifies every time-related resource deployed during the delay period (site staff, plant, temporary facilities, insurance, bonds, utilities) and prices each at its actual daily or weekly cost. This is the most granular and, when well-documented, the most persuasive method, and it aligns closely with BGB §650c’s reference to tatsächlich erforderliche Kosten (actually required costs).

Selecting the method, evidence and commercial realism for German courts

Industry observers expect German courts and tribunals to favour the actuals method where contemporaneous records support it, because it matches the statutory language of §650c and satisfies the general civil-law burden of proof under ZPO §286. The Hudson formula serves as a useful cross-check or fallback where site records are incomplete. The Emden & Ashworth approach occupies a middle ground and is frequently deployed in arbitration. Practitioners should present the primary claim on actuals and include a Hudson or Emden & Ashworth calculation as a reasonableness check. The table below illustrates a Hudson calculation for reference.

Evidence and proof of prolongation cost, forensically sound claims

A claim is only as strong as its proof. German civil procedure places the burden squarely on the claimant to demonstrate both the fact and the quantum of delay-related costs. Understanding what constitutes acceptable proof of prolongation cost is essential before assembling any calculation.

Forensic schedule basics and accepted formats

The foundation of any prolongation claim is a forensic schedule analysis that demonstrates the causal link between the employer-risk event and the extension of the project timeline. Accepted methodologies include Critical Path Method (CPM) analysis, particularly time impact analysis (TIA) or windows analysis, where the delay event is inserted into the baseline programme and its effect on the critical path is measured. German courts expect the schedule to be based on a realistic, contemporaneous baseline rather than a theoretical or reconstructed programme.

Primary documentary evidence

• Daily site diaries. Signed records of weather, workforce, plant on site, activities performed and disruptions encountered.
• Timesheets and attendance records. Individual or crew-level records showing who was on site, for how long, and on which activity.
• Supervisor and foreman reports. Narrative accounts of progress, instructions received and problems encountered.
• Plant and equipment logs. Records of machinery hours, idle time and deployment location.

Secondary and corroborative evidence

• Cost ledger and project accounts. Monthly or weekly cost reports that track actual expenditure against budget.
• Payroll records. Verified payroll data confirming wage rates, overtime and employer social contributions.
• Supplier and subcontractor invoices. Third-party documentation supporting claimed rates and quantities.

Expert reports, scope, methodology and court expectations

German courts frequently appoint their own expert (gerichtlicher Sachverständiger) under ZPO §404 to assess delay and quantum. Claimants should anticipate this by preparing their own expert report to a standard that will withstand scrutiny, using transparent methodology, cross-referencing contemporaneous records, and clearly distinguishing between proven and estimated cost items.

Worked example, step-by-step calculation for a 45-day prolongation

The following worked example demonstrates how to calculate prolongation cost for a hypothetical 45-day employer-caused delay on a German commercial building project. Assumed project parameters: contract sum €12,000,000; original contract period 18 months (540 calendar days); monthly site overheads €95,000; head-office overhead and profit percentage per contract 8.5 %.

Step 1, Establish the daily site overhead rate

Monthly site overheads of €95,000 convert to a daily rate of €95,000 ÷ 30 = €3,167 per day. This figure captures site staff (project manager, site engineer, foreman, administrator), temporary facilities (site offices, welfare, fencing, security), insurances, bonds and utilities.

Step 2, Calculate site-level prolongation cost

Site prolongation cost = daily site overhead × delay days = €3,167 × 45 = €142,500.

Step 3, Calculate head-office overhead contribution (Hudson cross-check)

Using the Hudson formula from the table above: €1,889 per day × 45 days = €85,000.

Step 4, Reconcile and present the total claim

The 80 % interim payment under BGB §650c(3) would be €233,950 × 80 % = €187,160, the amount the contractor could demand as an interim payment even before final agreement on quantum.

Practical claim drafting checklist, how to prepare a claim in construction

When assembling the prolongation claim submission, ensure the following documents and schedules are attached:

• Compliant VOB/B notice(s). Copies of all notices served, with proof of delivery (signed receipt or registered post confirmation).
• Forensic schedule extract. CPM analysis isolating the delay event and showing the critical-path impact in calendar days.
• Site overhead cost schedule. Monthly or weekly breakdown of time-related costs, tied to project accounting records.
• Payroll and wage records. Extracts showing the cost of site staff during the delay period, including social contributions.
• Plant utilisation logs. Equipment idle-time records with daily or hourly rates from hire agreements or depreciation schedules.
• Head-office overhead calculation. Supporting workpaper showing the Hudson or Emden & Ashworth computation, with source data identified.
• BGB §650c interim payment calculation. Separate schedule computing the 80 % figure for immediate enforcement if needed.
• Narrative claim report. A concise document linking the factual events, the programme impact, and the monetary claim, structured for a reader unfamiliar with the project.

Risks, mitigation and litigation posture

Even a well-prepared prolongation claim faces risks. Employers will challenge causation (arguing concurrent delay or contractor culpable delay), the reasonableness of rates, and the adequacy of notice. To mitigate these risks, contractors should: maintain contemporaneous records from day one rather than reconstructing them after the event; engage a delay analyst early enough to influence record-keeping; and consider negotiation or adjudication before formal litigation, which can take two to four years through the Landgericht and Oberlandesgericht. Employers facing prolongation claims should conduct an early independent review of the schedule analysis and consider whether the §650c interim payment exposure justifies early commercial settlement.

Both sides benefit from understanding that German courts scrutinise factual evidence rigorously, early expert instruction and disciplined documentation are the strongest forms of risk mitigation available.

Conclusion

Accurately understanding how to calculate prolongation cost in Germany requires mastery of three linked disciplines: legal compliance (choosing and executing the correct VOB/B or BGB §650c route), rigorous evidence collection (forensic scheduling and contemporaneous cost records), and sound calculation methodology (actuals supported by formula cross-checks). In the current 2026 environment of rising labour and material costs, the financial stakes of getting this process right, or wrong, are higher than ever. Whether you are a contractor seeking to recover legitimate delay costs or an employer assessing exposure, professional legal guidance from a specialist in German construction law is essential.

Explore the construction law practice directory to connect with qualified practitioners who can review your claim, advise on compliance, and help you secure the recovery you are entitled to.

Sources

1. German Civil Code (BGB), §650c (Gesetze im Internet)
2. OLG München, Decision re §650c applicability (Gesetze Bayern)
3. Bundesgerichtshof (BGH), Official jurisprudence database
4. DIN Media, VOB 2019 overview (official standards publisher)