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Phase 2 · Design · Step 2.4

Civil + stormwater design

A PE civil engineer produces the drainage plan and on-site stormwater BMP. Almost every DADU triggers this because the footprint plus walkways and parking pad cross 750 sq ft.

Who
Civil eng.
How long
2-3 weeks
Cost
$2,500-$7,500
You end up with
Stormwater site plan + drainage report

The trigger most owners don't see coming

Drainage review kicks in when a project disturbs more than 5,000 sq ft of land, or adds or replaces more than 750 sq ft of hard surface.

A typical 800–1,000 sq ft DADU plus a walkway, small patio, and any vehicle pad almost always crosses 750. So pretty much every DADU triggers drainage review, whether you planned for it or not.

What the civil engineer produces

  • Standard Drainage Control Plan — the stormwater submittal SDCI reviews.
  • On-Site Stormwater Management List Approach Calculator — proves your BMP meets the code requirement.
  • Drainage report — narrative explanation of the BMP choice and sizing.
  • For projects ≥ 5,000 sq ft of new + replaced hard surface: a PE-prepared and stamped plan (most single-DADU projects stay under this).

Common BMP choices

  • Bioretention rain garden (per SDCI Tip 532). The default for most projects with available yard space.
  • Cistern with sized overflow — works on tight sites.
  • Permeable paving for the walkway and parking pad. Avoids counting them as "hard surface" at all.

The free money: RainWise rebate

If your address is in a RainWise-eligible drainage basin, the rebate covers up to 100% of cisterns and rain gardens — averaging $4,000–$4,600. Outside RainWise basins, King County GSI Mini Grants offer up to $1,500. Check eligibility before you finalize the BMP. A free $4,000 rebate changes the math.

Cost

  • $2,500–$7,500 for civil + stormwater on a typical Seattle DADU.

Sequencing

Engage in parallel with structural (step 2.3) and energy (step 2.5), after schematic design is signed off.

Where this information came from

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