My builder mentioned a side-sewer issue. What does that mean and how much does it cost?

Question

Marcus Golen · Accepted Answer

In Seattle, a DADU can share the main house side-sewer if it has adequate capacity (verified by a CCTV inspection) — saves $15–30k. If a separate side-sewer is required (steep lots, long runs, capacity issues, or main-line replacement triggers), expect $18–45k including SPU permit, trenching, restoration, and tie-in. Always do the CCTV inspection in the feasibility phase — discovering this in construction is the #1 cause of budget overruns.

Answer

Realistic SDCI timing for a DADU in 2026 is 18–28 weeks total from intake to issuance: 2–3 weeks intake, 8–14 weeks first review, 4–6 weeks for our corrections response, 3–5 weeks for the second review cycle. Pre-approved DADU plans cut that to 10–14 weeks. The 8–12 week SDCI quote refers only to the first review cycle

Answer

Real and meaningful. Pre-approved DADU plans skip the architectural review portion of SDCI intake, cutting permit time from 18–28 weeks down to 10–14 weeks and saving roughly $6–10k in design fees. The trade-off: limited design flexibility (you pick from ~15 approved layouts) and minor site adaptation fees. If your lot

Answer

Bellevue is slightly faster (12–20 weeks total vs Seattle’s 18–28) and has clearer setback rules in R-4 and R-5 zones, but the lot-size minimum is higher (5,000 sqft for DADUs) and design review is more rigid for lots near critical areas. Bellevue allows one ADU per lot (not two like Seattle). Permit fees are comparabl

Answer

Seattle exempts ADUs from transportation impact fees and school impact fees — saving roughly $7–18k versus a full new home. You still pay SDCI plan-review fees ($6–11k), inspection fees ($3–6k), SPU water/sewer connection charges ($4–9k if utilities are upgraded), King County wastewater capacity charge ($2,800 one-time

Answer

Yes — every habitable bedroom in a basement AADU needs a code-compliant emergency escape and rescue opening (EERO): minimum 5.7 sqft net clear opening, 24 in min height, 20 in min width, sill no more than 44 in above floor. If your basement doesn’t already have this, expect $8–18k per egress window (saw-cut concrete, w

Answer

Seattle’s 2024 tree code (SMC 25.11) classifies trees by size and species. Tier 2 trees (24+ in DBH) require a permit + replacement to remove and often cannot be removed at all if they’re heritage species. Tier 3 (12–24 in) can usually be removed but require 2:1 replacement. Your DADU site plan must show all existing t

Do I need a separate side-sewer for a Seattle DADU?

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