Building without SuDS approval in Wales exposes you to SAB enforcement action: temporary stop notices, stop notices and enforcement notices requiring you to stop work or retrofit compliant drainage at your own cost. Failing to comply with a notice is a criminal offence carrying an unlimited fine.
Unlike planning enforcement, there is no retrospective approval route for SuDS. If you build without approval, you face enforcement notices and mandatory remediation, and prosecution if you fail to comply with a notice.