Chapter 7
Overlay Districts
Mapped overlay districts that add standards in addition to, or in lieu of, the underlying zoning district. Consolidates the federal-interface overlays (Airport Noise Influence, Wildland-Urban Interface) and provides the framework for future overlays added through the regular zoning text and map amendment processes.
11-7-1Purpose and applicability
This chapter establishes overlay districts. Overlay districts are mapped on the Official Zoning Map and apply in addition to the underlying zoning district.
The standards of an overlay district apply to development within the overlay's mapped boundaries. Where an overlay standard conflicts with a standard of the underlying zoning district, the overlay standard controls, except as Section 11-7-3 provides otherwise.
Overlay districts established under this chapter are
The Airport Noise Influence Overlay (Section 11-7-4)
The Wildland-Urban Interface Overlay (Section 11-7-5).
Additional overlay districts may be established under Section 11-7-6.
11-7-2Establishment of overlay districts
An overlay district is established by adoption of an overlay-specific section in this chapter and adoption of the overlay's boundaries on the Official Zoning Map. Both adoptions are accomplished under the zoning text amendment and zoning map amendment procedures in Chapter 3.
The section that establishes an overlay district states
the purpose of the overlay
the geographic basis for the overlay's boundaries, including the adopted source or finding from which the boundaries are derived
the standards that apply to development within the overlay; and
the relationship of the overlay's standards to the standards of the underlying zoning district, where the section provides a different relationship than the general rule in Section 11-7-3.
11-7-3Relationship to underlying zoning districts
The standards of an overlay district apply in addition to the standards of the underlying zoning district. Both sets of standards apply, and development must comply with both, except as subsection B provides.
Where an overlay standard and an underlying district standard cannot both be satisfied, the overlay standard controls.
An overlay-establishing section may provide a different rule for the relationship between the overlay's standards and the underlying district's standards. Where the section provides a different rule, that rule controls.
The overlay districts established under this chapter do not change the use-table calls of the underlying zoning district. Uses permitted in the underlying district remain permitted within the overlay, subject to the overlay's standards.
11-7-4Airport Noise Influence Overlay
Purpose. The Airport Noise Influence Overlay protects noise-sensitive land uses from the cumulative effects of aircraft noise and protects the operational viability of Helena Regional Airport.
Boundaries. The boundaries of the Airport Noise Influence Overlay are shown on the Official Zoning Map and are derived from the most recent Airport Master Plan or noise study adopted by the Helena Regional Airport Authority.
Standards. Development within the Airport Noise Influence Overlay complies with
Disclosure. A noise disclosure is recorded against the title of every parcel within the overlay at the time of any subdivision, transfer, or building permit. The disclosure states that the parcel is subject to aircraft noise and that residential use within the overlay is subject to a higher level of ambient noise than residential use elsewhere in the City.
Sound attenuation. New residential construction within the overlay incorporates sound attenuation measures sufficient to achieve an interior noise level of 45 dB Ldn or less in habitable rooms.
Avigation easement. An avigation easement runs in favor of the Helena Regional Airport Authority for every parcel within the overlay. The easement permits aircraft overflight, noise, vibration, and incidental airspace use customary to airport operation.
Use limitations. Within the overlay, new construction of schools, hospitals, and other noise-sensitive institutional uses is conditional. The applicant demonstrates that the proposed use can achieve interior noise levels appropriate to the use through sound attenuation, site planning, or both.
11-7-5Wildland-Urban Interface Overlay
Purpose. The Wildland-Urban Interface Overlay reduces the risk of structure ignition and fire spread in areas where development meets undeveloped wildland vegetation.
Boundaries. The boundaries of the Wildland-Urban Interface Overlay are shown on the Official Zoning Map. The boundaries encompass the corporate limits of the City and overlay all other zoning districts.
Roofing materials. A structure within the overlay does not have exposed wooden roofing material, treated or untreated. Roofing materials are noncombustible or are fire-resistant materials rated Class C or higher under ANSI/UL 790 or ASTM E 108 or an equivalent test.
Renovation triggers. Where an existing roof undergoes renovation, alteration, or repair that affects more than ten percent of the square footage of an affected roof plane, the affected plane meets subsection C. Where the renovation, alteration, or repair affects more than fifty percent of the square footage of the entire roof, the entire roof meets subsection C.
Vent screening. All vents in roofs, chimneys, gables, and eaves are screened with openings no larger than one-fourth inch in any dimension.
11-7-6Additional overlay districts
Additional overlay districts may be established by amendment of this chapter and amendment of the Official Zoning Map under the procedures in Chapter 3.
An overlay district established under this section conforms to the structure of Section 11-7-2 and the relationship rules of Section 11-7-3, except as the section establishing the overlay expressly provides otherwise.
Open items for this chapter (5)
- Section 11-7-4 (Airport Noise Influence Overlay): the avigation easement requirement in subsection C(3) needs review against current Helena practice and against Helena Regional Airport Authority requirements. Current Chapter 36 does not state the avigation easement as a code requirement, but the airport authority commonly requires one in practice. Planning staff propose stating it in the overlay so the practice is operating under adopted code rather than under practice alone.
- Section 11-7-4 (Airport Noise Influence Overlay): the 45 dB Ldn interior standard in subsection C(2) is the FAA-recommended threshold for habitable rooms. Current Chapter 36 does not specify a numeric interior threshold. Planning staff propose adopting the FAA threshold explicitly so the standard is measurable. Worth checking with airport authority and with state aviation administration.
- Section 11-7-5 (Wildland-Urban Interface Overlay): the renovation thresholds in subsection D (10% / 50%) carry forward from current Chapter 41. Worth reviewing against current wildfire code best practice (NFPA 1144, IWUIC) to see whether the thresholds reflect current understanding of structure ignition risk.
- Future overlays: the working file's deferred drafting list does not name additional overlays as items planning staff intend to draft. Section 11-7-6 establishes the procedural pathway; substantive proposals for floodplain, historic, viewshed, or neighborhood character overlays are deferred to later work or to the consultant's full rewrite.
- Relationship to subdivision: an overlay's standards can affect what subdivision can occur within the overlay (lot size, infrastructure requirements). Chapter 5 cross-reference to overlay districts to be drafted; current chapter does not state this cross-reference.