18
May
Seattle issued 987 ADU permits in 2023 alone — four times the number from five years earlier. With the city’s July 2025 reforms now allowing two ADUs per residential lot, eliminating the owner-occupancy requirement, and even permitting detached units to be subdivided for separate ownership, the ADU boom across the Seattle metro area is accelerating. Based on our 30,000+ inspections across Northwest Washington, Titan Inspection Services has seen firsthand how this growth is creating new opportunities for homeowners and investors — and new risks for buyers who skip the inspection.
Whether you are building a new DADU in the backyard, converting a basement or garage into a rental unit, or purchasing a property with an existing ADU, understanding what a professional inspection covers and why it matters is essential to protecting your investment. The legislative changes driving this boom — including Washington’s HB 1337 and HB 1110 — removed many of the barriers that previously limited ADU construction, but they did not change the building code requirements that determine whether a unit is safe and habitable.
Why ADU Inspections Are Different from Standard Home Inspections
An ADU is not simply a smaller version of a regular home. It operates with its own systems, its own code requirements, and its own set of potential defects that a standard home inspection may not fully address if the inspector does not understand the differences.
Separate electrical systems. ADU electrical circuits must be completely independent from the main dwelling. A separate panel is recommended, and if a shared panel is used, it must be in a common area accessible to both units. All new circuits require AFCI protection — they are considered new construction regardless of the age of the structure. We regularly find ADU conversions where the electrical was simply extended from the main home on existing circuits, creating both code violations and safety hazards. Brian, owner of Titan Inspection Services, covers the most common electrical issues we encounter in his Ask Brian: What Is The Most Common Electrical Flaw Found In Homes? video.
Fire separation requirements. Attached ADUs must have walls and floor-ceiling assemblies with a minimum one-hour fire-resistance rating separating them from the primary dwelling. This means proper gypsum board installation, draft stops in attics along the fire wall line, and continuous barriers between units. These requirements exist because two separate households sharing a structure changes the fire safety equation entirely.
Sound transmission standards. Common walls and floor-ceiling assemblies between an ADU and the primary dwelling must meet an STC 50 minimum for airborne sound and an IIC 50 minimum for impact sound, per building code Section 1207. Inadequate soundproofing between units is one of the most common complaints from both homeowners and tenants.
Independent HVAC and plumbing. Detached ADUs require their own heating, cooling, plumbing, and water heating systems. Utility connection work alone typically costs $15,000 to $40,000 in King County. When these systems are improperly installed or undersized, the problems show up quickly in a Pacific Northwest winter. Sewer connections for DADUs are a particularly common source of costly surprises — the same types of lateral line issues we cover in our sewer scope inspection guide apply to ADU sewer tie-ins, where builders sometimes take shortcuts on connections to existing lines.
Key Takeaway: ADU inspections require evaluating independent electrical, HVAC, plumbing, fire separation, and soundproofing systems that standard home inspections may not fully address. The inspection standards set by the American Society of Home Inspectors provide the baseline, but ADU-specific knowledge of local code requirements is what separates a thorough inspection from a generic one.
The Most Common Defects We Find in ADU Inspections
Garage Conversions
Garage conversions are the most popular ADU type in Seattle and also the most likely to have significant defects. Original garage wiring was designed for a single 20-amp circuit intended to run a light and a door opener — not a full dwelling unit with a kitchen, bathroom, and heating system. We frequently find electrical systems that were never upgraded to handle the increased load.
Beyond electrical, garage walls are often too thin to accommodate the insulation required by current energy code. Concrete slab floors typically lack moisture barriers and insulation. And the foundation itself may not extend deep enough below grade to meet residential requirements. These are not cosmetic issues — they affect habitability, safety, and code compliance.
Basement Conversions
The two most common disqualifying defects in basement ADU conversions are ceiling height and egress windows. Habitable rooms require a minimum of 7-foot ceilings, and many Seattle basements fall short. Egress windows must provide a minimum 5.7 square feet of opening with a maximum 44-inch sill height — requirements that many existing basement windows do not meet.
Moisture is the other major concern. Seattle’s high water table and persistent rainfall create conditions where basement waterproofing is not optional. We look for cracks, evidence of past water intrusion, and whether proper drainage and waterproofing systems are in place. These are the same moisture and structural indicators we evaluate during crawl space inspections — the conditions below grade in a basement conversion carry many of the same risks as a traditional crawl space, including vapor barrier integrity, drainage adequacy, and signs of pest activity.
Detached Units (DADUs)
New DADUs built with permits generally have fewer defects because they go through the city’s plan review and inspection process. The issues we most often find involve existing detached structures — converted sheds, workshops, or studios — that were repurposed as living spaces without proper permitting. These structures commonly have inadequate foundations, improper drainage and grading, and utility connections that do not meet code. If you are purchasing a property with a new-construction DADU, be aware that builder warranties may not cover everything you assume — our guide to new construction inspections explains the gaps between what builders warrant and what a professional inspection reveals.
Key Takeaway: Garage conversions are the most defect-prone ADU type in Seattle, with electrical, insulation, moisture barrier, and foundation issues being the most common findings. Basement conversions frequently fail on ceiling height and egress window requirements. Detached units built with permits tend to have fewer issues, but converted sheds and workshops often have fundamental habitability problems.
Unpermitted Work
This is the biggest risk factor in Seattle ADU inspections. An ADU is not legal unless it was established through a permit process, and Seattle SDCI actively enforces violations with fines up to $500 per day, stop-work orders, and required removal of unpermitted work. A permit without a final inspection is treated the same as no permit by title companies and insurers.
If you are purchasing a property with an existing ADU, verifying permit status through Seattle SDCI’s online permit portal should be one of the first steps — before the inspection, not after.
The Insurance Gap Most Buyers Miss
One of the most overlooked risks with ADUs is the insurance gap. Attached ADUs can typically be covered under the main homeowner’s policy, but detached ADUs fall under “other structures” coverage — usually limited to 10 percent of dwelling coverage.
Here is what that means in practice. If your home is insured for $350,000, your DADU coverage maxes out at $35,000. But DADU rebuild costs in Seattle routinely exceed $150,000. That is a $115,000 gap that most buyers never realize exists until they need to file a claim.
Rental use of an ADU generally requires a separate landlord insurance policy. And unpermitted ADUs risk insurance claim denials entirely — if the unit was not built with proper permits, your insurer may not cover damage to it regardless of your policy limits.
The Yes Home Warranty through Titan provides an additional layer of protection for inspection clients. When your ADU or DADU is inspected by Titan, warranty coverage can extend to that unit — giving you documented protection that standard homeowner policies may not provide.
Key Takeaway: Detached ADUs are typically covered at only 10 percent of your home’s dwelling coverage — often creating a six-figure gap between your insurance limit and actual rebuild cost. Unpermitted ADUs risk complete claim denial. Verify both permit status and insurance adequacy before closing on any property with an ADU.
What Seattle’s 2025 Reforms Mean for Buyers and Investors
The regulatory landscape for ADUs in Seattle changed significantly in July 2025, when the city implemented compliance reforms in response to Washington’s HB 1337 and HB 1110.
Two ADUs are now allowed per lot. You can have an attached ADU in the basement and a detached unit in the backyard on the same property.
Owner occupancy is no longer required. You do not need to live on the property to build or rent an ADU, which opens the door for pure investment properties.
DADUs can be subdivided for separate ownership. This condominiumization option creates entirely new equity strategies — you can build a DADU and sell it independently from the main home.
Size limits increased to 1,000 square feet across all zones, and height limits rose to 32 feet in Neighborhood Residential zones.
These reforms are driving increased construction, which means more ADUs entering the market — built to varying quality standards by contractors with varying levels of experience. A professional inspection protects buyers from inheriting someone else’s shortcuts. If you are a seller with an ADU on your property, getting ahead of buyer concerns with a pre-listing inspection can prevent deal-killing surprises during the transaction.
Key Takeaway: Seattle’s 2025 ADU reforms allow two units per lot, eliminate owner-occupancy requirements, and permit DADUs to be sold independently. These changes are accelerating construction and creating investment opportunities — but they also mean more units entering the market with varying build quality, making professional inspections more important than ever.
Frequently Asked Questions
Do I need a separate inspection for an ADU?
If you are purchasing a property with an ADU, the unit should absolutely be inspected. ADUs have independent systems and code requirements that differ from the main dwelling. Titan Inspection Services evaluates ADUs as part of our comprehensive inspection process, examining electrical independence, fire separation, HVAC adequacy, plumbing, and structural integrity.
How do I check if an ADU was built with permits?
You can verify permit status through Seattle SDCI’s online permit portal or by contacting their office directly. Look for both the building permit and the final inspection sign-off — a permit that was never finaled is treated the same as no permit by title companies and insurers.
What are the biggest red flags in an ADU inspection?
The most concerning findings are unpermitted construction, electrical systems that are not independent from the main dwelling, missing fire separation between attached units, inadequate egress windows in basement conversions, and foundation deficiencies in garage conversions. Any of these can represent significant safety hazards and financial exposure.
How much does an ADU add to property value in Seattle?
ADUs add approximately $200 to $300 per square foot to home appraisals. An analysis of over 1,000 ADU projects in competitive markets, including Seattle, found that property values rose by an average of 39 percent. One-bedroom ADUs in Seattle currently rent for approximately $1,800 to $2,500 per month.
Will my homeowner’s insurance cover a detached ADU?
Standard homeowner policies typically cover detached structures at only 10 percent of dwelling coverage — often far below actual rebuild costs. If your DADU would cost $150,000 or more to rebuild, supplemental coverage is almost certainly needed. Rental use generally requires a separate landlord policy. Unpermitted units risk claim denials entirely.
Can I convert my garage to an ADU without a permit?
No. All ADU conversions in Seattle require a building permit, and depending on the scope, you may also need electrical, plumbing, and side sewer permits. Unpermitted ADU conversions carry fines up to $500 per day and can complicate future sales and insurance claims.
Schedule Your ADU Inspection
Whether you are purchasing a property with an existing ADU, converting a garage or basement, or evaluating a new DADU investment, Titan Inspection Services has the experience and local knowledge to identify issues before they become expensive problems. With more than 30,000 inspections and over 3,000 five-star reviews across Northwest Washington, our team understands Seattle’s ADU landscape and the specific code requirements that apply.
Call us at 206.451.1120 or visit titaninspectionservices.com to schedule your inspection. We are available 7 days a week with a 1- to 3-day turnaround.