I’m torn between a 600 sqft ADU and a 600 sqft addition. Which costs less?

Question

Marcus Golen · Accepted Answer

Per square foot, an addition is 20–30% cheaper ($380–480/sqft vs $520–700/sqft for a DADU) because it shares roof, walls, foundation, and utilities. But additions don’t add a separate rentable unit, won’t separate-meter for refinancing, and won’t qualify for ADU-specific tax treatment. If your goal is more space for your family, addition wins. If your goal is rental income or future caregiver/multi-generational use, DADU wins despite the higher cost.

Answer

For a fully detached, code-built DADU in Seattle (2026), realistic all-in pricing is $340k–$520k for 600–1,000 sqft. That includes design ($18–35k), SDCI permit fees ($14–22k), site work and utility runs ($30–70k), construction ($240–380k), and a 10–15% contingency. Quotes under $250k almost always exclude utility sepa

Answer

AADU wins on upfront cost (typically $180–$300k for a basement conversion) but rents for 15–25% less and is harder to refinance separately. DADU costs more ($340–$520k) but commands premium rent ($2,400–$3,400/mo in Seattle), is easier to refinance with separate utility meters, and adds more resale value. ROI math: a D

Answer

In 2026, a new 600–800 sqft DADU in core Seattle (Ballard, Capitol Hill, Wallingford, West Seattle) rents for $2,400–$3,200/mo unfurnished and $3,200–$4,500/mo furnished/short-term. North Seattle (Lake City, Northgate) is 10–15% lower. Outside Seattle, expect Bellevue $2,600–$3,400, Kirkland $2,400–$3,100, Redmond $2,5

Answer

Detached garage-to-DADU conversion in Seattle typically runs $220–$340k for 400–600 sqft. The garage shell saves you ~$40–60k versus ground-up but you almost always need new foundation work (most garages don’t meet residential code), full insulation, new electrical service, water/sewer extensions, and full Energy Code

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Recent (2024–2026) Seattle appraisal data shows DADUs add 65–85% of build cost to appraised value at first appraisal — so a $400k DADU adds roughly $260–340k. Within 3–5 years, as comps mature, that ratio rises to 85–110%. AADUs add less in absolute dollars but a higher ratio (~90% of cost) because they don’t require n

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For a custom DADU in Seattle, design + structural + civil + energy modeling typically runs $22–38k (roughly 6–8% of construction cost). Pre-approved plans drop that to $8–14k (site adaptation only). Beware quotes under $15k for custom — they usually exclude structural calcs, energy compliance, or revisions, all of whic

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