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Detached ADU (Backyard Cottage) on the North Shore

A detached ADU is a separate small home in your yard. It offers the most privacy, and it also involves the most site work: a foundation, utility trenches, and setbacks from lot lines. Some builders offer modular or prefab units. We connect you with independent local pros who can quote either approach.

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Detached backyard cottage with a small porch

Why the 2025 ADU law matters

The rules changed statewide. Here is what that means for a North Shore lot.

Allowed by right

Since February 2, 2025, one ADU per lot is allowed by right in single-family zoning districts. Towns cannot require a special permit for a protected ADU.(Mass.gov: Accessory dwelling units (ADUs))

No owner-occupancy or family requirement

Towns cannot require the owner to live on the property or require that the ADU be occupied by family.(Mass.gov: Accessory dwelling units (ADUs), 760 CMR 71.00: Protected use accessory dwelling units)

Size and entrance rules

A protected ADU can be no larger than half the main house's gross floor area or 900 square feet, whichever is smaller, and must have its own entrance.(760 CMR 71.00: Protected use accessory dwelling units)

Local rules still apply

Towns may still use site plan review, setbacks and dimensional rules, short-term rental limits, and Title 5 septic requirements.(Mass.gov: Accessory dwelling units (ADUs), MassDEP: Guidance on Title 5 compliance for accessory dwelling units (PDF))

From idea to move-in

  1. Site check. Setbacks, lot coverage, slopes, trees, and where water, sewer or septic, and power will come from.
  2. Design or select a model. Custom design or a modular unit, sized under the state cap.
  3. Permits and site work. Building permit, any site plan review, then foundation and utility trenches.
  4. Build or set, then inspect. Construction or crane-set of a modular unit, connections, final inspections and certificate of occupancy.

Is this a good fit?

  • You have a larger lot with room to meet setbacks
  • You want the most privacy between households
  • You are considering a modular or prefab cottage
  • You may want to rent the unit
  • Your house itself has no space to convert

Further reading: Mass.gov: Accessory dwelling units (ADUs) · 760 CMR 71.00: Protected use accessory dwelling units · MassDEP: Guidance on Title 5 compliance for accessory dwelling units (PDF) · Mass.gov: Check a home improvement contractor registration · Mass.gov: When is a construction supervisor license required?

Every job is different. The pro you speak with will explain their own approach and pricing. North Shore ADU Pros does not perform, supervise, or guarantee any work.

ADU questions

Is a detached ADU allowed by right?

Under state law, one ADU per lot is allowed by right in single-family zoning districts, and it can be attached, detached or within the house, subject to reasonable local rules such as setbacks.

Can I rent it out?

Towns cannot require you to live on the property or rent only to family, but they may restrict short-term rentals.

How big can it be?

Under state rules a protected ADU can be no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller.

How many ADUs have been approved in Massachusetts?

The state's Executive Office of Housing and Livable Communities reported 2,084 ADUs approved and 2,824 applications statewide from 2025 through the first half of 2026.

How much parking can a town require?

No more than one space for a protected ADU, and none if the lot is within half a mile of a commuter rail, subway, ferry or bus station.

Other ADU types

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