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PDFWAC 246-272A-0320

Developments, subdivisions, and minimum land area requirements.

(1) A person proposing a subdivision where the use of OSS is planned shall obtain a recommendation for approval from the local health officer as required by RCW 58.17.150.
(2) The local health officer shall require the following prior to approving any development:
(a) Site evaluations as required under WAC 246-272A-0220, excluding subsections (3)(a)(i) and (4)(d);
(b) Where a subdivision with individual wells is proposed:
(i) Configuration of each lot to allow a one hundred-foot radius water supply protection zone to fit within the lot lines; or
(ii) Establishment of a one hundred-foot protection zone around each existing and proposed well site;
(c) Where preliminary approval of a subdivision is requested, provision of at least one soil log per proposed lot, unless the local health officer determines existing soils information allows fewer soil logs;
(d) Determination of the minimum lot size or minimum land area required for the development using Method I and/or Method II:
METHOD I. Table X, Single-Family Residence Minimum Lot Size or Minimum Land Area Required Per Unit Volume of Sewage, shows the minimum lot size required per single-family residence. For developments other than single-family residences, the minimum land areas shown are required for each unit volume of sewage. However, the local health officer may require larger lot sizes where the local health officer has identified nitrogen as a concern either through planning activities described in WAC 246-272A-0015 or another process.
Minimum Land Area Requirement
Single-Family Residence or Unit Volume of Sewage
Type of Water Supply
Soil Type (defined by WAC 246-272A-0220)
0.5 acre
12,500 sq. ft.
15,000 sq. ft.
18,000 sq. ft.
20,000 sq. ft.
22,000 sq. ft.
2.5 acre1
Individual, on each lot
1.0 acre
1 acre
1 acre
1 acre
2 acres
2 acres
2.5 acres1
1See WAC 246-272A-0234(6).
METHOD II. A minimum land area proposal using Method II is acceptable only when the applicant:
(i) Justifies the proposal through a written analysis of the:
(A) Soil type and depth;
(B) Area drainage, and/or lot drainage;
(C) Public health impact on ground and surface water quality;
(D) Setbacks from property lines, water supplies, etc.;
(E) Source of domestic water;
(F) Topography, geology, and ground cover;
(G) Climatic conditions;
(H) Availability of public sewers;
(I) Activity or land use, present, and anticipated;
(J) Growth patterns;
(K) Reserve areas for additional subsurface treatment and dispersal;
(L) Anticipated sewage volume;
(M) Compliance with current planning and zoning requirements;
(N) Types of proposed systems or designs, including the use of systems designed for removal of nitrogen;
(O) Existing encumbrances, such as those listed in WAC 246-272A-0200 (1)(c)(v) and 246-272A-0220 (2)(a)(vii); and
(P) Estimated nitrogen loading from OSS effluent to existing ground and surface water;
(Q) Any other information required by the local health officer.
(ii) Shows development with public water supplies having:
(A) At least twelve thousand five hundred square feet lot sizes per single-family residence;
(B) No more than 3.5 unit volumes of sewage per day per acre for developments other than single-family residences; and
(iii) Shows development with individual water supplies having at least one acre per unit volume of sewage; and
(iv) Shows land area under surface water is not included in the minimum land area calculation; and
(e) Regardless of which method is used for determining required minimum lot sizes or minimum land area, submittal to the health officer of information consisting of field data, plans, and reports supporting a conclusion the land area provided is sufficient to:
(i) Install conforming OSS;
(ii) Assure preservation of reserve areas for proposed and existing OSS;
(iii) Properly treat and dispose of the sewage; and
(iv) Minimize public health effects from the accumulation of contaminants in surface and groundwater.
(3) The department shall develop guidelines for the application of Method II by (insert date one year from the effective date).
(4) The local health officer shall require lot areas of twelve thousand five hundred square feet or larger except when a person proposes:
(a) OSS within the boundaries of a recognized sewer utility having a finalized assessment roll; or
(b) A planned unit development with:
(i) A signed, notarized, and recorded deed covenant restricting any development of lots or parcels above the approved density with the overall density meeting the minimum land area requirements of subsection (2)(d) of this section;
(ii) A public entity responsible for operation and maintenance of the OSS, or a single individual owning the OSS;
(iii) Management requirements under chapter 246-272B WAC when installing a LOSS; and
(iv) Extinguishment of the deed covenant and higher density development allowed only when the development connects to public sewers.
(5) The local health officer may:
(a) Allow inclusion of the area to the centerline of a road or street right of way in a Method II determination under subsection (2)(d) of this section to be included in the minimum land area calculation if:
(i) The dedicated road or street right of ways are along the perimeter of the development;
(ii) The road or street right of ways are dedicated as part of the proposed development; and
(iii) Lots are at least twelve thousand five hundred square feet in size.
(b) Require detailed plot plans and OSS designs prior to final approval of subdivision proposals;
(c) Require larger land areas or lot sizes to achieve public health protection;
(d) Prohibit development on individual lots within the boundaries of an approved subdivision if the proposed OSS design does not protect public health by meeting requirements of these regulations; and
(e) Permit the installation of an OSS, where the minimum land area requirements or lot sizes cannot be met, only when all of the following criteria are met:
(i) The lot is registered as a legal lot of record created prior to the effective date of this chapter;
(ii) The lot is outside an area identified by the local plan developed under WAC 246-272A-0015 where minimum land area has been listed as a design parameter necessary for public health protection; and
(iii) The proposed system meets all requirements of these regulations other than minimum land area.
(6) The use of a reduced-sized SSAS does not provide for a reduction in the minimum land area requirements established in this section. Site development incorporating reduced-sized SSAS must meet the minimum land area requirements established in state and local codes.
[Statutory Authority: RCW 43.20.050. WSR 05-15-119, § 246-272A-0320, filed 7/18/05, effective 7/1/07.]
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