Thinking about a multifamily project in Oregon or Washington? We break down the stormwater rules, green building mandates, and density regulations that...

Multifamily work in Oregon and Washington runs on stormwater manuals written by cities that assume you can infiltrate and building codes that assume you can't. Reconciling those two assumptions on a sloped infill lot is the real site planning exercise.

Density and Site Coverage

Multifamily site planning starts with the density calculation. How many units can the zoning allow? What's the maximum lot coverage? How much open space is required? These numbers set the upper bound on what the architect can design, and they cascade directly into the civil engineer's scope: more units means more parking, more impervious surface, more stormwater treatment, more utility demand, and more grading complexity.

On typical Pacific Northwest infill sites -- quarter-acre to half-acre parcels in established neighborhoods -- state-level housing reforms are reshaping what's possible. Oregon's HB 2001 legalized middle housing statewide, and Washington's HB 1110 followed suit. These laws, combined with local density incentives for affordable housing, often result in higher unit counts than the base zoning historically allowed. From the civil engineer's perspective, the additional units add load to every system: water, sewer, storm, electrical, and parking.

Parking and Access

Parking is the most space-intensive element of a multifamily site plan. At 1.5 spaces per unit — a common suburban requirement — a 60-unit project needs 90 parking spaces. At 300 square feet per space (including drive aisle share), that's 27,000 square feet of paving: nearly two-thirds of an acre dedicated entirely to car storage. On a tight site, parking often dictates the building footprint more than the architecture does.

Oregon and Washington have been at the forefront of parking reform. Oregon's Climate-Friendly Areas rules and Portland's removal of parking minimums in the central city mean many transit-adjacent multifamily projects can reduce or eliminate required parking. This has transformed site planning: without a parking mandate, the site plan can prioritize open space, stormwater treatment, and building coverage. But even with reduced parking, some spaces are still typically provided, and each one needs to meet dimensional, accessibility, and EV readiness requirements.

Stormwater on Tight Sites

Stormwater compliance on a multifamily infill site is one of the most challenging aspects of the civil design. In Portland, BES requires onsite stormwater management for all new development, and in Seattle, the Stormwater Code applies to projects creating 750 SF or more of new plus replaced impervious surface. The impervious area ratio is often 75-90%, which means the stormwater treatment area can consume 3-4% of the total site. Finding room for a facility that's large enough to treat the required area, receives runoff by gravity, and doesn't conflict with foundation setbacks or utility corridors requires careful coordination with the architect and landscape architect from day one.

Flow-through planters along building frontages are a common solution on constrained multifamily sites. They treat runoff from the adjacent roof and sidewalk, occupy linear space that would otherwise be landscape buffer, and can be integrated into the architectural aesthetic. The civil engineer designs the planter dimensions, media depth, underdrain system, and overflow connection; the landscape architect selects the plants and surface treatment.

Utility Coordination

A 60-unit multifamily building needs domestic water, fire service, sanitary sewer, storm drain, gas, electrical, and telecom. Each utility has its own lateral from the street, its own meter or point of connection, and its own set of design standards. On a 40-foot-wide street frontage, fitting all of these laterals without violating the minimum horizontal separation requirements between utilities is a spatial puzzle that the civil engineer solves on the composite utility plan.

The Bottom Line

Every project has its own constraints — site geometry, soil conditions, agency jurisdiction, schedule pressure. What doesn't change is the physics: water flows downhill, utilities need clearance, and code requirements aren't negotiable. The projects that go smoothly are the ones where the civil engineer is involved early enough to shape the site plan around these realities rather than retrofitting solutions after the architecture is locked.

At Calichi Design Group, we've built our practice around getting these details right the first time. Our team has permitted projects in dozens of jurisdictions across the West Coast and Pacific, and we know which agencies want what, which reviewers flag what, and which shortcuts actually cost more time than they save.

If you're starting a project and want to avoid the most common civil engineering pitfalls, reach out for a conversation. We'll give you an honest assessment of what your site needs and a fixed-fee proposal — usually within a week.