A trial de novo is a brand new trial, held as though a prior trial never happened. In such trials, the facts of a case are considered all over again and a new verdict is rendered. This type of trial may be held after a successful appeal arguing that a prior case was not considered properly. It is ordered by a judge, typically an appellate judge, with the goal of preventing miscarriages of justice and ensuring that people get fair and reasonable trials when they enter the legal system.
The trial de novo proceeds like no trial has been held to decide the matter before. Both sides of the case are required to present their evidence and arguments anew, and it is possible to introduce new material to the trial, as long as it conforms with the rules of evidence. The trial may be held in front of a judge alone or before a judge and jury, depending on the nature of the case. Once all the facts have been brought up in court, deliberations can begin to reach a verdict.
This is in contrast with an appeal, where people attempt to argue that a verdict in a prior case was erroneous because of a matter of law. In appeals, new factual information cannot be introduced and the case is not judged on its merits, but on whether the law was followed while the case was tried. People can appeal verdicts they feel are unjust and the appeal may ask for a trial de novo if it is believed that a new trial is the only way to achieve a just outcome in the case.
The provision of a trial de novo is not considered a violation of laws against double jeopardy, laws prohibiting people from being tried for the same crime twice, because the prior trial is treated under the law as though it never existed. The slate is wiped clean and the case starts all over again. Legally, the outcome of the prior trial cannot be considered in the new trial and the court must reach an entirely new verdict on its own.
Holding an entirely new trial can be a costly endeavor for all parties involved. The people in the case must pay legal fees, consulting fees for witnesses, and other costs related to the trial. Meanwhile, the government must pay the judge and set aside a time in the court's schedule for the new trial to be held. The added costs are considered when evaluating a request for a trial de novo and people must show that a new trial is clearly legally necessary.