ECHO units can be a fairly affordable housing option when compared to, say, a few years in an assisted-living center. A new, basic 500-square-foot unit might go for $25,000, while a bigger, higher-end carriage house can cost as much as $100,000. Expect to pay extra to pour a foundation and set up the utility connections.

In some places, the cost can be offset slightly by a state "caregiver" tax credit, available to adults who provide ongoing care to their aging parent. (In California, for example, this is $500 per year per parent.) Depending on your area's zoning laws, ECHO housing units can also be rented out and used to generate monthly income after a family member moves out.

ECHOs can be bought or leased, new or used. You may be able to find a used unit through your local classifieds or an online buy-sell resource like Craigslist.org. Search for phrases like "modular home," "manufactured home," "carriage house," and "mobile home."