Building a granny flat has a few downsides, the most notable being the cost. While granny flats can range in price depending on the size and style you choose, they typically cost more than an apartment or condo. Additionally, granny flats often require a higher level of maintenance than other types of housing, so be prepared to budget for ongoing repairs and upkeep. Finally, because granny flats are typically built on family property, there can be some legal complications if you decide to sell your home down the line. Be sure to consult with a real estate attorney to understand all the potential risks involved before signing any contracts.