Woodsville

New Hampshire has long-standing statewide ADU preemption originally enacted in 2016 (SB 146 / RSA 674:71-73, effective 2017-06-01) and substantially expanded in 2025 by HB 577 (Chapter 197, Laws of 2025), signed by Governor Ayotte 2025-07-15 and effective 2025-07-01. Under the original 2016 law, every NH municipality with a single-family residential zone must allow at least one ATTACHED ADU by right (within or attached to the primary single-family dwelling). HB 577 expanded the mandate so municipalities must ALSO permit at least one DETACHED ADU per single-family lot, redefined ADU as 'located on a lot containing a single-family dwelling' (rather than 'within or attached to'), and required that municipalities allow ADUs to be created from existing structures including detached garages even where the existing structure violates current dimensional requirements for setbacks or lot coverage. HB 577 set ADU size: ordinances may not limit an ADU to less than 750 sq ft, and may not exceed 950 sq ft unless the municipality authorizes a larger size. Cities may apply otherwise-applicable RSA 674:72 IV-IX requirements (state building/fire code, public-health, septic). New Hampshire is one of the strongest ADU-preemption regimes in the country.

New Hampshire Housing Finance Authority (NHHFA, dba 'New Hampshire Housing'), a quasi-governmental authority operating under RSA Chapter 204-C, does not operate a stand-alone homeowner ADU loan or grant product as of 2026-04-26 — but the agency has been the country's most active state HFA in writing concrete ADU policy and program models. The April 2023 NHHFA report 'Policy and Program Models for Creating Accessory Dwelling Units in New Hampshire' lays out the state's current toolset and gaps. ADU construction can be financed through NHHFA's general homeowner programs (Home Flex Plus first-mortgage product with up to 4% non-repayable cash assistance, Home First with up to $10,000 in down-payment assistance) when the underlying primary-residence purchase or refinance qualifies. The InvestNH Housing Opportunity Planning (HOP) grants under InvestNH provided $30M+ in capital and an additional $2.9M in 2024 for municipal planning consultants — supporting cities to update zoning to comply with HB 577 and similar reforms. None of this is a per-ADU homeowner subsidy; the Granite State Homeowner Repair Loan and Septic Loan programs occasionally finance ADU-adjacent work.

• NHHFA Home Flex Plus
• NHHFA Home First
• InvestNH Housing Opportunity Planning (HOP) Grants

New Hampshire's ADU programmatic posture pairs the strong RSA 674:71-73 / HB 577 preemption framework with the NHHFA-led InvestNH Housing Opportunity Planning grant program (≈$30M in capital plus $2.9M FY2024 expansion) helping municipalities update their zoning and master plans to comply. NHHFA's New Hampshire Housing Toolbox (https://nhhousingtoolbox.org/) hosts a model ADU ordinance, sample architectural drawings, and a homeowner-facing ADU resource page. The state does not operate a centralized pre-approved ADU plan catalog (e.g., a CalHFA-style design library), nor a per-ADU rebate program. The NH Office of Energy and Planning (within DBEDC / Department of Business and Economic Affairs) publishes 'Planning for Accessory Dwellings' guidance for local code amendments. Impact-fee waivers are NOT statutorily mandated; ordinance authority for fees remains with municipalities subject to the RSA 674:71-73 framework.

• InvestNH Housing Opportunity Planning (HOP) Grants
• NH Housing Toolbox (Model ADU Ordinance + ADU Resources)
• Office of Energy and Planning — Planning for Accessory Dwellings guidance