On van Berckenrode's mapping we see these small structures at the back of Lange Houtstraat 16 and 14. Most likely both or one of them housed the earliest attested synagogue on Vlooienburg (Hell 2024, 66). The Jewish congregation rented it from the owner of the house until 1609, after which they moved to a larger location. They were immediately replaced by a small group of Brownists, puritan christian seperatists from England, who were prosecuted for their religious insubordination in England. A little down the street (Lange Houtstraat 26) they had a larger church for several decades, but some of them had temporarily split off due to some disputes and ended up in this shed.
All over Vlooienburg, and Amsterdam, one could find such synagogues and small churches for various religious groups and split offs, hidden from the streets. By city law it was not allowed to have public places of worship for any other religion than the Dutch Reformed Church (Nederduits Hervormde Kerk). Other religions were allowed to have their places of worship, only their structures should not be recognizable nor be accessible from the main street. Therefore, buildings with side entrances on alleys or structures on the inside of the blocks of housing had to be used.
An alley running through the house gave access to the backyard, and would have been used first by the Jewish worshippers and later the Brownists to access their churches.
It is well possible that the structures are not accurately represented on van Berckenrodes map of Amsterdam. Considering their use, they are likely to have been larger than shown here.
Hell, M., 2024. Verloren wereld in de Amstelbocht. Leven op Vlooienburg, 1600-1815. Walburg pers.