N.H. Rev. Stat. Ann. § 677:6

In an appeal to the court, the burden of proof shall be upon the party seeking to set aside any order or decision of the zoning board of adjustment or any decision of the local legislative body to show that the order or decision is unlawful or unreasonable. All findings of the zoning board of adjustment or the local legislative body upon all questions of fact properly before the court shall be prima facie lawful and reasonable. The order or decision appealed from shall not be set aside or vacated, except for errors of law, unless the court is persuaded by the balance of probabilities, on the evidence before it, that said order or decision is unreasonable.