According to planning law, carrying out building works or a change of use to a property without the necessary planning permission isn't a criminal act. The only type of works that might be are unauthorised advertisements and works to listed buildings.
This would mean that initially most breaches of planning regulations wouldn't result in fines or prison.
If an enforcement notice is served and isn't complied with then it can lead to court action or fines.
Planning enforcement is a power we use with discretion. This means we only use them to put right any harm caused by not complying with planning control.
When there's little or no harm we normally wouldn't take enforcement action. We would take action if the breach of planning means that the original planning application would have been refused.