New Hampshire ADU Law Update: What HB 577 Means for Homeowners

In keeping with our recent updates on Maine's evolving ADU laws, we want to share exciting news for our neighbors in New Hampshire. HB 577, signed into law on July 15, 2025, and now in effect, marks a major step forward for accessory dwelling unit (ADU) development across the state. This new law makes it easier, clearer, and more flexible for homeowners to add ADUs—a much-needed solution to the growing housing demand in New England.

Example of an Accessory Dwelling Unit (ADU)

Key Changes Under HB 577

• Detached ADUs Allowed by Right

  Municipalities must now allow detached ADUs in zones where single-family homes are permitted. Previously, most towns required that ADUs be attached to the main home.

  
  

• Increased Maximum ADU Size

  HB 577 raises the cap on ADU square footage—municipalities may now permit ADUs up to 950 square feet.

  
  

• Fewer Local Restrictions

  Towns can no longer impose more restrictive requirements (like additional setbacks, lot size, or design rules) on a property just because it has an ADU. ADUs and primary dwellings must be treated the same under zoning standards.

  
  

• Interior Connection Requirement Removed

  Municipalities may no longer require ADUs to be connected to the primary home by an internal doorway.

  
  

• Clarity on Definitions and Process

  The law better defines “attached” and “detached” ADUs, ensuring consistent interpretation and implementation across towns.

  
  

• One ADU Required if No Local Ordinance Exists

  If a town has no ADU ordinance in place, it must allow at least one ADU by right, subject only to standard building permits and codes.

Why HB 577 Matters to Our New Hampshire Customers

For NH homeowners and property owners exploring ADU options, this law opens more doors:

• More Flexibility in Design and Use

  Detached ADUs open doors to backyard cottages, studios, and in-law units with greater privacy and flexibility.

  
  

• Reduced Delays and Red Tape

  By limiting excessive local restrictions, HB 577 makes the path to building an ADU faster and more predictable.

  
  

• Expanded Housing Options

  Whether you're looking to house aging parents, generate rental income, or create space for grown children, the law gives you more ways to make your property work for your family.

• Support for Community Needs

  With housing demand on the rise across New Hampshire, this law provides a practical, homeowner-friendly way to contribute to local solutions.

Canbury Homes’ Perspective

At Canbury Homes, we believe HB 577 is a thoughtful, forward-looking law that balances individual property rights with broader community needs. It empowers homeowners, eases the housing crunch, and creates new opportunities for creative, small-scale development.

That said, we also understand that every town is different—and every property is unique. Our team is here to help you interpret how the new law applies in your area and walk you through the entire process of planning, permitting, and building your ADU.

Ready to Explore Your Options?

Whether you're just starting to explore ideas or ready to build, now is a great time to take advantage of New Hampshire's updated ADU laws. Our team of building experts are here to help you understand how New Hampshire ADU Law HB 577 applies to your property and guide you through every step—from planning to completion. Contact us today to start the conversation. Let’s build something great together!

Related Reading:

• Maine's ADU Law Explained – LD 2003

• Maine ADU Law Update – LD 1829