Deposits
There is no standard percentage. Match the firm name before you pay.
There is no standard percentage
Some firms ask for an Anzahlung before they order materials. German law does not print one percentage for every trade on a page like this. A large cash sum to a person who will not match the firm name on the card is a reason to stop.
Match the name
The account name should match the firm on the card, or the firm should say in writing why it differs. The firm's own Impressum, the legal notice on its website, is the place it publishes its name and address. Read that before you transfer.
Pay against the written offer. Keep the transfer reference equal to the offer number they gave you.
A locksmith at the door
Agree the total, trip included, before the lock is touched. Do not hand over cash on the landing to someone who arrived without being the firm you called. Night work does not cancel that step.