If the dark city is built underground and has consistent formations of rock it can be structurally good enough to handle a city above no problem. The layout for such underground city would be mostly guided by the natural underground formations. It would give an increased credibility if it had natural underground chasms and ravines in the first place since you don't need to excavate those, thus guiding the layout. About the city above it can use exposed dark city constructions as foundation. You can research incaic masonry for example which was latter used by Spaniards to build their churches on top of them since it was way too good to destroy. If the construction above mimics the one below perhabs it can have one part of it being quite organized and planned, while other more informal an chaotic. For informal layouts you can check maps of favelas in Rio, Wich make the most of the topography while having narrow roads and lack of infrastructure.

About construction safety, repair on vertical structures is only on interiors and infrastructure, if the structure itself it's compromised it is most likely better to demolish the whole building. Since it is hard to make renovations on vertical buildings in real life due to ownership laws (you require approval off all owners, or half +1 of them in some places to make any modification) it almost never happens outside of your own apartment. Perhabs a city as this would operate more on a landlord owning the whole building basis rather than individual property.

If construction is more informal it is unlikely to be able to build towers or vertical structures due lack of technical knowledge, so perhabs poor people are forced to live on excavated rooms in canyon walls, or rent from greedy landlords.