When homeowners plan an ADU, the first question is almost always cost. The second should be time. A backyard unit is a construction project layered on top of a government approval process, and the calendar is where most projects surprise their owners. The build itself is rarely the long pole. Design decisions, plan review, and inspections quietly eat months before and around the actual hammering.
There's no single number that fits every project, because a garage conversion and a from-scratch detached cottage live on completely different timelines. But the phases are the same everywhere in Washington, and once you understand what each one involves, you can estimate your own project with far more confidence and spot the steps worth getting ahead of.
The Four Phases of an ADU Timeline
Every ADU, whether it's an attached basement unit or a standalone DADU, moves through the same four stages. The total time is the sum of these phases, and where you lose weeks is usually the handoff between them.
1. Design and planning. This is where you decide what you're building: unit type, size, layout, and whether you're using a custom design or a pre-drawn plan set. Custom architectural design takes the longest because it's iterative, you review drawings, request changes, and wait for revisions. Choosing a builder's existing plan or a jurisdiction's pre-approved design compresses this phase dramatically. This is also when you should have your financing lined up; our guide to financing an ADU in Washington covers the options so this step doesn't stall the rest.
2. Permitting and plan review. You submit your plans to the local permitting authority, and they review them for code compliance, setbacks, utilities, and zoning. This phase is the least predictable because it depends on the jurisdiction's workload, the completeness of your submission, and how many correction cycles your plans go through. A clean, complete application reviewed by a fast department moves quickly; an incomplete one that bounces back for revisions can add weeks each round.
3. Construction. Once the permit is issued, the actual build begins: site prep and foundation, framing, roofing, then mechanical, electrical, and plumbing rough-ins, followed by insulation, drywall, and finishes. A conversion of existing space is faster because the shell already exists. A detached, from-the-ground-up unit takes longer because it includes foundation and full weatherproofing.
4. Inspections and final approval. Inspections happen throughout construction at key milestones (foundation, framing, rough-ins), not just at the end. The final inspection and certificate of occupancy are what legally let someone live in the unit. Scheduling inspections around the department's availability can add small delays, so a builder who coordinates them tightly keeps this phase short.
What a Realistic Timeline Looks Like
Because the phases stack, most owners should think in terms of several months to roughly a year from first drawings to a finished, occupiable unit, not the construction window alone. A garage or basement conversion sits at the shorter end: the structure exists, so design and construction are both compressed. A custom detached DADU sits at the longer end because it carries a full design phase, ground-up construction, and more inspection milestones.
The single most common planning mistake is measuring only the construction phase. A builder might quote a construction window that sounds short, but that clock doesn't start until the permit is in hand, and getting there took design plus review. When you set expectations, add the front-end phases in. The build is the visible part; the paperwork is the part that runs in the background for months.
What Speeds a Washington ADU Up
Several levers meaningfully shorten the calendar, and most of them live in the first two phases.
- Pre-approved or pre-drawn plans. Some Washington jurisdictions, Seattle among them, offer pre-approved DADU plan sets that have already cleared design review. Starting from one of these can cut significant time off both design and permitting. Many builders also have their own repeatable plans that sail through review.
- A complete, correct permit application. The fastest permits are the ones that don't come back for corrections. An experienced local builder who knows what the reviewer wants submits a clean package the first time, which is often the biggest single time-saver.
- HB 1337's statewide streamlining. Washington's HB 1337 standardized and eased ADU rules across the state, removing owner-occupancy requirements and curbing some local restrictions that used to slow projects down. It doesn't set a universal approval clock, but it has reduced friction in jurisdictions that previously made ADUs hard to permit.
- Choosing a conversion over new construction. If you have a usable basement, garage, or attached space, converting it skips foundation and shell work entirely.
What Slows It Down
The delays are just as predictable as the accelerators, and knowing them lets you plan around them.
- Custom design cycles. Every round of "can we move this wall" adds days or weeks. Deciding the layout early and sticking to it protects the timeline.
- Permit corrections. Incomplete drawings, missing details, or code issues send the application back. Each correction cycle is another wait in the review queue.
- Site conditions and utilities. Sloped lots, difficult soil, or long sewer and utility runs add engineering and construction time. Utility connections in particular can involve their own approvals.
- Jurisdiction backlog. Review speed varies by city and by season. A department buried in permits reviews yours slower, and that's outside your control, which is exactly why the front-end steps you can control matter.
- Change orders during construction. Changing your mind mid-build ripples through the schedule and often the budget too. Our guide on questions to ask an ADU builder helps you lock down scope before ground breaks.
Plan the Calendar Alongside the Budget
The takeaway isn't a magic number; it's a sequence. Design, permit, build, inspect, in that order, each with its own clock, and the total is what determines when you actually collect rent or move family in. Treat the timeline like the budget: something you plan deliberately rather than discover as you go. If income is the goal, every month of the timeline is a month before the unit starts paying for itself, which is worth modeling right alongside the cost of building an ADU in Washington.
The most reliable way to get a real timeline for your specific lot is to talk to builders who work in your city and know the local review process. Get free quotes from verified Washington ADU builders, or browse the directory of vetted local builders to start your shortlist and ask each one how long they'd expect your project to take.
This article is general information, not legal or construction advice. Permitting timelines and requirements vary by jurisdiction; confirm specifics with your local permitting authority and a licensed Washington builder.