https://www.jw.org/en/news/releases/by-region/united-states/towers-brooklyn-for-sale/
https://www.jw.org/en/news/releases/by-region/united-states/jw-headquarters-peak-construction/
No doubt the construction was done largely by volunteer labour as has occurred here in Australia.
Population centers in Australia are widely scattered; so, most of the 3,000 who volunteered to work on the branch facilities at Ingleburn between 1978 and 1983 had to travel at least 1,000 miles [1,600 km]. However, bus travel for groups of volunteers was coordinated, and congregations en route hospitably offered to supply meals and association for the brothers at rest stops. Some of the brothers sold homes, closed businesses, took vacations, and made other sacrifices in order to share in the project. Teams of experienced tradesmen came in—some of them more than once—to pour concrete, hang ceilings, put up fences. Others donated materials.
The majority of volunteers on these projects were unskilled, but with a little training, some of them took on big responsibilities and did excellent work. They learned how to fabricate windows, operate tractors, mix concrete, and lay bricks. They enjoyed a definite advantage over non-Witnesses who do the same kind of work commercially. In what way? Those who were experienced were willing to share their knowledge. No one was afraid that someone else would take his job; there was plenty for everyone to do. And there was strong motivation to do high-quality work, because it was being done as an expression of love for God.
