
In-Law Suite & ADU Additions in Southern New Hampshire: 2026 Cost, Permits & Design Guide
Multigenerational living is reshaping Southern NH homes. Here is the 2026 guide to ADU laws (RSA 674:71-73 and the new HB 577 detached mandate), realistic costs by build type, septic and permitting hurdles, and the design choices that actually work for in-law suites in Merrimack, Bedford, Nashua, and Manchester.








