The amount of the costs of remedying defects must be conclusive. Lump sum offers and offers for work on an hourly rate are also admissible if they are described in concrete terms. If the purchaser has estimated the costs of remedying the defect, the submission of evidence in the form of an expert opinion is generally sufficient to assert the claim in court (BGH ruling of 8 May 2003 - VII ZR 407/01).