ADU FAQs — Greater Boston
Horizon ADU was started out of a desire to help families in the Boston area. Our mission is to help homeowners transform unused space on their property into valuable housing for a wide range of people. This approach expands access to high-quality schools, local amenities, job centers, and well connected neighborhoods.
General
What exactly is an ADU under Massachusetts law?
An Accessory Dwelling Unit (ADU) is a self-contained living space—complete with a kitchen, bathroom, and separate entrance—located on the same lot as a primary residence. ADUs may be internal (inside the home), attached (adding space), or detached (standalone cottage).
Are ADUs legal in Greater Boston?
Yes. As of August 2024, Massachusetts law (Chapter 150 of the Acts of 2024) allows one ADU per property in cities or towns permitting single-family homes, and this is now permitted by right—meaning no discretionary zoning approval is needed. Municipalities cannot impose unnecessary restrictions and cannot require owner occupancy or family residency.
How large can an ADU be?
ADUs must be no larger than 900 square feet or 50% of the primary home’s gross floor area, whichever is smaller. These limits are uniform across all Massachusetts municipalities .
Where can I build an ADU in Boston?
In Boston, internal ADUs are allowed on all owner-occupied lots with 1–3 family homes. Detached and attached ADUs are generally subject to zoning approval, except in some pilot neighborhoods like Mattapan where detached ADUs may be allowed by right.
Can towns restrict who lives in the unit?
No. Massachusetts law prohibits municipalities from restricting ADU occupancy based on family relationships, tenant type, or renter identity.
Do ADUs need parking or impact septic systems?
Only reasonable parking requirements are allowed. No parking is required within 0.5 miles of transit stations. If septic capacity is insufficient, Title 5 regulations may require an upgrade—but towns cannot enforce stricter rules beyond state standards.
Will building an ADU increase my property taxes?
Yes. Since an ADU is considered a property improvement, your assessed value may rise—but this is often offset by rental income potential or increased home equity.
What is the ADU building timeline?
Design & Construction
Can I customize the design of my ADU?
Yes—many builders offer fully customizable layouts, finishes, and exterior styles to match your main home.
What are the size limits for detached ADUs in Massachusetts?
Most detached units are limited to 900 sq ft or 50% of the main home’s size, whichever is smaller.
How long does construction take once it starts?
Typically 4–6 months, depending on site access, weather, and design complexity.
Will an ADU match the style of my existing home?
Yes—materials and finishes can be selected to blend seamlessly with your primary residence.
Financing & Costs
Can I use rental income from the ADU to help qualify for financing?
Some lenders will count projected rental income toward your loan qualification—especially with ADU-specific loan products.
Is there any state or city assistance for building an ADU?
Some cities like Boston offer forgivable or low-interest loans for homeowners building ADUs for affordable housing or senior family members.
Are there any hidden costs I should be aware of?
Site work, utility connections, permitting fees, and design changes are common cost variables.
Permitting & Legal
Can I convert an existing garage into an ADU?
Yes, if it meets zoning and building codes. Garage conversions are popular but may require structural upgrades.
What happens if my lot doesn’t meet current zoning rules?
You may need a zoning variance or special permit. A feasibility study can identify your options.
Do I need to notify my neighbors before building an ADU?
In most cases, no—but if a variance is required, a public notice or hearing may be part of the process.
Usage & Lifestyle
Can I build an ADU to use as a home office or studio?
Yes, ADUs can be used as guest space, offices, studios, or rental units—as long as they include a kitchen and bath.
Can my parents or in-laws live in the ADU?
Absolutely. ADUs are ideal for multigenerational living and offer privacy while keeping family close.
Can I live in the ADU and rent out the main house?
Yes—many homeowners choose to downsize into the ADU and rent out their primary residence.
Maintenance & Operations
Who handles utilities for the ADU?
ADUs can have separate or shared utility meters depending on your preferences and local utility regulations.
What kind of maintenance does an ADU require?
Similar to a small home—roofing, HVAC, plumbing, and exterior upkeep. New builds often require little maintenance in the first 5–10 years.