The latest housing craze might be smaller than you thought.

It’s the cool new thing in real estate, with a decidedly uncool name: the ADU, or accessory dwelling unit. Truth be told, there’s nothing “new” about it—homeowners have been converting their garages and building small secondary homes on their properties for decades. But the phenomenon has gotten undeniably buzzy in recent years. Hipster architects are designing them; prefab startups are raising venture capital millions to build them. ADUs are even finding their way into the pages of glossy shelter magazines.

Design trends are sometimes driven by vague, big-picture cultural factors, but the reason for the rise of these dwellings is simple: the nation’s housing crisis. As cities across the country struggle with a lack of affordable housing, easing permit restrictions on ADUs is seen as a way to increase stock without the political challenges of building low-income developments. “Look at any major metropolitan city, and they either have passed an ADU ordinance or are looking to pass one,” says Caitlin Bigelow, the co-founder of Maxable, a startup that connects curious homeowners with ADU resources. “At the end of the day, it’s a supply and demand issue.”