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Planning commission discusses ADU height, setbacks and impervious-surface options in workshop
Summary
Planning staff presented possible code amendments for ADUs and impervious-surface standards and the commission offered preliminary support for increasing setbacks and simplifying the ADU second-story stepback rule.
Planning staff presented possible code changes for accessory dwelling units (ADUs), accessory structures and impervious-surface standards at a June 10 Planning and Zoning Commission workshop, and commissioners provided preliminary guidance but made no formal changes.
Staff member Hillary led the discussion and said administering the current ADU second-story stepback is time-consuming for staff and applicants, requiring detailed elevation drawings and repeated measurements. City engineer Chris Bosley attended to advise on stormwater and impervious-surface issues, and the city’s building official (Ted Lancey) was referenced regarding snow-shedding and roof details.
Staff outlined several topics for commission input: (1) remove the ADU-specific second-story stepback and instead increase side-yard setbacks for rear-yard ADUs (proposed 10-foot side yards, with a reduced 5-foot rear setback where a rear alley exists); (2) limit ADU height to lower maximums within the principal building envelope unless the ADU is attached to the principal dwelling (the code currently allows up to 32 feet within the principal envelope); (3) whether all accessory structures should be subject to the same increased setbacks; (4) whether windows on ADU garage structures that face neighboring residential lots should be restricted; and (5) whether to change from the current ADU requirement of 30% pervious surface to an impervious-surface approach and, if so, what percentage thresholds should apply.
Commissioners generally favored simplifying the rules for clarity and easier administration. Several spoke in favor of moving from a complex second-story stepback calculation to a straightforward setback standard (10 feet on both side yards was suggested for consistency), and of limiting ADU height within the principal envelope unless the accessory unit is attached to the main dwelling. Commissioners and staff noted that small lots and PUDs could be constrained by any larger setback or pervious/impervious requirement and that some lots might be effectively unable to accommodate ADUs.
On impervious surfaces and stormwater, staff said the current ADU section requires 30% pervious surface and that the prior city code used an impervious-based table (different limits by zoning) that was repealed. Engineering staff said a single, simple threshold (for example, an increase from 30% to 40% pervious or a 60% impervious cap) would be easier to administer than complex formulas; engineering also recommended a two-tiered approach where projects above a reasonable threshold would submit a simple stormwater checklist or a basic drainage plan that demonstrates how roof and hard-surface runoff will be managed on site. Staff and commissioners discussed requiring snow-retention devices on roofs near property lines, gutters and downspout routing, French drains and dry wells as feasible mitigation measures, and the practical enforcement limits for small lots.
No formal code amendments were adopted at the workshop. Staff said they will draft concrete amendment language based on the commission’s input and provide options for setbacks, ADU height limits, and a simplified impervious/pervious threshold (including a proposed checklist or trigger for a drainage plan when projects exceed that threshold).
