Constructing buildings on a planet is weird in Stellaris, and not in a good way. You get 1 building slot per city district, but there are also building slots from other weird sources that make no sense like technologies, traditions etcetera, and then there is the worst problem that really makes the system wonky: building slots have a hard cap. This leads to all sorts of weird situations, such as the fact that you can't build any more buildings on a size 25 planet than you can on a size 12 planet, which not only doesn't make any sense, but also makes planet balance quite wonky and weird.
So my suggestion is to make the whole thing more comprehensible by simply having 1 city district provide 1 building slot, remove building slots from all other sources and remove the hard cap completely, so that if you have a size 25 planet and build 25 city districts on it, you'll get 25 building slots. This will require a larger area dedicated for building slots in the UI, but it would make much more sense gameplay and fantasy-wise.
Naturally you'd need a habitat version of city districts added to habitats, and perhaps change ringworlds to just act like size 25 planets with special districts that are not any larger than normal planet districts. Either that or ringworld "city" districts would add 6 building slots so that you'd get them to act like size 24 planets if filled with buildings instead of other districts.
Specialized districts for science/alloys/consumer goods/etc. would still have their place as they'd be cheaper to produce than a city district + building + upgrades, although this change would make it possible to remove those districts from the game altogether.
So my suggestion is to make the whole thing more comprehensible by simply having 1 city district provide 1 building slot, remove building slots from all other sources and remove the hard cap completely, so that if you have a size 25 planet and build 25 city districts on it, you'll get 25 building slots. This will require a larger area dedicated for building slots in the UI, but it would make much more sense gameplay and fantasy-wise.
Naturally you'd need a habitat version of city districts added to habitats, and perhaps change ringworlds to just act like size 25 planets with special districts that are not any larger than normal planet districts. Either that or ringworld "city" districts would add 6 building slots so that you'd get them to act like size 24 planets if filled with buildings instead of other districts.
Specialized districts for science/alloys/consumer goods/etc. would still have their place as they'd be cheaper to produce than a city district + building + upgrades, although this change would make it possible to remove those districts from the game altogether.