Accessory Dwelling Units (ADUs), in-law apartments, backyard cottages, and tiny homes are gaining popularity around Massachusetts. A new statewide ADU law has opened up zoning access in many communities, and an increasing number of Massachusetts homeowners are exploring ADUs for multigenerational living, long-term rental income, and greater flexibility on their property. We at Fagone Plumbing, Heating, and HVAC are beginning to see the demand. Homeowners are calling and emailing with questions about mechanical design, heating and cooling options, plumbing layout, radiant heat, and code requirements to build a safe and efficient ADU. We put together this comprehensive, easy-to-read guide to ADUs in Massachusetts based on current legislation, building code practice, state incentives, industry research, and our own experience providing mechanical systems for ADUs throughout the North Shore and Greater Boston.