Olympia, WA Building Setbacks & Zoning: Lot Coverage Limits (2026)

Olympia caps impervious and hard surface coverage on residential lots through Table 4.04 in OMC 18.04.080, and subsection (K), Surface Coverage Limits, controls how those caps can be exceeded or worked around. The table's hard surface column is labeled as a performance based bonus that is not guaranteed, and it states that hard surfaces are treated as impervious unless shown workable through an approved design.

The increased impervious coverage bonus allows up to a 20 percent increase above what Table 4.04 lists, but only for three kinds of projects: a non-residential use such as a school, park or place of worship permitted in the residential zone; a development with accessory dwelling units or middle housing units that exceeds minimum density on the underlying lot; or a land division that achieves the maximum allowed density in its zone. A stand-alone single-family unit on a lot is ineligible, as is any project in the Residential Low Impact Zone.

Eligibility also requires an engineered drainage design. The area above the Table 4.04 figure must meet Drainage Design and Erosion Control Manual core requirement #5 for onsite stormwater treatment and core requirement #7 for flow control, regardless of minimum area thresholds. Projects that would otherwise be exempt from flow control must still demonstrate compliance. Pervious pavement, pervious geogrid, bioretention, infiltration, green roofs and micro-detention systems are the listed examples, permeable pavement and geogrid on Type D soils are ineligible, and the code says that if compliance cannot be provided, the bonus will be denied. A maintenance agreement for the flood control system is required before permit issuance.

The hard surface coverage bonus has its own test. All K.1 criteria must be satisfied, projects on type D soils are ineligible, and total hard surface coverage cannot exceed the site area minus the minimum open space requirement. Existing legally built lots that exceed current limits can be replaced within the existing footprint but not expanded in a way that increases the nonconformity, and replacement must follow the Drainage Design and Erosion Control Manual. Ingress and egress easements serving a neighboring property, flag lot panhandles and code-driven driveway extensions beyond the setback are excluded from the calculation.