The most obvious cost of detached homes is the completely unsustainable amounts of infrastructure required to maintain them. Roads, sewage, electric, etc.
It’s a well documented fact that suburbs of sprawling suburban homes are bankrupting towns/cities all across America and only the densely built downtown cores are keeping these cities afloat because the tax revenue of dense mixed-use areas is substantially higher than the cost of maintaining the infrastructure for these places. Check out Strong Towns if you’d like to know more and see the studies showing all this.
My preferred variation of this is to make it an open question that leaves them in the position of authority, and assumes that they made a deliberate decision.
For example, instead of “Why aren’t you using StandardLib that does 90% of this?”, I would try “Could this be achieved with StandardLib? Seems like it would cover 90% of this”.