A building permit is required in Portland for retaining walls over 4 feet measured from the bottom of the footing to the top of the wall, walls supporting a surcharge load (driveways, structures, slopes), and any wall within the shoreland zone. Lower freestanding garden walls are typically exempt.
Portland follows the Maine Uniform Building and Energy Code (MUBEC) baseline under 25 MRS § 2451 et seq. and the International Residential Code adopted thereunder. Under MUBEC/IRC, a permit is required for any retaining wall more than 4 feet in height measured from the bottom of the footing to the top of the wall, OR any retaining wall (regardless of height) supporting a surcharge — meaning a load from an adjacent driveway, structure, or sloped ground. Engineered design (stamped drawings) is generally required at higher heights. In the Shoreland Overlay (within 250 ft of tidal waters, the Fore River, Back Cove, Presumpscot River), retaining walls may require additional review under 38 MRS § 435 et seq. and Chapter 14's shoreland implementation. Retaining walls also factor into Portland's stormwater and erosion controls administered by the Department of Public Works.
Building without a required permit triggers stop-work orders and after-the-fact permit fees from Portland Permitting & Inspections, plus civil penalties for code violations. Unpermitted walls in the shoreland zone can require removal and restoration of the vegetated buffer.
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