Responsible ADU building starts by proving what the current rules and property conditions allow before a favorite plan is treated as feasible. A detached backyard unit, attached conversion, basement unit, or addition can involve different site, structure, utility, access, energy, and construction questions. Rules also change, so source and date belong beside every early conclusion.
This guide is intentionally bounded to a Seattle workflow rather than pretending one summary governs surrounding cities. Confirm jurisdiction and use current agency material for the parcel. The purpose of early due diligence is not to design the entire project; it is to expose constraints, alternatives, decision owners, and information still needed.
Create a parcel evidence sheet
Record the address, parcel number, jurisdiction, lot dimensions, zoning, existing structures, easements, environmentally critical areas, shorelines, trees, slopes, access, and known utilities. Gather surveys, title material, permits, plans, and prior correspondence. Mark the source and retrieval date for each fact, and separate verified conditions from owner recollection or map interpretation.
A Seattle addition and ADU builder can organize that evidence for design and specialist review. Do not scale a concept from an online aerial image or assume a fence marks a legal boundary. Decide when a survey, arborist, geotechnical, utility, or other focused investigation is warranted.
Read current Seattle requirements as a system
Seattle’s official Accessory Dwelling Unit guidance states that an ADU must meet current residential, building, mechanical, electrical, energy, land-use, environmentally critical area, and shoreline standards. Use the live page and linked code resources; this article does not freeze a project’s rules at publication.
Build a rule matrix with the issue, source, date checked, evidence, preliminary conclusion, reviewer, and open question. Cover permitted unit type and count, size or form limits, yards, height, lot coverage, entries, parking where applicable, and constraints unique to the parcel. Obtain confirmation when interpretation materially controls purchase, layout, or cost.
Compare attached and detached paths
An attached unit may reuse enclosure, foundation, or utilities but can expose fire separation, sound, egress, ceiling height, moisture, and unknown-condition work. A detached unit may offer privacy and cleaner planning but adds site construction, foundation, exterior envelope, utility routing, and delivery access. Compare the actual property rather than assuming one format is inherently easier.
The ADU design and new-construction guide focuses on turning feasibility into a livable small-home plan. Keep that design work downstream of the rule check. If two locations remain plausible, carry both with explicit investigation needs until evidence supports a controlled choice.
Trace utilities from source to fixture
Map water, sewer, stormwater, power, gas if considered, communications, heating, ventilation, and fire-safety systems. Identify capacities, routes, meters, shutoffs, slopes, trench conflicts, easements, restoration, and service access. A nearby main house connection on a sketch may be inaccessible, undersized, or costly once elevation and existing construction are measured.
Ask each utility or reviewing authority for current process and documentation when their approval is required. Coordinate energy and equipment choices with enclosure design and available service. Preserve alternative routes until site, structural, and landscape impacts are compared; a shorter trench is not automatically the best route if it conflicts with roots or future access.
Test people, emergency access, and construction access
Draw the daily route from street to entry, including gates, lighting, grade, privacy, waste, mail, bicycles, and mobility needs. Separately trace emergency and service access. Then model excavation, concrete, framing, crane or pump needs, material staging, worker parking, neighbor protection, and restoration. Construction access can eliminate a concept that fits geometrically on the lot.
Use the Seattle small-home design guide for interior performance after the viable footprint and approach are known. “Luxury” in a compact unit should mean dependable comfort, storage, daylight, acoustics, and serviceability—not features that crowd circulation or prevent replacement.
Budget uncertainty before finishes
Create early cost categories for surveys and consultants, design, permits, utility work, site protection, demolition, earthwork, foundation, structure, enclosure, systems, interiors, equipment, access, logistics, testing, and closeout. Link each category to the current concept and assumptions. Avoid presenting a single area-based number as though site and connection work were already solved.
Maintain a feasibility risk register with the question, why it matters, least-cost test, decision owner, due date, and possible responses. Examples include sewer elevation, tree impacts, electrical capacity, existing foundation, access width, or an unverified record. Resolve high-consequence unknowns before investing in detailed finishes or nonrefundable orders.
Release design from a dated feasibility record
At the feasibility gate, reconcile parcel evidence, current rule matrix, surveys, site constraints, unit type, utility concepts, access, specialist recommendations, preliminary budget, and unresolved risks. State what has been confirmed by an authority and what remains a design assumption. Freeze the source date so later code checks can identify what changed.
Send the parcel information, available survey, intended occupancy, unit options, and known constraints through a Seattle ADU estimate request. The initial response should distinguish due diligence and design work from construction pricing, with clear dependencies for any preliminary range.
Continue checking rules at defined project gates: concept selection, permit preparation, submission, correction response, construction release, and approved field change. Retain issued documents, inspections, utility records, photographs, manuals, and as-built information at closeout. A durable ADU record preserves both the design and the basis on which it was allowed.