Module 1: Zoning & Allowed Uses
Can You Legally Build an ADU on Your Parcel?
Before falling in love with a specific floor plan or hiring a contractor, your first step is verifying what local municipal codes allow on your lot.
While accessory dwelling unit (ADU) regulations across Washington State and the Pacific Northwest have expanded significantly to encourage housing density, every city and county enforces specific limits on zoning designation, maximum lot coverage, and floor area ratios (FAR).
Follow these three steps to evaluate your parcel's zoning and footprint capacity.
Step 1: Look Up Your Parcel Number & Zoning Code
To check your property’s rules, you need your parcel’s official zoning designation (such as NR-1, NR-2, R-6, or SF 5000).
Open Your Local GIS Parcel Viewer: Search online for your county's GIS map tool (e.g., King County iMap, Seattle Services Portal, or Snohomish County Parcel Viewer).
Enter Your Address: Locate your property boundaries and copy your 10-digit Parcel Number (APN).
Identify the Zoning Classification: Look under the property details tab for "Zoning" or "Land Use Designation."
Confirm ADU/DADU Allowance: Most single-family residential zones in PNW cities legally permit both attached ADUs and detached DADUs, but always check if your lot is within a specific historic district, HOA, or shoreline overlay that adds restriction.
Step 2: Calculate Your Max Lot Coverage & Floor Area Ratio (FAR)
Municipalities restrict how much of your property can be covered by structures to manage stormwater runoff, tree canopy, and neighborhood density.
[ Image Placeholder: Graphic illustrating "Lot Coverage" vs "Unbuilt Buildable Area" ]
How Lot Coverage Works:
Lot Coverage Ratio: The percentage of your total lot area covered by roofed structures (your primary home, garage, shed, and proposed DADU).
The Calculation: Total Allowed Coverage Sq Ft = Total Parcel Sq Ft x Max Allowed Coverage
Example: If your lot is 6,000 sq ft and your local zone allows 35% lot coverage, your total roofed footprint across all structures cannot exceed 2,100 sq ft. If your main house and garage take up 1,400 sq ft, you have 700 sq ft of ground-level footprint remaining for a DADU.
Matching Footprints to Understory Models:
Understory offers pre-designed architectural plan sets specifically optimized for common Pacific Northwest lot sizes [1]:
Compact Single-Story Footprints: Perfect for tighter lot coverage limits.
Two-Story Designs (e.g., Nurse Log or Twinflower): Maximize living space (up to 1,000 sq ft) while keeping the ground-level building footprint compact [1].
Step 3: Review Local Footprint & Height Limits
In addition to lot coverage percentages, local jurisdictions cap the absolute maximum size of an ADU:
Maximum Square Footage: Many PNW jurisdictions cap detached ADUs at 1,000 sq ft of habitable space (excluding garages or covered porches).
Owner-Occupancy Rules: Washington State legislation has largely removed owner-occupancy mandates for standard ADUs, meaning you generally do not need to live on-site to rent out the main home or the ADU (always double-check your local city ordinance).
Height Restrictions: Base height limits for DADUs typically range between 18 to 25 feet, depending on roof pitch and proximity to property lines.
Module 1 Self-Assessment Checklist
Before moving on to site placement, confirm you can check off these items for your parcel:
[x] Zoning Confirmation: My parcel zoning legally allows an ADU/DADU.
[x] Square Footage Budget: My remaining lot coverage capacity can accommodate my desired ADU footprint.
[x] Size Allowance: My target design falls within local square footage caps (typically 1,000 sq ft).
What’s Next?
Now that you know your zoning allows an ADU and you have room on your lot, the next step is determining where on the property it can physically sit.
Need Help Evaluating Your Site?
If you prefer an expert review of your property before purchasing a plan set, you may schedule a feasibility consultation with Waldron Designs.
