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ADU RESOURCE & BUILDER · PUGET SOUND, WA

What can you build on your lot? And what will an ADU cost?

Washington now lets nearly every homeowner add two accessory dwelling units. Prism turns state law, your city’s code and real construction costs into one clear answer for your property — from Everett to Tacoma.

QUICK ESTIMATE 3 INPUTS
3001,200 SF
02 — ADU TYPE
ESTIMATED ALL-IN COST $330k – $510k $414–639 / sq ft · build, design, permits, utilities, finishes
Full breakdown
ALLEY STREET ADU 02 DETACHED MAIN HOUSE EXISTING ADU 01 ATTACHED 50'-0" 100'-0" ≤ 1,000 SF 5'-0" SETBACK N
SHEETA-01
SITE PLAN50' × 100' LOT
STATE BASELINE2 ADUS / LOT
Black backyard cottage among trees
DETACHED ADU
Wood-paneled ADU living room with fireplace
INTERIOR
Small prefabricated house with a red roof
PREFAB
Compact modern kitchen
KITCHEN
Bedroom with warm lighting
BEDROOM
WASHINGTON ADU LAW · HB 1337

The statewide floor every city has to meet.

Read the law in plain English →
2 ADUs per residential lot — cities inside urban growth areas must allow them.
1,000SF The lowest maximum size a city may set for an ADU.
24FT Minimum height cities must allow for a detached ADU.
0 Owner-occupancy requirements. Cities can no longer make you live on site.
02RULES BY CITY

What’s allowed in your city.

State law sets the floor; each city adds its own setbacks, fees and permit steps. Every city page puts them in one table, with a link to the official code.

See all cities on the map →
SeattleSEATTLE 01 BellevueEASTSIDE 02 RedmondEASTSIDE 03 KirklandEASTSIDE 04 SammamishEASTSIDE 05 IssaquahEASTSIDE 06 Mercer IslandEASTSIDE 07 BothellEASTSIDE 08 ShorelineNORTH 09 LynnwoodNORTH 10 EdmondsNORTH 11 EverettNORTH 12 RentonSOUTH KING 13 KentSOUTH KING 14 AuburnSOUTH KING 15 Federal WaySOUTH KING 16 BurienSOUTH KING 17 TacomaPIERCE 18 King CountyUNINCORPORATED 19 Snohomish CountyUNINCORPORATED 20 Pierce CountyUNINCORPORATED 21
03ADU TYPES

Five ways to add a home to your lot.

Compare all types →
01 A-frame backyard cottage Detached ADUA backyard cottage with its own entry, roof and utilities. 02 Modern wood-clad addition Attached ADUAn addition that shares a wall with the main house. 03 Compact kitchen in a converted garage Garage conversionAn existing garage rebuilt as a finished, legal unit. 04 Living room in a finished lower level Basement conversionA lower level turned into a separate apartment. 05 Prefabricated modular house Prefab / modularFactory-built modules set on a site-built foundation.
04PROCESS

From idea to keys, in five steps.

  1. 01
    Feasibility Confirm what your lot allows: size, height, setbacks and lot coverage. Pencil on a site plan
  2. 02
    Design Start from a pre-approved plan or design custom, sized to your city’s rules. Architect drawing plans
  3. 03
    Permit Submit to the city, answer plan-review corrections, pay the fees. Architectural blueprint
  4. 04
    Build Foundation to final inspection, including sewer, water and power connections. Framer on a wall frame
  5. 05
    Live in it or rent it House family, downsize into it, or register it as a long-term rental. Hand holding house keys
Get an Estimate Read the step-by-step guide
05PLANS & IDEAS

Layouts sized to the rules.

Browse all plans →
Studio≈ 350–500 SF Price this plan →
One bedroom≈ 500–700 SF Price this plan →
Two bedroom≈ 750–1,000 SF Price this plan →
06INSIDE

Small footprint. A whole home.

See the layouts →
Cabin living room with hanging fireplace Bright compact kitchen Bathroom with glass shower Minimal living room with large windows Bedroom with city view Kitchen island with stools
07METHODOLOGY

Every number here has a source.

We are builders, so our estimates have to survive a real bid. The rules and the cost model behind this site are published, line by line.

Methodology & Sources
STATE LAW
Washington’s ADU statute (HB 1337) and middle-housing law (HB 1110), summarized in plain English.
CITY CODE
Each city page links straight to the municipal code section it summarizes.
COSTS
An itemized model — construction, design, permits, utilities, finishes — with every assumption shown.
08JOURNAL

Field notes.

All articles →
Dark wood-clad small house OWNERSHIP Can you sell an ADU separately as a condo in Washington? Read → Drafting a plan by hand PERMITS Pre-approved ADU plans: what they save, and what they don’t. Read → House keys FINANCING HELOC, cash-out refi or renovation loan: paying for an ADU. Read →
GET AN ESTIMATE

Bring your lot. We’ll bring the build plan.

Send your address and calculator result. We check it against your city’s code and come back with a scoped estimate.

Get an Estimate → Open the calculator