William Playfair's building — like its neighbour, the Royal Scottish Academy — was designed in the form of an Ancient Greek temple. While Playfair designed the RSA in the Doric order, the National Gallery building is in the Ionic order. The main east and west elevations have plain pilastrading with the higher central transverse block having hexastyle Ionic porticoes. Paired Ionic columns in antis are flanked by tetrastyle Ionic porticoes at north and south. The design reflects the building's original dual purpose being divided longitudinally with the exhibition galleries of the RSA to the east and the National Gallery to the west. Playfair worked to a much more limited budget than the RSA project, and this is reflected in his comparatively austere architectural style. He may have drawn inspiration from an 1829 scheme for an arcade of shops by Archibald Elliot II, son of Archibald Elliot. Playfair's National Gallery was laid out in a cruciform plan; he originally planned to build towers at the corners of the transverse central block, but these were abandoned during the project. When the RSA moved into the former Royal Institution building in 1912, the Office of Works Architect for Scotland, William Thomas Oldrieve remodelled the NGS interior to house the National Gallery collection exclusively. In the 1970s, when the gallery was under the direction of the Department of the Environment, the internal accommodation was extended. An upper floor was added at the south end in 1972, creating five new small galleries, and in 1978 a new gallery was opened in the basement to house the Gallery's Scottish Collection. The new Princes Street Gardens entrance and underground space opened in 2004 was designed by John Miller and Partners. Construction took five years and cost £32 million. The area contains a lecture theatre, education area, shop, restaurant, an interactive gallery, and a link to the RSA building. In January 2019, construction work began on a project to alter the lower level areas and to create extended exhibition space. It is planned that the Princes Street Gardens entrance will become the main entrance of the gallery; to facilitate access, East Princes Street Gardens is being re-landscaped with sloping paths and 52 trees have been felled, to be replaced with 22 newly planted saplings. The research facilities at the Scottish National Gallery include the Prints and Drawings Collection of over 30,000 works on paper, from the early Renaissance to the late nineteenth century; and the reference-only Research Library. The Research Library covers the period from 1300 to 1900 and holds approximately 50,000 volumes of books, journals, slides, and microfiches, as well as some archival material relating to the collections, exhibitions and history of the National Gallery.