Yes, a mother-in-law suite is a good investment. Although building one can easily cost $40,000–$130,000 according to HomeAdvisor, that number pales in comparison to the average cost of assisted living facilities and nursing homes. According to Genworth, the median cost of an assisted living facility was $5,350 per month in 2023. The addition may also increase your home’s value, and you can rent out the space when unoccupied for extra income.