A building can only be built and then upgraded for a total of 6 times (construction site -> small house/trailer -> big house -> small apartments -> big apartments -> small tower -> big tower/skyscraper). After that, the building is done and can't be upgraded any further. If your land value is low, fully upgraded buildings will look like old stone sky scrapers from the '30s and '40s. If you've since increased land value that the building sits on (through booster buildings like Education, Gambling, Enterntainment, Parks, etc), fully upgraded buildings will be stuck as the old buildings they previously were. Like in real life, you need to bulldoze them down and rebuild them from scratch to get taller glass covered skyscrapers.
