Good ADU design and new construction starts by asking whether a specific property, use, unit type, and delivery method can work together. A concept image cannot confirm development standards, utility capacity, access, drainage, structure, privacy, or construction logistics. Feasibility should narrow the design before detailed selections consume time and budget.
Rules and application procedures can change, and conditions differ by address. This guide does not declare what one lot is allowed to build. It gives homeowners a disciplined way to assemble property evidence, ask the right jurisdictional questions, and connect the accepted concept to documents and field work.
Define the intended household use
State who may occupy the unit, desired privacy, accessibility needs, number of sleeping spaces, cooking expectations, storage, laundry, outdoor access, parking assumptions, and relationship to the main home. A family suite, long-term rental, guest space, and flexible future unit may lead to different priorities.
A Seattle ADU builder can convert that use brief into questions about location, size, systems, and construction. Avoid designing a generic small house before deciding which daily activities and separations the property must support.
Compare unit types against the site
Evaluate an attached addition, internal conversion, or detached structure using the same property record. Show existing buildings, lot lines, topography, trees, drainage, utilities, easements, access, parking, outdoor areas, and neighboring conditions. Each type changes the balance of disruption, independence, and site work.
The attached addition versus detached ADU comparison helps organize those tradeoffs. The preferred form should emerge from evidence and household goals, not from assuming that detached always means better or that conversion always means simpler.
Verify current rules by address
Seattle’s official common-project permit directory includes accessory dwelling units and related residential work. Use current City resources and project contacts to confirm the applicable path, required information, and any land-use or construction questions for the property.
Create a code-and-property question log rather than copying numeric limits into the concept prematurely. Record the source, date checked, interpretation owner, and design effect. If a condition remains unresolved, identify which dimensions, uses, or costs cannot yet be fixed.
Map access before drawing rooms
Show survey and investigation access, excavation and equipment routes, worker parking, material storage, crane or pump needs if relevant, debris removal, and protection for the main home and landscape. A design may fit on paper while being difficult to build without a realistic path to the work area.
Plan resident and emergency routes during construction as well as after occupancy. Gates, stairs, slopes, narrow side yards, overhead lines, fences, and shared driveways can affect equipment and schedule. Treat temporary access restoration as work, not an invisible contractor assumption.
Resolve utility strategy early
Investigate water, sewer or septic as applicable, stormwater, electrical service, heating, cooling, ventilation, communications, and metering goals. Map route lengths, elevations, connection points, panels, equipment locations, shutoffs, and maintenance access. Utility feasibility can control the unit’s placement and floor elevation.
The Seattle ADU permit planning guide explains why utility decisions belong before drawings are final. Obtain the information required by the responsible agencies and design professionals rather than assuming the nearest visible line has capacity or permission.
Design a small home, not leftover space
Test furniture, storage, doors, windows, kitchen work, bathroom clearances, laundry, mechanical equipment, and routes together. Natural light and privacy should be reviewed from inside both dwellings and from neighboring properties. Every circulation area should earn its place while preserving dignified daily use.
Coordinate structure, insulation, moisture control, acoustics, fire separation, ventilation, equipment, and service access as complete assemblies. Compact design increases the consequences of unresolved conflicts; a few inches lost to ducts, chases, or wall build-up can change cabinetry and clearances throughout the unit.
Build the budget around development gates
Separate due diligence, survey, design, engineering, utility investigation, permits and fees, site preparation, structure, enclosure, interiors, equipment, landscape restoration, management, and contingency. Distinguish construction quantities from unresolved approvals. A cost range without those boundaries can conceal the stage of knowledge.
Create decision gates for property feasibility, concept acceptance, utility strategy, developed design, permit set, issued approvals, procurement, and construction release. Do not purchase project-defining products or schedule disruptive work while an earlier gate can still alter the building footprint or system path.
Prepare for construction and handover
The field plan should coordinate staging, inspections, weather protection, neighbor communication, utility interruptions, and protection of occupied areas. At completion, reconcile final approvals, testing, manuals, warranties, finish records, spare materials, utility information, and responsibilities shared with the primary dwelling.
Provide the property address, intended use, photographs, available survey or plans, known constraints, and preferred unit type through an ADU construction estimate request. The first useful output is a defined feasibility path, not an unsupported promise that the lot can accept a chosen design.
Preserve the feasibility record with the accepted concept. If the unit’s use, footprint, openings, utilities, or access later changes, revisit the assumptions and approvals affected. This prevents an attractive revision from silently invalidating the evidence that allowed design and pricing to advance.
At handover, explain shared systems, shutoffs, meters, drainage, maintenance access, address or occupancy records where applicable, and emergency routes to both households. A compact second dwelling needs clear operational ownership after the construction team leaves.