Wylie

Collin County administers or co-administers several overlay regimes relevant to ADU siting on unincorporated parcels, though the scope is narrower than in geographically-diverse states. Unlike California or Oregon, Texas has no Coastal Commission jurisdiction (Collin County is landlocked, ~250 miles from the Gulf), no CAL FIRE-equivalent Very High Fire Hazard Severity Zone regime (DFW-area fire risk is primarily grass / wildland-urban-interface rather than chaparral, managed locally by volunteer / municipal fire departments with no statewide overlay), and no seismic-retrofit overlay (north Texas is seismically quiet). The principal county-administered overlays are (1) FEMA Special Flood Hazard Areas along the East Fork Trinity River, Sister Grove Creek, Wilson Creek, Rowlett Creek, and tributaries, administered by the County Engineer as Floodplain Administrator under 44 CFR 60 and the 2024-11-04 Collin County Floodplain Management Regulations; and (2) coordination jurisdiction over airport-land-use compatibility around McKinney National Airport (TKI, county-adjacent but owned by the City of McKinney) and the handful of smaller general-aviation airports, though Texas lacks a California-style ALUCP / ALUC statutory regime and airport-compatibility review is primarily municipal. There is no county historic-preservation overlay; historic districts are municipal. Endangered-species consultations (primarily for the Golden-cheeked Warbler in adjacent counties) generally do not reach Collin County.

• FEMA Special Flood Hazard Areas (SFHA) — Collin County Floodplain Management Regulations — ADUs in an SFHA require lowest-floor elevation at or above Base Flood Elevation plus county freeboard (typically +1 ft), flood vents on enclosures below BFE, anchoring, and a post-construction Elevation Certificate (FEMA Form 086-0-33). For developments of 5+ acres or 50+ lots with any SFHA portion, a flood study demonstrating no adverse upstream / downstream impact is required. Development Permit fee: $50. For parcels inside city limits, the city's floodplain administrator applies (most Collin County cities participate in NFIP and have Community Rating System enrollments that affect flood-insurance premiums).
• Lavon Lake / U.S. Army Corps of Engineers Regulated Shoreline — Homeowners planning an ADU on a Lavon Lake-adjacent parcel should separately consult USACE Fort Worth District regarding any shoreline work; the county Development Services counter does not handle USACE permits.
• Airport Land Use Coordination (McKinney National TKI, Aero Country, Collin County Regional) — A two-story ADU (height in excess of FAA Part 77 imaginary surfaces) near an airport approach would trigger Form 7460-1 notice and potentially an obstruction determination. Single-story ADUs and typical second-story heights (~18-22 ft) almost never trigger Part 77. No county permit required; FAA determination is advisory to the permitting authority.
• Collin County Subdivision Regulations (Tex. LGC ch. 232) — Plat review covers road frontage, drainage, utility service, OSSF suitability, and floodplain compliance. A simple single-ADU addition to an already-platted parcel does NOT re-trigger platting; subdividing a parcel to sell an ADU separately (e.g., under a hypothetical SB 673-style condo-ADU regime — not currently authorized in Texas) WOULD trigger replatting and associated review. Plat applications processed within 30 days.

Collin County Development Services (part of County Engineering, headquartered at 4690 Community Ave., McKinney, TX 75071) administers the narrow slate of permits available to unincorporated parcels in Texas counties. Unlike California or Oregon counties, Texas counties do NOT run a broad building-code plan-check program: there is no county residential building permit, no county electrical/mechanical/plumbing permit, and no county design review. A homeowner constructing a detached dwelling or ADU on an unincorporated Collin County parcel files only the hazard-and-infrastructure permits the county is statutorily authorized to issue: a Development Permit ($50) if the parcel is in a regulated floodplain, an OSSF (On-Site Sewage Facility) septic permit if the parcel is not on a central sewer system, a 911 rural-address assignment for new structures on vacant lots, and a Culvert / right-of-way permit ($250) if driveway frontage work on a county road is involved. Commercial work in unincorporated areas is reviewed by the Fire Marshal for compliance with the 2021 International Fire Code / International Building Code; residential work is NOT. For parcels inside a city ETJ (Extraterritorial Jurisdiction) — which covers most of Collin County outside established municipal boundaries — the city's platting authority applies (Tex. LGC ch. 212) and city rules may add building-code review via interlocal agreement with the county. The county's effective policy is: if your ADU is on a parcel that has a city limit or ETJ, consult the city; if it's in true unincorporated Collin County outside every ETJ (a small and shrinking geography in a rapidly-incorporating county), the county permits septic, 911 address, floodplain compliance, and driveway, and otherwise leaves construction to the owner.