When it comes to building an Accessory Dwelling Unit (ADU) in the Puget Sound region, pre-approved ADU plans have become an increasingly popular choice for homeowners who want to move from idea to reality with fewer headaches, faster approvals, and significant savings.
A pre-approved plan is a “ready-to-go” design — already reviewed and approved by the local permitting department — that you can license for your property without going through the lengthy back-and-forth of a traditional plan review.
For homeowners in 2025, these plans promise faster permitting, more predictable costs, and a clear path to compliance. But they’re not perfect for every property or every project. This guide will walk you through exactly where they’re available, how much money and time you can realistically save, when they might not work, and the hidden pitfalls to watch for before committing.
What Are Pre-Approved ADU Plans?
A pre-approved ADU plan is an architectural design that a city or jurisdiction has already evaluated for compliance with local building codes, zoning laws, and — in many cases — sustainability requirements. Instead of hiring an architect to start from scratch, you license one of these designs for your property.
Because they’ve already passed the review process, you avoid the months-long delays that normally occur when plans bounce back and forth between your architect and the permitting department.
Pre-approved ADU plans are often:
- Designed to fit typical neighborhood lot sizes and shapes
- Built to meet (or exceed) energy efficiency standards
- Eligible for faster permitting and sometimes reduced fees
- Created by local architects familiar with Pacific Northwest weather, terrain, and housing styles
The concept is simple: instead of reinventing the wheel for every ADU, use a trusted, tested design that the city already knows works. If you’re considering a rental unit, make sure to check out 5 Common Mistakes to Avoid When Building a Rental ADU in Seattle so you don’t fall into the same traps.
Where Are Pre-Approved ADU Plans Offered in 2025?
As of early 2025, several cities in the Puget Sound region have embraced pre-approved ADU programs as part of a larger effort to encourage backyard housing and ease the regional housing shortage:
- Kirkland – Connects pre-approved plan use to green building incentives, reducing fees and often speeding up construction approval.
- Redmond – Encourages pre-approved designs that align with new Washington State ADU legislation.
- Bellevue – Offers pre-approved designs integrated directly into the online permitting portal.
- Tacoma – Features a detailed library of pre-approved plans along with homeowner guidance.
- Everett – Provides select pre-approved layouts with expedited review timelines.
- Burien, Renton, Shoreline – Rolling out or actively promoting pre-approved plan options.
This regional adoption makes ADUs more accessible for homeowners looking to add value, rental income, or multi-generational living space.
Pros of Pre-Approved ADU Plans
1. Substantial Cost Savings — Often 30–50% vs. Custom Plans
Average Cost Comparison
| Feature | Custom ADU Plans | Pre-Approved ADU Plans | Avg. Savings |
| Architectural Design Fees | $10,000 | $4,000 | $6,000 |
| Permit Review Time | 3–9 months | 2–8 weeks | 1–8 months |
| Total Typical Savings | — | — | $6,000 |
With a pre-approved design, architectural fees are often just a fraction of what you’d pay for a custom build. You also save on soft costs like engineering adjustments and interest on construction loans.
2. Faster, Predictable Permitting and Build Timelines
Custom plans can require 3–9 months for permitting. Pre-approved plans often cut this to just 2–8 weeks.
Faster permitting means:
- Lower financing costs — Shorter timelines mean less interest on loans.
- Earlier rental income — Begin generating income sooner.
- Quicker move-in for family — Ideal for multi-generational needs.
3. Built-In Compliance and Incentives
Pre-approved plans already meet zoning and building codes, reducing the risk of rejection. Some cities also offer:
- Green building incentives — Fee reductions or grants for sustainable design.
- Priority processing — Moves your permit to the front of the line.
- Simplified paperwork — One streamlined application instead of multiple submissions.
4. Reduced Risk and Stress
Knowing your plan already complies with local requirements minimizes redesigns, uncertainty, and costly delays.
Cons of Pre-Approved ADU Plans
1. Limited Customization Options
While you can adjust finishes or colors, major changes like footprint alterations, second stories, or wall relocations often require full plan re-approval, wiping out time and cost savings.
2. Not All Properties Are Ideal for Pre-Approved Plans
| Constraint | Impact on ADU | Common in Region? |
| Retaining Walls | May limit buildable area or require downsizing | Yes |
| Sloped Terrain | Can require costly foundation work | Yes |
| Overhead Power Lines | Setbacks (10–15 ft) can restrict placement or size | Yes |
| Main House Size Limits | Capped at ~60% of primary home footprint | Yes |
| Narrow or Irregular Lot | Layout may not fit efficiently | Yes |
3. Design Limitations
Pre-approved plans are designed to “fit in,” not stand out. This can be a drawback for homeowners wanting distinctive architecture, a custom studio space, or special accessibility features.
4. Potential Site-Related Costs
Even with a pre-approved plan, your lot may need:
- Slope-specific foundation work
- Drainage compliance upgrades
- Utility capacity improvements
- HOA or neighborhood design requirement adjustments
These can reduce overall savings.
Can You Modify a Pre-Approved Plan to Save Money?
Yes — but only within limits.
| Modification Type | Keeps Pre-Approval? | Time Impact | Cost Impact |
| Changing finishes | Yes | None | Minimal |
| Moving walls/windows | Sometimes | +2–4 weeks | $500–$2,000 |
| Changing footprint/layout | No | +3–6 months | $5,000+ |
| Adding floor/height | No | +4–8 months | $8,000+ |
Who Benefits Most from Pre-Approved Plans?
- Standard-sized, rectangular lots
- Flat terrain without major grading needs
- Projects with tight budgets and timelines
- Homeowners seeking rentals, in-law suites, or guest homes
- Owners prioritizing function over custom design flair
Summary: Is a Pre-Approved ADU Plan Right for You?
Pre-approved ADU plans can cut design costs by up to 50% and reduce permitting timelines by nearly 40%. For many Puget Sound homeowners, they’re a smart, low-stress path to a new backyard living space.
However, unique site conditions or custom design needs may make a fully custom plan the better route.
Selecting the right contractor is the most important step in any ADU project. Our post How to Hire the Right ADU Contractor in Seattle (2025 Guide) breaks down what to look for in 2025.
