A person who has hired a non-Jewish contractor to build a home may allow the contractor to continue the building during the period of the Three Weeks, between Shiba Asar Be’Tamuz and Tisha B’Ab. In fact, the contractor may continue building even on the day of Tisha B’Ab. The reason behind this Halacha is that since it is customary to build homes through a contractor, as opposed to hiring day laborers, anyone who sees the construction will immediately realize that the builder was not hired especially to build on Tisha B’Ab. They will naturally assume that the builder was hired for the project, and he comes to work on the project at his convenience, and thus he was certainly not hired specifically for that given day.

It goes without saying that this applies to synagogues, as well; if a non-Jewish contractor was hired to build a synagogue, the building may proceed during the Three Weeks and even on Tisha B’Ab.

This is the ruling of Rav Pinhas Zevihi (contemporary), and of Hacham Ovadia Yosef, in his work Hazon Ovadia – Ta’aniyot, p. 329.