Module 2: Setbacks
Where Can Your ADU Physically Sit on Your Lot?
Once you know your parcel zoning permits an ADU and you have sufficient lot coverage capacity, your next step is identifying where on the property the building can actually sit.
A property may look wide open on a satellite view, but strict municipal zoning laws mandate minimum distances between your new structure and property lines, existing buildings, utility lines, and recorded easements. This unbuilt interior space is known as your buildable envelope.
Follow these three steps to map your property's placement envelope.
Step 1: Measure Front, Side, and Rear Yard Setbacks
Setbacks are mandatory buffer zones along property lines where no permanent above-ground structures can be built.
[ Image Placeholder: Diagram illustrating Front Setback, Side Yard Setbacks, Rear Setback, and the inner Buildable Envelope ]
Locate Property Lines: Consult a boundary survey or property tax map. Do not rely on existing fence lines, as fences are frequently installed off actual legal property boundaries.
Apply Local Minimum Setbacks:
Side Yard Setbacks: Most PNW jurisdictions mandate a minimum 5-foot side setback for detached ADUs (though some urban zones allow reduced setbacks for alley-facing structures or converted accessory buildings).
Rear Yard Setbacks: Typically 5 feet from the rear property line.
Front Yard Setbacks: DADUs are generally required to sit behind the primary residence, adhering to the standard principal structure front yard setback (often 15–20+ feet).
Check Corner Lot & Alley Exceptions: If your lot sits on a corner or borders a platted alleyway, special sightline or alley-dedication setbacks may apply.
Step 2: Account for Building Separation & Power Line Clearances
In addition to property line buffers, your local building code enforces physical separation distances between structures on the same lot:
Separation from Primary Residence: Local building codes usually require a minimum 10-foot separation between the exterior walls of the main house and a detached ADU for fire safety and emergency access.
Overhead Power Line Clearances: Utility providers (such as Seattle City Light or Puget Sound Energy) enforce strict vertical and radial clearances—typically maintaining 10 feet of clear radial distance from high-voltage overhead service lines to any part of a new roof structure.
Eaves and Overhangs: Remember that roof eaves, gutters, and cantilevered decks often count toward setback encroachments if they project more than 18–24 inches into a required buffer.
Step 3: Identify Recorded Easements & Utility Rights-of-Way
Easements give utility companies, neighbors, or municipalities legal rights to access portions of your land. You cannot build a permanent foundation over a recorded easement.
[ Image Placeholder: Sample site plan depicting a utility easement running along a rear property line ]
Common Easement Types: Look for drainage easements, public utility easements (PUE), or shared driveway rights-of-way on your property title report or plat map.
Underground Line Locates: Before finalizing where your foundation will sit, call 811 (Utility Locate) to map underground gas, water, electric, and fiber optic lines traversing your yard.
Matching Your Envelope to Understory Designs
Once you measure your buildable envelope width and depth, you can select an Understory pre-designed plan set optimized for your yard dimensions:
Narrow Lot Envelopes: Designs with compact footprints fit within tight side-yard constraints.
Two-Story Designs (e.g., Nurse Log or Twinflower): Keep ground footprint small while doubling living space upstairs, helping maintain required yard buffers and building separation [1].
Flexible Single-Story Models (e.g., Foxglove): Ideal for wider rear yards with generous building envelopes [1].
Module 2 Self-Assessment Checklist
Before moving on to critical areas and topography, confirm you can check off these placement items:
[x] Property Line Buffers: My proposed location honors required front, side (5 ft), and rear (5 ft) yard setbacks.
[x] Building Separation: I have maintained the required separation distance (typically 10 ft) from the main house.
[x] Easement Clear: The proposed footprint does not encroach on recorded utility easements or power line clearances.
What’s Next?
Now that you’ve mapped out a clear buildable envelope for your ADU, the next step is evaluating site topography, steep slopes, and environmental triggers.
