Is a completion bond necessary for the completion of construction?

The retention must be "reasonable." Reasonable, for example, is 200% of the cost of defect remediation. So, this has hardly anything to do with the total construction costs.

If your roof is complete botch work, you of course withhold the entire payment for the roof. At least.

A guarantee is not unusual. Many, including me, have a warranty guarantee. In my case, the amount is 10%. But I don't know anyone who has a completion guarantee. That can be better secured through the payment plan.

I quote from the notary contract:

"the buyer is entitled pursuant to §632a III Baugesetzbuch to withhold once 5% of the purchase price already at the first installment to secure the timely completion of the work without significant defects."