In this world, there exists a type of material that has mass, but is incredibly buoyant in atmosphere. 1 kg of this material attached to 1 kg of any other material will be neutrally buoyant at sea level and act more like a beach ball than a large clod, excepting the effect of wind (it takes a lot of force to move more mass than a beach ball).
This material is available everywhere thanks to its incorrigible habit of bonding structurally and mixing with other materials and it is as common as most other forms of earth. Its buoyancy is logarithmic with respect to elevation so that even at the treeline on mountains it still retains something like 95% buoyancy, but achieves neutral buoyancy approximately in the range storm and rain clouds form and pass through, bringing it back to earth with the water cycle and re-bonding it to the now wet earth (this seems like a good way to keep it from both permanently floating up to where humans can't use it and from forming an opaque layer in the atmosphere, killing all plant life).
We'll call it floatstone.
A human analogue civilization has arisen on this planet and evolved to the point of approximately modern Earth humanity, including the ability to separate floatstone from other materials and form it into any shape we need it to be, much like clay. I don't think firing would be necessary, I'm envisioning a concrete like material, more curing to enable larger structural use.
How would modern city planning most effectively use this kind of material?
Would they float whole cities and place infrastructure underneath? Would they float individual buildings, chain them to the ground and join them by rope bridges? would it look much like our current infrastructure, just with the capability to build super high thanks to lighter bonded materials? Larger buildings and skyscrapers in relation to modern city planning are the goal, but including other buildings like factories or heavy industry would be great.