The standards relating to the right to refuse acceptance (Section 640 (1) sentence 2 BGB, Section 12 (3) VOB/B) state that acceptance of the work may not be refused due to insignificant defects. There is no statement on the required degree of completion of the work as a prerequisite for acceptance.
However, the question of what degree of completion the work must have reached in order to be considered ready for acceptance is very important, particularly in the case of plant construction, which is usually very complex.