The library building is the most monumental of Plečnik's works in Slovenia. In its size and form, it models the former Ducal Court (), which was at the same location and was destroyed in the 1895 earthquake. The building has a square ground plan and is a massive block with a court. The front façade, oriented toward Auersperg Street, was designed as a combination of brick and stone embeddings, some of them archaeological remains from the place. It was modelled in the manner of the Italian palazzo, similar to the house of the Italian architect Federico Zuccari. The handles of the main door end with a little head of Pegasus. The symbiosis of the fragile glass and massive walls in other parts of exterior is entirely original. The side walls are oriented towards Gentry Street and Vega Street. The side entrance from Gentry Street is decorated with a sculpture of Moses, created by Lojze Dolinar.