My main panel is 100A. Will I need to upgrade to add an ADU?

Question

Marcus Golen · Accepted Answer

Usually yes. A modern ADU typically draws 60–100A by itself once you include heat pump, induction range, EV-ready circuit, and laundry. With a 100A main, you’ll almost certainly need to upgrade to 200A or 400A ($3–6k for 200A, $7–12k for 400A including SCL service drop). Some projects qualify for a smart load-management system (SPAN panel, etc.) that lets you avoid the upgrade — saves $4–8k but adds $3,500 in equipment. Worth the analysis.

Answer

For a DADU: 5–8 months of active construction once permits are issued. Breakdown: site work and foundation 4–6 weeks, framing and dry-in 4–6 weeks, MEP rough-in 3–4 weeks, insulation and drywall 3–4 weeks, finishes 6–8 weeks, final inspections and certificate of occupancy 2–3 weeks. AADU conversions: 3–5 months. Add 4–

Answer

Three options: (1) Slab-on-grade — cheapest ($18–28k), fastest, but no under-floor mechanical access. Best for flat lots. (2) Crawl space — mid-cost ($28–42k), gives plumbing access, standard choice for most Seattle lots. (3) Full basement — adds 30–60% more usable sqft and $80–140k. Only worth it if zoning lets you co

Answer

Top 5 mistakes we see: (1) Skipping CCTV side-sewer inspection — finding a collapsed line mid-construction adds $25–40k. (2) Underestimating utility separation — separate water meter alone runs $8–15k. (3) Ignoring tree retention — losing a designated tree mid-project triggers $5–25k mitigation. (4) Picking the cheapes

Answer

Six checks: (1) Verify WA L&I license + bond on lni.wa.gov. (2) Ask for 3 completed ADU projects in the last 24 months and call those owners. (3) Confirm they’ve permitted with SDCI (or your city) at least 5 times — shows they understand local quirks. (4) Insist on a fixed-price contract with itemized allowances, not c

Answer

Yes if (a) you plan to rent the ADU long-term, (b) you might sell or refinance the ADU as a separate income unit, or (c) tenants will pay their own utilities. Separate meters cost $8–15k for water (SPU) + $4–8k for electric (SCL) but increase appraisal value by 1.5–2x the install cost and eliminate landlord-tenant util

Answer

Code requires STC 50 between dwelling units. Practical approach: double-stud or staggered-stud wall + Roxul Safe’n’Sound + 5/8" Type X drywall on resilient channels each side gets you STC 56–62. For floor/ceiling assemblies between AADU and main house, use Green Glue between two layers of 5/8" drywall on resilient chan

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