The archive comprises 25 meters, including 10 meters of file boxes, 10 meters of roller boxes, 3 meters of photo boxes and 2 meters of other packaging. About three-fifths of the archive consists of designs and works. A small portion of these concern urban planning, the vast majority architecture. The early works are mainly present in the form of folded drawings. These are mainly the specification drawings. Most of the postwar works are present in rolled-up form. These often include working drawings. Sketches are rare, sketch plans and preliminary plans are somewhat more common. Documentation of a number of works is also present in the form of articles from newspapers or magazines. Written documents, such as correspondence, are mainly there for the most important works. Of the other works, there are few if any written records. The virtual absence of written specifications is striking. There is no photo documentation of most of the works, only of the most important ones.
Remarkable in the Siebers archive is the large number of documents concerning participations, memberships, publications, trips and lectureships, as well as a series of notebooks on various subjects (including notes on philosophy, theology and religious architecture). Also in the archive is a fine series of training drawings by both Siebers and Van Dael.
The archive also contains works from after Siebers left the firm (1966). All works for which there are no documents in the archive are listed in Appendix I. These are for the most part works from after Siebers' departure, continuing up to and including the Ruys and Bolder period.