Methodology & sources.
Where every rule on this site comes from, how the calculator builds a number, and how we keep both current.
01HOW RULES ARE SUMMARIZED
From code to a city page, in four steps
- 01Read the codeState statute first, then the city’s current ADU section.
- 02Fit the templateEvery city gets the same rule table, so cities compare line for line.
- 03Link the sourceEach page links to the exact code section it summarizes.
- 04Re-check on changeWhen a city amends its code, the page is reviewed and re-dated.
02THE COST MODEL
How the calculator builds a number
ALL-INRange
CONSTRUCTIONSF × base rate × city factor
FINISHESSF × finish rate
DESIGNFixed, by path
PERMITS3–5% of construction
UTILITIESSum of chosen connections
Each line is computed twice — once at the low rate, once at the high — and the two totals form the range. Land, financing, furniture and unusual site conditions are excluded.
03RATES & FIXED COSTS
Base construction rate
| Detached ADU | $300–420 / SF |
|---|---|
| Attached ADU | $280–400 / SF |
| Prefab / modular | $260–380 / SF |
| Garage conversion | $150–250 / SF |
| Basement conversion | $120–220 / SF |
Finish rate
| Standard | $40–70 / SF |
|---|---|
| Upgraded | $70–110 / SF |
| Premium | $110–170 / SF |
Fixed costs
| Design — pre-approved plan | $8–15k |
|---|---|
| Design — custom | $20–40k |
| Design — conversions | $6–25k |
| New side sewer | $15–35k |
| Separate electric meter | $4–10k |
| Separate water meter | $5–14k |
| Tie-ins only (conversions) | $3–8k |
| Permits & fees | 3–5% OF BUILD |
04CITY FACTORS
Local labor and permitting, as one multiplier
Mercer Island×1.12
Bellevue×1.10
Seattle×1.08
Kirkland×1.08
Redmond×1.06
Sammamish×1.05
Issaquah×1.04
Bothell×1.02
Shoreline×1.02
Edmonds×1.02
King County×1.00
Renton×0.98
Lynnwood×0.98
Burien×0.97
Snohomish County×0.96
Kent×0.95
Everett×0.95
Auburn×0.94
Federal Way×0.94
Tacoma×0.93
Pierce County×0.93
05SOURCES
STATERCW 36.70A.681 — Accessory dwelling units
STATERCW 36.70A.635 — Middle housing
STATEDepartment of Commerce — ADU and middle-housing guidance
COUNTYKing County Code, Title 21A — Zoning
COUNTYSnohomish County Code, Title 30 — Unified Development Code
COUNTYPierce County Code, Title 18A — Development Regulations
CITYMunicipal codes for all 18 cities — linked from each city page
REVIEWED BY
[NAME]
[ROLE], Prism General Contractors
CORRECTIONS
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