Frequently Asked Questions

An accessory dwelling unit (ADU) offers homeowners a flexible, future-proof solution that adds both value and versatility to their property. For many families, an ADU supports multi-generational living, creating a comfortable, independent space for parents, adult children, or extended family while keeping everyone close. It also provides the opportunity for ageing in place, allowing homeowners or relatives to maintain their independence in a smaller, accessible home without leaving their community or support network.

Beyond lifestyle benefits, an ADU can serve as a smart financial investment. Adding an ADU to your property adds immediate and long term value to your home. If you choose to rent out the space, ADU or original home, homeowners have the potential to generate additional income that can offset mortgage payments or supplement retirement savings.

In short, an ADU offers flexibility today and peace of mind for the future: a practical way to house loved ones, stay rooted as needs change, and create meaningful financial value.

Here’s a clear summary of the state rules you should know — and then we’ll dig into how they may apply locally.

Massachusetts

• Under the Affordable Homes Act (Chapter 150, 2024), ADUs up to 900 sq ft are allowed “by right” in single-family zones. McGregor

• The ADU may not exceed 50% of the gross floor area of the main dwelling (if that is less than 900 sq ft). In some cases the basement ar of your home is considered in the gross floor area.

• Municipalities may still impose reasonable restrictions on setbacks, building height, bulk, lot coverage, and site plan review.

• Importantly, towns cannot require that either the primary or accessory dwelling be owner-occupied, nor can they demand a familial relationship between occupants.

You can read the official MA ADU rules here: Mass.gov – Accessory Dwelling Units

New Hampshire

• State law (RSA 674:71–73) mandates that municipalities allow one ADU that is detached or attached in any district that allows single-family homes. Revised Law on Accessory Dwelling Units

• Local rules may set minimum/maximum square footages (but it may not limit an ADU to less than 950 sq ft as of 7/1/25 per changes in HB 577) and may impose parking or owner-occupancy requirements.

• The ADU must comply with the same dimensional, setback, height, and lot requirements applicable to a single-family home, unless local rules differ for detached units.

• An ADU must have adequate water and wastewater capacity, but the law does not require a totally separate system.

The NH rules can be found here: https://www.nhmunicipal.org/sites/default/files/uploads/Guidance_Documents/adu-do-s-and-don-ts-2025.pdf

The short answer is YES.  Both New Hampshire and Massachusetts  require that cities and towns allow for ADUs in residential zones.  However,local regulations and setbacks may influence how this will affect your property We’ll review your address and local codes to confirm exactly what’s allowed for your site

The size of the ADU that will fit on your property depends on your lot size, zoning, and setback requirements. All jurisdictions must allow a minimum of 950sf in NH and 900sf or 1/2 gross area in Mass. but the exact limit varies by local code and available space. We’ll evaluate your property layout and zoning to determine the maximum size and best design options for your site.

Projects using one of our four set designs typically take about 10-14 weeks from start to finish. If you choose to customize your design, the timeline may vary depending on the scope of changes and permitting requirements. We’ll provide a clear schedule upfront so you know exactly what to expect.

Absolutely—you can customize your ADU to fit your needs and style. We’ve spent our careers building custom homes, and we know no two properties or families are exactly alike. Whether you’d like to adjust the layout, finishes, or overall design, we’ll work with you to make sure your ADU feels like it truly belongs on your property and fits your specific needs.

Call or email us to discuss your project.  We can give you an instant assessment of the scope of your ADU and determine how your local zoning may affect your project.  We look forward to helping you create a space that meets your needs and goals.   

If you intend on financing the construction of your ADU, we have a mortgage professional that can guide you through the process.

Darlene Pearl

https://www.harboronemortgage.com/loan-officers/darlene-pearl

Mortgage Loan Officer-

NMLS 39842

NLMS ID 2561

Construction/Renovation Loan Officer

p.   603-319-4305

c.   603-475-4858

f.    866-845-5316

e.  dpearl@HarborOne.com

Lending in NH, ME, MA, CT, VT, RI, NJ, PA, FL, NC and SC