BOSTON — Accessory dwelling units are now allowed by right in single-family zoning districts across most of Massachusetts, under a law Gov. Maura Healey signed in August. The rule went into effect on Sunday.

The law stipulated that the policy would go into effect 180 days after it was signed, which was on Aug. 6, to allow small, independent living spaces to be built on the same property as a primary residence by right regardless of local single-family zoning ordinances.

The Healey administration estimates the law could lead to between 8,000 and 10,000 ADUs over five years.

The law applies to 350 municipalities in Massachusetts. Boston is exempt, and is pursuing its own city-specific ADU policy.

ADUs must have a separate entrance from the main house to qualify under the law, and must be no larger than half the gross floor area of the principal residence, or 900 square feet, whichever is smaller. They can be internal, like converting a basement into an apartment; attached to a house by adding a new addition; or completely detached but on the same property.

The Executive Office of Housing and Livable Communities spent the last six months drafting more specific regulations to accompany the law, to allow cities and towns to “impose reasonable restrictions and requirements.”

Those newly drafted regulations allow municipalities more control over site plan review; short-term rentals; and regulations concerning septic systems and setbacks, as well as the bulk and height of structures.

Control over short-term rental policies was especially important for towns on Cape Cod, Martha’s Vineyard and Nantucket, an official from the housing office said during a background briefing Thursday. The tourism economy in the region has already turned a lot of the Cape and islands’ housing into seasonal rentals, pushing full-time residents out.

By allowing towns to restrict ADUs from being used mainly as short-term vacation rentals, the housing agency official said the Cape and islands can focus on creating year-round housing opportunities for residents who actually live in their towns.

The regulations also explicitly outline some restrictions that municipalities cannot put in place.

For example, cities and towns cannot require that the owner of the primary property be familially related to the occupant of the ADU.

ADUs have sometimes colloquially been called “granny flats,” where aging parents of middle-age adults could live in their senior years. Alternatively, ADUs have also been used for adult children who may want to move out of their parents’ house but cannot afford to live on their own, or need to stay close to home.

The Healey administration has said ADUs can help address the state’s housing crisis. If aging adults move out of their single-family home, it frees up that house for another family to move into.

However, housing advocates, lawmakers and administration officials have also talked about ADUs being an alternative housing option outside of just familial ties.

They have said they can create another option for renters, and an opportunity for homeowners to earn rental income that can offset maintenance and repair costs, without necessarily needing a relationship with the ADU’s occupant.

The administration’s regulations also seek to encourage more housing near public transit.

They say municipalities cannot require that ADUs have a parking spot if they are located within half a mile from a commuter rail station, subway station, bus station or all-season ferry terminal. They can’t require more than one parking spot for ADUs outside of that half-mile radius.