Those of you Familiar With San Francisco

I live in San Francisco and if you've never been, it's extremely congested. There is very little green space and this includes spacing between buildings be they residential or commercial. In the areas where the bulk of the Dirty Harry movies are filmed, structures are built right next to each other. They don't share side walls, but the side walls of each building are built right up against the other structure. In most parts of the city, there are no side yards or ways to get to the back of the building from the front without going through the building or around the block to an alley (if any). In some areas there are blocks of free standing buildings with walkable perimeters, but specific to filming areas (the Mission, North Beach, Tenderloin, Civic Center, SOMA, parts of the FIDI and Marina etc.) it's more rare. It's entirely feasible to walk the length of an entire block and not have a gap between buildings. Obviously it also depends on the structure. Victorians have peaked roofs and Edwardians have flat ones. Since these different designs coexist next to each other it may be feasible to walk across the roofs, but not very safe or easy.