Yona

Although Guam has no county tier, a set of island-wide regulatory overlays effectively functions at what would be the county scale for ADU siting — they cut across all 19 villages uniformly. Chief among them are the typhoon wind-hazard regime (the entire territory sits in the Pacific typhoon belt and is treated as a single high-wind design-wind-speed zone under the Guam Building Code, IBC 2018 adaptation), the coastal and bay flood-hazard overlays administered under the FEMA NFIP (Agana Bay, Pago Bay, Tumon Bay, Apra Harbor tidal reaches, plus riverine Special Flood Hazard Areas along the Talofofo, Ylig, Pago, and Agana Rivers), and a patchwork of wartime-era military real-estate easements and Department of Defense encroachment-zone overlays (Andersen AFB APZ/noise zones in northern Guam, Naval Base Guam encroachment zones around Apra Harbor, plus residual Civil Reserve Air Fleet land-use restrictions). Two territory-wide utility providers — Guam Power Authority (GPA) and Guam Waterworks Authority (GWA) — function as single-provider utility overlays for any ADU add-on since their service-capacity and line-extension rules apply uniformly island-wide.

• Guam Typhoon Wind-Hazard Zone (island-wide) — The entire territory of Guam is treated as a single high-wind design zone with design wind speeds at the upper end of the US hurricane-prone range. ADU construction must satisfy hurricane-clip, sheathing, and opening-protection requirements as a condition of building-permit issuance, materially raising unit cost over mainland US comparables. There is no sub-island variation — the wind-hazard overlay functions uniformly.
• FEMA NFIP Coastal and Riverine Flood Zones — Coastal Special Flood Hazard Areas run along Agana Bay (Hagatna), Pago Bay, Tumon Bay, Apra Harbor tidal reaches, and the mouths of the Talofofo, Ylig, Pago, and Agana Rivers. Detached ADUs in these zones face lowest-floor-elevation, venting, and anchoring requirements under the Guam floodplain-management program, administered at the territorial level by the Guam Bureau of Statistics and Plans. NFIP participation is active and federally backed flood insurance is available.
• DoD Military Encroachment and Wartime-Easement Overlays — Andersen AFB in the north (Yigo/Dededo sectors) imposes Accident Potential Zones (APZ-I, APZ-II) and noise-contour overlays that restrict residential intensification — an ADU in an APZ-I area is typically disallowed or severely conditioned. Naval Base Guam and the Apra Harbor area carry encroachment zones affecting Santa Rita, Piti, and Agat. A broader class of wartime-era real-estate easements (WWII-era land takings later partially returned under the Guam Ancestral Lands Commission process) can create unusual title conditions that surface during ADU permitting title-review — owners on ancestral-return parcels should confirm clear title before siting an ADU.
• Guam Power Authority (GPA) — single-provider electric utility — GPA is the sole electric provider for the territory. An ADU requires either a sub-meter off the primary dwelling's service or a new service drop; line-extension, transformer-capacity, and demand-meter rules apply uniformly island-wide. There is no municipal or county utility alternative. GPA maintains standardized ADU-relevant service-application forms at its customer-service offices (Gloria B. Nelson Public Service Building, Fadian).
• Guam Waterworks Authority (GWA) — single-provider water and wastewater utility — GWA is the sole water and sewer provider for the territory. Sewered areas concentrate around Hagatna, Tamuning, Dededo, and the military-adjacent population centers; rural and mountain-village parcels commonly operate on on-site wastewater (septic) systems subject to Guam EPA review. An ADU typically triggers either a sub-meter application or a second service tap; septic-served parcels require a Guam EPA on-site wastewater permit before a second dwelling unit can be legally occupied.