BG3 comprises two building sections, which were constructed in different eras. The oldest part, on the Oudezijds Achterburgwal side, is the original Administratiegebouw (Administration Building), dating from 1916 and designed by J.M. van der Meij. The oval part, on the square called Binnengasthuisplein, is the later addition designed by Theo Bosch, dating from 1995.
The Administratiegebouw comprises a central middle section with a ground floor, two upper floors and an attic under a hip roof. The extensions on either side are two or three floors with a flat roof. The building used to be connected to the Kinderkliniek, originally a children's hospital, which was demolished and replaced with an extension with a floor plan shaped more or less like an oval, with a basement, ground floor and two upper floors under a flat roof.
The Administratiegebouw was built in 1913-1916, based on a design by J.M. van der Meij. It was originally connected to the Kinderkliniek, the children's hospital behind it, which was demolished in 1991 to make room for a building designed by Theo Bosch (1940-1994). This was posthumously completed in 1995. Pressure from society and an administrative recommendation led to politicians deciding in 1982 that retaining Van der Meij's building was of significant interest. The building is an early Amsterdam School design, and the visible use of concrete was lauded as exceptional.
Most of the Administratiegebouw interior was demolished, however, although the stairwells are still largely in keeping with Van der Meij's design. With the exception of a chimney to the left of the building facade, the exterior still strongly resembles the original design, although Bosch did build on the balconies on the corners.
The new construction is oval in shape, with floors supported by pilotis and with an open floor plan. The old and new parts of the building are connected by an open building section, with a footbridge connecting the second floor of the new part with the old part of the building. The interiors of both the old and the new part have similar floor plans and finishing. As it is today, the new section of the building is largely the same as the original Bosch design.
Source: 2019 P. Vlaardingerbroek, Monumenten en Archeologie gemeente Amsterdam, Beschrijving van Binnengasthuisstraat 9 [Monuments, Historical Buildings and Archaeology Department of the City of Amsterdam, description of Binnengasthuisstraat 9], internal document, compiled on 14 February 2019. With references to, among other things, building files from the Monuments, Historical Buildings and Archaeology Department, Amsterdam City Archives, the latter's image bank, Stadsdeel Centrum (City Centre urban district), Archief Bouw- en Woningtoezicht (Building Control Department Archives).