Getting approval for an ECHO can be a Herculean task. Zoning departments tend to frown on the idea, and neighborhood associations sometimes fear that ECHOs compromise the quality of the neighborhood by attracting renters and creating a higher density environment. Some even consider ECHOs eyesores. Although manufactured housing has improved in recent years, and newer styles include attractive touches like rounded sheet rock and tiled roofs, these inexpensive units sometimes look more like trailer homes.

"Each city has a different take on zoning," says Bob Clay, who sells ECHOs senior housing units through www.grannyflats.net. "But typically you need to have side yard clearance, you can't face a busy street, and you can't convert the two properties to condos and sell the granny flat separately."

Some zoning departments stipulate that the unit can only be used for a family member. If your city places restrictions on who can live in the unit, an ECHO can reduce the value of your property, so be sure to sell the ECHO and move it off your lot before you put your home on the market.

ECHO housing is easiest to install in settings where zoning laws accommodate alternative housing options. These include both high-density areas with rental housing shortages (Santa Cruz, California, or Seattle, for example) and rural areas with minimal zoning laws and large lots. Families with smaller lots may want to consider other senior housing options.

Finally, ECHO housing can be hard to remove. Finding a service that's willing and able to haul the unit away without damaging it may be a challenge in areas where ECHOs are rare. And the price of removal can add considerably to the overall cost of the unit.