If a homeowner is planning to build an ADU for an adult child returning hoe, an aging parent, or even to bring in a little additional income, consider offering these recommendations:

Incorporate Universal Design  Units with universal design can accommodate the special needs of the handicapped, those aging in place, and even individuals with a temporary medical condition that makes mobility challenging.  It will not diminish the use of the dwelling for individuals without special challenges, but will make aging in place possible for the homeowner in future years.

Go Low Maintenance  Unless you love home-maintenance chores and want to expand your to-do list, use materials and building methods that create a high-efficiency, low-maintenance dwelling.  It may cost a bit more, initially, but will pay off in the long run.  This type of building also makes a more attractive option for the homeowner, should they decide to age in place in the ADU, while renting out the main home to increase income.

Design to Increase Privacy  Make sure your ADU is positioned to provide privacy for the inhabitants AND the neighbors.  You don't want to provoke resentment with windows that peer into the neighbor's windows, or staring down into the yard they fenced off for privacy.  When building, think "oasis," and create a dwelling that has charm, privacy, and is respectful of your neighbor's desire for the same.

(Excerpt from The SRES Professional)