Stormwater rules sound dry until you realize almost every DADU triggers them — a new roof plus walkways plus a parking pad easily crosses the threshold. Good news: for most projects a $3K drywell or rain garden handles it, and BES's Simplified Approach skips the full engineering calc.
What governs
Bureau of Environmental Services (BES) requires every project that adds impervious surface (roof + paving) to manage that stormwater on-site. The rules live in the Stormwater Management Manual (SWMM). For a typical DADU you'll use the Simplified Approach, which skips a full hydraulic calculation.
The three common DADU solutions
- Drywell. Perforated underground chamber that infiltrates roof runoff. Cheapest. Works in most Portland soils. ~$2,500-$4,500 installed.
- Infiltration planter. Above-grade box with engineered soil that takes downspout runoff and infiltrates. Needs a sliver of yard space. ~$3,000-$5,000.
- Rain garden. Depressed planted area. Most landscape value. Takes the most space. ~$2,500-$5,000.
You may need a geotechnical infiltration test to size the system. Some east-side and west-hill soils don't infiltrate well; in those cases a flow-through planter discharging to the combined sewer (with BES approval) is the fallback.
What the deliverable contains
- A scaled site plan showing the stormwater facility
- Sizing calculations per the SWMM
- Notes that key the construction details
This usually lives on the architectural sheet set. Some designers do the calcs themselves; some bring in a civil engineer.
Where this information came from
- BES — 2025 Stormwater Management Manual · retrieved April 23, 2026