Accessory Dwelling Unit

Accessory Dwelling Units (ADUs) are secondary housing units which are located on the same lot as a primary residence. ADUs offer additional living space or rental income opportunities. These units can be attached to, or can be detached from, the main house, and are often referred to as “in-law suites,” “granny flats,” or “secondary suites”.

🏡 Key Provisions of the New ADU Law

By-Right Construction: Homeowners in single-family zoning districts can now build one ADU by right—no special permit required. 
Size Limits: ADUs must be no larger than 900 sq ft or half the gross floor area of the primary dwelling, whichever is smaller. 
Separate Entrance Required: ADUs must have a separate entrance, either directly from outside or via a shared corridor that meets state building code for safe egress. 
No Owner-Occupancy Requirement: Municipalities cannot require the homeowner to live on the property. 
No Minimum Lot Size: ADUs are allowed even on non-conforming lots; towns cannot impose minimum lot size requirements. 
Utility Hookups: Separate water, sewer, or electric connections cannot be required unless mandated by state law or utility providers. 
Short-Term Rentals: Municipalities can regulate or prohibit ADUs from being used as short-term rentals (under 31 days). 
Dimensional Standards: Local rules on setbacks, lot coverage, and height must be no stricter than those for single-family homes. 

Purpose:
ADUs provide housing for family members, generate rental income, or offer a space for downsizing homeowners. The law is really trying to aim to address the housing crisis by facilitating the addition of ADUs, which can provide affordable housing options and enable older adults to age in place.

Types:
ADUs can be internal (within an existing structure, like a basement or attic conversion), attached (as an addition to the main house), or detached (a separate structure on the property, such as a garage conversion or a new cottage).

Benefits:
ADUs can also help address housing shortages, offer affordable housing options, increase property values, and provide flexibility for multi-generational living – my favorite.

Regulations:
ADUs construction is not free for all and for safety reason are subject to local zoning regulations and building codes. In Massachusetts, this new state laws has made it easier to build ADUs by reducing permitting requirements but NOT eliminating building code.

Cost:
Personal choice on return. According to Boston.gov, building an ADU can range from price typically in range of $75,000 to $165,000 if internal, $150,000 to $350,000 if  attached, $250,00 to $500,000 if in a detached unit; all depending on the type and size. Estimated ROI is 8–12% annually; ~13–14 years simple payback.  At some point, it is not worth spending much more that affects house value with little to no marginal returns.

🛠️ Types of ADUs Allowed

Internal: Basement or attic conversions
Attached: Garage conversions or additions
Detached: Backyard cottages or standalone units

As an Investment

The ADU law applies to investment properties also. The law does allow homeowners to create ADUs, which can be rented out, even if the owner doesn’t live on the property, in either the primary dwelling or the ADU. This is big Investment opportunity too because it means that investors can add an ADU to a single-family home they own and then rent it out, which effectively creates a two-unit rental property; perhaps a significant and potential opportunity for higher income for real estate investors to now expand their rental portfolios by adding ADUs to existing properties, increasing their number of rentable units without needing to purchase a separate multi-family building.

Compliance with regulations:
Once again, while the law allows for ADUs on investment properties, it’s crucial for owners to ensure compliance with all relevant regulations, including size restrictions, building codes, and zoning requirements. Always talk to the town the property is in to find out other applicable requirements, if any. Short-term rental restrictions:
Owners will have to check with their town rules for compliance on length of rentals. While long-term rentals are generally permitted, short-term rentals (like Airbnb) may have additional restrictions. However, in many cases, ADUs are permitted “by-right” in zoning districts where single-family homes are allowed. This means that they don’t require a special permit even if the principal residence may require a special permit.

Interested in more, here is a link to a well written article: Accessory Dwelling Units New MA Law Change on Feb 2, 2025

For additional FAQ’s please visit https://www.mass.gov/info-details/accessory-dwelling-unit-adu-faqs

ADU Guide –  ADU