One of the things that has puzzled me for a long time is how little thought we have applied over time towards designing and building better homes. Energy and water usage are two elements of this.
Regarding water. Every time I wash my hands I wish that water went into a gray water tank instead of being wasted. This water, perhaps with some filtration (or maybe not [0]) could then be used for irrigation. The same is the case for the shower.
We try to take walks around the neighborhood every day. I am always amazed at the amount of water almost constantly running into storm drains due to overflowing/unmanaged lawn watering. If we designed homes to store water secondary use we would likely save enough water to make a huge difference. Washing the car or hosing-down the patio or driveway would also benefit from this immensely.
This water would most definitely be good for flushing the toilet. Why do we need clean drinking water (that's what's coming into your house) to flush the toilet? !!!
In other words, just about the only use for "first generation" water at the home would be drinking or cooking.
Why can't I dump the water I used to cook spaghetti into a sink that routes it into a holding tank? Why can't that water be used to flush the toilet?
All of this would require a design rethink and creating the incentives necessary to compel people to retrofit. New construction, by default, would come with these enhancements.
To go beyond this, imagine the water wasted by large buildings. A 5, 10, 20+ floor building a hotel perhaps or an apartment building with 300 units. The numbers are staggering.
As a general comment, everything in government or public infrastructure always seems to be about "more" rather than "more efficient" first. More lanes on the highway, rather than coming up with ways to make traffic more efficient (like punishing the guy doing 45 in the middle of a sea of cars who have to go around him, creating a traffic jam that ripples back for miles). Efficiency never seems to be discussed or even considered. Saving is never a parameter in the public discourse.
And so, neighborhoods like mine, built 20-some years ago, have, every day, perfectly good water flowing like rivers down the edge of the street to eventually find a storm drain.
Energy is another one. Dark roofs. Unbelievable. A few blocks away someone pained their house black! Wow. Air conditioning systems that do not have the ability or option to bring in cool filtered outside air in the evening in order to cool down the house "for free" (compared to running a 5 ton compressor, a fan would be free). Interesting that cars have had the option to recirculate inside air or suck-in outside air for as long as I can remember but we don't require this from homes by law. Or how about systems that cool down or warm-up walls as required in order to store energy rather than just circulate cold air, which has horrible thermal capacity?
Etc.
[0] A friend mentioned that hand and dish soap in low concentrations doesn't really harm lawns, trees or plants. I did not research this. If this isn't the case, I'm sure some form of simple filtration could resolve the matter.