The plant is being built by Darlington-based J and RM Richardson and was designed by Darlington’s Architects Design Group.
Bosses say it will be twice the size of Subsea’s current home, offering scope to double its workforce to more than 70 staff and handle bigger contracts for oil, gas and energy clients.
The 40,000sq ft site is expected to be finished in August.
The company employs 44 workers and specialises in launch and recovery systems that deploy remotely operated vehicles for subsea work across the world.
It also designs and makes pipeline repair and subsea sealing systems used in the offshore industry, and construction of the factory comes just over a year after Mr Moon set a target to double sales of £7.5m in three years.
He said: "The new building will allow us to build bigger machines and carry out increased testing, so we don’t have to use external facilities.
"The fact the new building will have extra height means we can put larger cranage in, and it will also allow us to dedicate space to research and development.
"I always envisaged the company would double in size, and by increasing the production facility it means we can look at larger work and achieve this goal.
"Last year, I believe we would won a large order from a client if we had a bigger facility, so this move is a very positive one.
"We wanted to stay in the region and looked at other places, but Darlington had everything to keep us local.
"You have to be flexible and ready to move rapidly in this industry, and such an investment will allow us to do that and strengthen our future. It is all about getting your customers and keeping them for life."
He added the firm will continue to use its existing site for some production, refurbishments and storage.