If payment has been agreed upon based on a calculation with the contractor’s explicit guarantee for its accuracy, the contractor cannot demand a higher payment, even if the job required more work and incurred higher costs than anticipated. This does not exclude the application of rules regarding contract termination and modification due to changed circumstances. If payment has been agreed upon based on a calculation without the contractor’s explicit guarantee for its accuracy, and during the work, it becomes evident that an overrun is unavoidable, the contractor must immediately notify the client; otherwise, they lose the right to claim additional costs (Article 643 of the OZ).

If the payment amount in the service contract is determined based on a calculation for which the contractor did not guarantee accuracy, the contractor is entitled to request additional payment if it turns out that the execution of the job will incur higher costs than initially estimated or that a greater scope of work will be required than originally planned.²⁶⁴