The best way to minimize wasting hot water is to locate the hot water tank next to all the place that use it, or at least keep it as close as possible.  The next best option is to make the most used fixtures as close to the tank as possible (those would be the main bathroom and the kitchen).  After doing the best you can there, you also want to use as small a diameter pipe as possible.  Unfortunately, plumbing code does just the opposite: is designed so that when multiple faucets are turned on, each delivers the same amount of water anyhow.   Partly this is a convenience, but partly is was done before the general availability of pressure-balanced (or anti-scald) valves that keep the temperature of the water constant as the pressure in the hot or cold changes.  Of course not all valves are single handle, and two handle valves do not have the pressure balanced feature, so to some extent the problem could still exist (check this!).

Statistically, it is actually rare that more than one faucet is turned on at once, so most of the time the plumbing is just wasting water.  Changing from copper, to PEX solves this problem--although it could be solved with copper-it would just be very expensive. PEX piping is cross-linked polyethylene: it has been chemical treated to be more resistant to chemical attack and does not produce the plastic taste that normal polyethylene does.  It comes in long rolls and is much cheaper than copper both in material and installation.  The typical system uses 3/8 line for most fixtures in a "home run" configuration.  In this configuration, PEX plumbing is installed like electrical wiring: there is a manifold at the hot water heater that has one big input from the hot water heater and many outputs-each of which typically serves one fixture, although sometimes two are served when it is more convenient. With this smaller diameter pipe, much less water is left in the line.

(chart: gallons of water per 100' for pipe sizes).

One of the keys to saving hot water with PEX is that the manifold is located very close to the hot water tank, since this line is typically a 3/4" line.  Since code requires the the manifold be accessible, the only logical place to put the manifold for the ADU would be in the basement, implying that the hot water tank should go down there also (for more on placement of utilities see HVAC page).

Layout wise, our design isn't terrible, but its not very good either.  The ADU kitchen,  bathroom and laundry are all close to each other, and the guest bathroom and our laundry are clustered with the ADU fixtures.  That's the good part.  Unfortunately our bathroom, kitchen and powder room are all on the opposite side the house and not even all that close to each other.  Since we are using PEX, having rooms near each other isn't so important, especially since most fixtures have their own line back to the manifold (i.e. the "home run" configuration).  All that matters is how far each is from the hot water heater.  Luckily, a centrally located hot water heater in the basement makes all the runs relatively short (locating the heater in the attic results in somewhat longer runs because most of the fixtures are on the first floor).

While state law only allows toilets with 1.6gal/flush to be sold, a recent study by the National Association of Homebuilding (NAHB), definitely found that all toilets are not created equal.  In water use, not all toilets actually used 1.6 gallons out of the box, varying from 1.45 to 1.89; and further when the toilets we fitted with a generic  replacement flapper, many used dramatically more water, averaging nearly 3 gallons/flush (and hence it is important to use replacement parts compatible with the toilets manufacturer).  The NAHB study also address a common complaint about the "flush performance" of low flush toilets: each of the 35 toilets tests was ranked according to how well it cleared various types of solid material from the bowl.   We will be using one of the better rated models, but have not chosen one at this point.  For a summary see Environmental Building News V12#4 (Apr 2003) pg 4, and for the complete study, see www.savingwater.org/toilettest.htm on the web.