Contract law is the same everywhere in Switzerland, maybe it is interpreted differently in Ticino but I think it's worth it to become familiar with the Obligationenrecht.
I'd never let a construction company do any work at a place I own without written contracts. It's just asking for troubles on both the financial aspect and the exact extent of construction work.
I think you should kindly insist on a contract "for your files". I don't know about cultural differences but wouldn't overrate them anyway. In situations where costs can't be estimated to the last Rappen in advance (like it might be the case in construction and similar handicraft businesses), it's also possible to specify a bandwidth for the costs and this would be a binding offer unless the company states otherwise in a clause. Time to read that OR...