A survey of 700 firms in building services engineering revealed that only one in five feared leaving the EU will have negative implications.
Almost half said they believed the cost of materials will increase, while most said they did not rely on EU migrant workers.
Building Engineering Services Association chief executive Paul McLaughlin said: "Our survey clearly shows that many contractors are cautiously optimistic in the wake of the Brexit vote.
"In fact, putting these findings alongside the brighter than expected data for the UK economy as a whole gives us a much more upbeat feel than could reasonably have been predicted back in June."
Steve Bratt, chief executive of the Electrical Contractors' Association, added: "Contractors are telling us that they want the UK to maintain a single market, while they are less concerned about ensuring freedom of movement."