It is for the party who seeks

Practical application:

Under French law (Cass. civ., 5 Dec. 2011, n° 11-03.7), you must prove both the existence and cost of damage. Without clear before-and-after photos, a detailed check-in inventory, and professional repair quotes or invoices, your claim will fail. Immediately gather all evidence—take date-stamped photos, obtain written estimates, and keep receipts. If you lack proof, consider requesting a judicial expert appraisal, but act quickly; legal delays apply.

Practical application:

The 2011 Cour de cassation ruling means you must present solid evidence of the defect and its repair cost. For hidden defects, you need a timely expert report (within two years of discovery) and all relevant documents: pre-purchase diagnostics, correspondence with the seller, and detailed quotes. Without these, a judge will dismiss your claim. Retain every document linked to the property and, upon discovering a defect, immediately commission a certified expert to assess and quantify the damage.

Practical application:

The legal principle here is clear: a party claiming compensation must prove the damage with concrete evidence. As a co-owner, you should document damage immediately with time-stamped photos, notify the property manager (syndic) in writing, and obtain multiple repair quotes before starting work. After repairs, keep all invoices and bank statements. If another party is liable, you must send a formal demand letter (mise en demeure) within the legal time limit. Without such proof, even a valid claim will be rejected.