Our most valued source, Rabbi Feinstein, in Ir Tehila, does not give a date for the construction of the first synagogue.
The only source we have on this point is Zonnenberg, in his book The History of the City of Brest-Litovsk (1016-1907). On page 25 he mentions the construction of a synagogue around 1411 with much support of King Witold:
Witold ordered in his decree of 25th September 1411 that clay
and bricks be provided, free of charge, to the
Brest Jews.
Zonnenberg does not provide a source and we believe he was mistaken. King Zigmond I is known to have given almost identical support to a synagogue in Brest exactly 100 years later. Probably Zonnenberg attributed to King Witold the later actions of King Zigmond.
It stands to reason that this first synagogue was constructed within a reasonably short span of years after it was approved, so we choose 1411 to approximate the completion date.
