Retaining walls in Nashua require a building permit when the wall exceeds 4 feet in height above the grade base of footing, per NRO Sec. 105-22.A.(10) of the Building Permit Fee Schedule. Residential retaining walls cost $25 minimum or $0.15 per linear foot (whichever is greater); commercial walls cost $50 minimum or $0.30 per linear foot. Walls under 4 feet do not require a stand-alone building permit but must still comply with the New Hampshire State Building Code (2021 IRC and 2021 IBC) adopted by RSA 155-A. The Building Official may require third party peer review for 'extensive retaining walls and complicated foundation designs' under NRO Sec. 105-9.B.
NRO Sec. 105-22.A.(10) (as amended by Ordinance O-21-045) provides: 'Retaining walls over four feet in height above grade base of footing: (a) Residential: minimum $25 or $0.15 per linear foot (whichever is greater). (b) Commercial: minimum $50 or $0.30 per linear foot (whichever is greater).' NRO Sec. 105-9.B incorporates IBC Section 107.3 and authorizes the Building Official to 'require third party peer review where applications for unusual designs or magnitude of construction are filed, such as high-rise structures, elaborate framing systems, extensive retaining walls and complicated foundation designs.' NRO Sec. 105-9.2.B applies the same peer-review authority under the International Existing Building Code. The 2021 International Residential Code Section R404 (Foundation and Retaining Walls) requires engineered design for retaining walls supporting more than 24 inches of unbalanced backfill with a surcharge, or more than 48 inches unbalanced backfill without surcharge, as adopted statewide under RSA 155-A (HB 1681, eff. 12-31-2024). Retaining walls cannot encroach on public rights-of-way and must respect recorded easements. The Department of Building Safety (229 Main St, 2nd Floor, 603-589-3080) reviews retaining wall permits and inspections.
Constructing a retaining wall over 4 feet in height (measured grade base of footing to top of wall) without a building permit from the Department of Building Safety violates NRO Sec. 105-22.A.(10) and Sec. 105-6 (penalties under NRO Chapter 74 and RSA 676:15-17-b). Building a retaining wall supporting more than 48 inches of unbalanced backfill without engineered plans violates the 2021 IRC R404 as adopted under RSA 155-A. Walls placed in a public right-of-way or in recorded utility/drainage easements must be removed at the owner's expense. Permits issued after construction started without a permit carry a 100% surcharge up to $275 residential / $750 commercial under NRO Sec. 105-22.A.(14).
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