The parish church in Kościan was built in the gothic style in the years 1333-1356. The first one was the central nave, two narrow side aisles and a tower from the west side, then after some time the chancel. Two rows of chapels were added to the temple aisles from the end of the 14th century to the 16th century. According to the then habits, they were funded by guilds and brotherhoods operating in the city. After the great fire of the city in 1503, chancel was expanded, by the chapel of St. Anne. In 1547 the tower was burnt, which was rebuilt at the end of the 16th century. After another destruction caused by a hurricane in 1711, it was preserved only to the height of the walls of the nave. Due to the impoverishment of the city and its residents in the second half of the seventeenth and eighteenth centuries, the temple has not been subject to major changes since that period.
The church is orientated, made of brick in monk and flemish bonds, hall and three-nave. It was built on the projection of an irregular rectangle ended from the east by an irregular seven-sided part, with a four-sided tower from the west, in which there is a porch. The main corpus is a four-span with very narrow aisles. Along with them, there are rows of chapels, of varying heights and depths, on a plan generally similar to a square. In the chapels on the third to the west, instead of chapels there are porches: north rectangular and southern on the plan of the trapezoid, both with chapels on the first floor. The chapels are connected with aisles by pointed archs of different heights. To the east is a four-span chancel, slightly wider than the central nave. In two western spans of the aisle of the northern chancel, the sacristy with the chapel of St. Anne on the floor were added.
Several types of vaults were used in the Kościan temple: a rare net vault in the central nave, the chapel of St. Anna and in one span of the chancel, the stellar vault in the chancel, the aisles and in one of the chapels, and the rib and barrel vaults in the remaining parts of the temple. The most valuable equipment in the church is a late gothic altar from 1507.
Webpage zabytek.pl, Kościół pw. NMP Wniebowziętej Kościan.