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Planning commission narrowly approves small Del Casa addition over Warner Canyon Creek with restoration and construction protections
Summary
After extensive public comment and debate about precedent and creek protection, the Planning Commission approved a 182 sq ft second‑story addition at 81 Del Casa, finding the site’s constraints and proposed native landscaping and bioswales justified a variance; the approval includes strict construction‑management conditions to protect the creek and requires the project to return to the commission if foundation work in the setback becomes necessary.
The Mill Valley Planning Commission voted 3–2 to approve a variance allowing a modest 182‑square‑foot second‑story addition at 81 Del Casa that encroaches into the city’s 30‑foot creek setback for Warner Canyon Creek. The property—an L‑shaped house constructed in 1948—already lies within the creek setback and partially spans the creek; staff described the parcel as unusually constrained with little buildable area outside the setback.
Daisy Allen, senior planner, told the commission staff’s recommended approval rests on four findings required for a variance: (a) an exceptional circumstance uniquely constrains the parcel, (b) the need for a variance was not created by the applicant, (c) the requested variance is necessary for reasonable development on the property (the proposal does not alter the existing building footprint or require new foundations), and (d) mitigation measures and conditions will avoid adverse impacts to neighboring properties and the creek. The updated proposal removed an upper deck and other elements that had prompted the commission to deny an earlier version of the project and added a native riparian planting plan and bioswales intended to improve stormwater management and habitat adjacent to the creek.
The applicant team—represented at the hearing by Adrienne Fodderhans—said it had worked with local nonprofit StreamKeepers and a biological consultant, and that StreamKeepers reversed an earlier opposition after the deck was removed and the landscaping plan revised. Architect Steve Geisler explained the project retains the house’s footprint and relies on a structural review indicating no new foundations are required.
Downstream neighbors urged denial during public comment, citing a two‑year‑old storm video showing fast flows in a concrete‑lined channel, visible pilings under the house and a concern that introducing more built area near the creek sets a precedent for future encroachments. "This is about protecting Warner Canyon Creek," one neighbor said, asking the commission to require an on‑site engineer inspection and robust erosion controls.
Commissioners split on the central policy tradeoff: some argued the creek setback ordinance exists to limit development near riparian habitat and that granting a variance that permits additional over‑creek building risks harmful precedent; others said the applicant materially reduced the scope of the previous proposal, the parcel is uniquely constrained, and the added native landscaping and bioswales present net ecological benefits compared with the current condition.
The approval includes several specific construction‑management and mitigation conditions: a construction management plan (CMP) requiring netting over the creek during demolition, surgical hand removal of the existing sunroom and controlled debris capture, prohibition of sawing and cutting exterior framing and siding over the creek (such work must be done inside or in the front yard), planting of native riparian species as shown in the approved landscape plan (subject to Southern Marin Fire District spacing requirements), preparation of a stormwater control and erosion‑control plan for review by the Public Works Department and relevant resource agencies (California Department of Fish and Wildlife and the San Francisco Bay Regional Water Quality Control Board), and a requirement that any need for new foundation work in the creek setback will nullify the approved variance and require the applicant to return to the commission.
The motion to approve passed 3–2. The decision is appealable to the City Council within 10 calendar days.
