Most of the great museums built around the turn of the 20th century have been added to, some numerous times. The 640,000-square-foot 1928 Philadelphia Museum of Art is an exception. Although many of its interior spaces had been altered, the honey-toned Kasota-limestone exterior remains almost unchanged from when it was completed by Horace Trumbauer and his senior designer, the African-American architect Julian Abele, in collaboration with Zantzinger, Borie, and Medary. Presiding over the city from its highest point, a Greco-Roman temple, enlivened by polychrome terra cotta, forms the heart of the museum, from which extend two gallery wings that enclose a terrace atop a monumental stair that faces downtown. As Philadelphia’s most prominent building, adding to it is deemed inconceivable.