Allan Jacobs in his book Great Streets features a section in which he presents painstakingly drawn figure-ground maps of several dozen of the world's best-known cities. Although he calculates the intersections and number of blocks per square mile, both indicators of connectivity and pedestrian accessibility (see Laurence Aurbach's site for more on this topic), no figures are given for the proportion of city land occupied by buildings and rights-of-way (and other public spaces) for each city.
A few quick notes: St. Nicholas Houses is a public housing project in central Harlem composed of cruciform towers on a superblock. New York's figures include Central Park. Measuring park and plaza spaces in organic cities like Paris or Vienna is a bit of a challenge: depending on the area examined, for instance, Paris' figure for parks might be somewhat higher. Fes is just an estimate, as the narrow footpaths of the city's old town are barely visible from above, but in any event it appears to be about the maximum possible built coverage possible without moving to an entirely streetless city.
Hopefully these numbers can provide one additional angle on the density question. I have some ideas how they might be paired with Jacobs' figures and some other measures to provide an even fuller picture of both density and connectivity, but I'll save that for another post.
Related posts: Charting the Grid