Tiny buildings called accessory dwelling units (ADU) are popping up in backyards across the nation as space for in-laws, offices or rental units.

Logan Havens been working on his backyard casita since early January, starting a few days after a state law expanding ADUs — or accessory dwelling units— went into effect.

When mapping out your plans ... of an ADU to an existing property is at least equal to the cost of the work,” Cantor says. The fully furnished ADUs his company builds include design ...