Key legal question
Whether accessory charges not clearly listed in the written lease could be charged to the tenants
Extracted holding
Only accessory charges specifically agreed in the lease could be imposed; the disputed additional charges were not owed.
Extracted reasoning
Art. 257a(2) CO requires a specific agreement on accessory charges. Conduct over many years cannot replace the lack of a sufficient contractual basis, and later acceptance of invoices without the official procedure cannot amend the contract against the tenants.