Garage Conversion ADU: Turning Your Garage Into a Living Space

Converting your garage into an Accessory Dwelling Unit (ADU) represents one of the smartest home improvement investments available to Pacific Northwest homeowners. With housing costs continuing to rise across Seattle, Portland, and surrounding areas, garage conversions offer a cost-effective way to create additional living space, generate rental income, or accommodate multigenerational living arrangements.

Why Garage Conversions Make Financial Sense

Compared to building a new detached structure, converting an existing garage typically costs 40-60% less. You already have the foundation, walls, and roof in place. The average garage conversion ADU in the Pacific Northwest ranges from $80,000 to $150,000, while new construction ADUs typically start at $150,000 and can exceed $300,000.

The rental income potential is substantial. A well-designed garage ADU in the Seattle metro area can command $1,500-$2,500 per month in rent. In Portland, expect $1,200-$2,000 monthly. This income stream can offset your mortgage, fund retirement, or provide financial flexibility during uncertain times.

Permit Requirements in the Pacific Northwest

Both Washington and Oregon have passed state legislation encouraging ADU construction, but local requirements still vary significantly. In Seattle, garage conversions require building permits and must meet the city’s ADU regulations, including owner-occupancy requirements that were recently relaxed. Portland has eliminated many barriers, including parking requirements and system development charges for ADUs under 750 square feet.

Expect the permit process to take 4-12 weeks depending on your jurisdiction. You will need architectural drawings, structural engineering reports if you are making significant modifications, and energy code compliance documentation. Working with a contractor experienced in PNW ADU conversions can significantly streamline this process.

Critical Design Considerations

Ceiling Height: Most building codes require a minimum ceiling height of 7 feet 6 inches for habitable space. Standard garage ceiling heights are often just at or below this threshold. If your garage has a low ceiling, options include lowering the floor (expensive) or applying for a variance (not always granted).

Egress Windows: Bedrooms must have emergency escape windows meeting specific size requirements. These windows need a minimum opening of 5.7 square feet with a sill height no more than 44 inches from the floor. In a garage conversion, this often means replacing the existing windows entirely.

Insulation: Pacific Northwest climate demands proper insulation for year-round comfort. Plan for R-21 wall insulation and R-38 ceiling insulation at minimum. The concrete floor should receive rigid foam insulation beneath a new subfloor. This investment pays dividends in tenant comfort and reduced utility costs.

Utility Connections: The Technical Details

Electrical: A garage ADU typically requires its own subpanel, usually 60-100 amps. You will need circuits for lighting, outlets, appliances, and HVAC equipment. Budget $5,000-$10,000 for electrical work depending on your main panel’s capacity and the distance to the garage.

Plumbing: Adding a bathroom and kitchenette requires extending water supply and drain lines. If your garage is close to your home’s existing plumbing stack, costs stay reasonable at $8,000-$15,000. Greater distances or the need for a grinder pump (common with below-grade conversions) can push costs to $20,000 or more.

HVAC: Options include extending your home’s existing system (rarely cost-effective), installing a mini-split heat pump ($4,000-$8,000 installed), or using electric baseboard heat with a wall-mounted AC unit. Mini-splits are the preferred choice for PNW ADUs due to their efficiency and ability to both heat and cool.

Common Challenges and Solutions

Moisture Issues: Garages were designed for cars, not people. Expect to address moisture intrusion through floor sealing, proper vapor barriers, and potentially French drains around the foundation.

Parking Replacement: Some jurisdictions still require off-street parking. You may need to create a new parking pad in your driveway or yard.

Fire Separation: If the ADU shares a wall with your main house, fire-rated construction between the units is mandatory. This typically means 5/8-inch Type X drywall on the shared wall.

Cost Breakdown and ROI

A typical garage ADU conversion budget includes: permits and design ($5,000-$10,000), structural work ($5,000-$15,000), electrical ($5,000-$10,000), plumbing ($8,000-$20,000), HVAC ($4,000-$8,000), insulation and drywall ($8,000-$15,000), flooring ($3,000-$6,000), kitchen and bath finishes ($15,000-$30,000), and exterior work ($5,000-$10,000).

With rental income of $1,500-$2,000 monthly, most garage ADU conversions pay for themselves within 5-8 years while adding $100,000 or more to your property value.

Timeline Expectations

Plan for 4-6 months from initial design to move-in ready. This includes 4-12 weeks for permits, 2-3 weeks for demolition and structural work, 3-4 weeks for rough-ins (electrical, plumbing, HVAC), 2-3 weeks for insulation and drywall, and 3-4 weeks for finishes and inspections.

Working with experienced professionals who understand Pacific Northwest building codes and climate considerations can make the difference between a successful project and a costly mistake. Your garage conversion ADU can provide decades of value when planned and executed properly.