How Much Does an ADU Cost to Build in the Tri-Cities, WA? A Realistic 2026 Breakdown

'How much does an ADU cost?' is hands-down the first question we get from almost every homeowner in the Tri-Cities who's seriously thinking about adding one. And honestly, it's the right question to start with because the answer tells you a lot about what's actually involved and helps you figure out which type of ADU makes sense for your situation.

The short answer is that ADU costs in Kennewick, Richland, and Pasco can range from around $40,000 for a straightforward garage conversion all the way up to $200,000 or more for a fully custom detached ADU with high-end finishes. That's a wide range, and the difference comes down to the type of ADU, the condition of the existing structure, site-specific factors, and your finish choices.

We've built ADUs and done garage and basement conversions throughout Benton and Franklin County. Here's a real, honest breakdown of what you can expect to spend and what drives the cost up or down so you're not going into your project blind.

Garage Conversion ADUs: The Most Affordable Starting Point

If you already have an attached or detached garage, converting it into a livable ADU is typically the most cost-effective route in the Tri-Cities. You're starting with an existing structure that already has a foundation, a roof, and walls which takes a big chunk of the cost out of the equation compared to building something from scratch.

A basic garage-to-ADU conversion in Kennewick or Richland thinks insulation, drywall, flooring, a bathroom, a small kitchen area, electrical upgrades, and HVAC typically runs between $40,000 and $90,000 depending on the size of the garage and the condition it's starting in. A larger two-car garage with good bones on the lower end of that range can become a genuinely comfortable studio or one-bedroom unit without pushing into six figures.

The factors that push a garage conversion toward the higher end are things like the need for a new electrical panel, adding a full bathroom from scratch in a space that doesn't already have plumbing nearby, or upgrading the structure to meet current Benton County building code for habitable space things like minimum ceiling heights and egress windows. Your builder should walk through the existing space with you and flag any of these before giving you a quote.

Typical Garage Conversion Cost in Tri-Cities, WA: $40,000 – $90,000

Basement Finishing & Conversion: Mid-Range With a Lot of Upside

Basement conversions in Richland, Kennewick, and Pasco follow a similar logic: you're working with an existing structure, so you're not starting from zero. But basements have their own set of considerations that can affect the budget. Waterproofing, egress window installation, ceiling height clearance, and HVAC extension are the big ones that tend to surprise homeowners who've never done a basement finish before.

A basement conversion into a functional living space or ADU in the Tri-Cities area typically runs between $50,000 and $100,000 for a full finish with a bathroom, kitchen or kitchenette, and separate entry. If the basement already has some rough-in plumbing in place and good ceiling height usually 7 feet minimum for a habitable space under Washington state code you're in good shape to come in on the lower end of that range.

One thing worth knowing: if you're planning to rent the finished basement as a legal accessory dwelling unit rather than just using it as extra family space, Benton and Franklin County have specific requirements around separate entrances, egress windows in sleeping areas, and smoke and CO detection. Your contractor should be pulling the right permits and building to ADU standards from the start not just finishing the space as a general living area.

Typical Basement Conversion Cost in Tri-Cities, WA: $50,000 – $100,000

Detached ADU Construction: The Biggest Investment, The Biggest Return

A fully detached ADU, a separate structure built in the backyard of an existing home, is the most involved and most expensive ADU option. You're building a complete home from the ground up: foundation, framing, roofing, all utilities, full kitchen and bath, HVAC, electrical, and finishes. In the Tri-Cities market, a detached ADU typically runs between $120,000 and $220,000 depending on size, design complexity, and finish level.

Smaller detached ADUs in the 400 to 600 square foot range can come in closer to the $120,000 to $150,000 range with standard finishes. Larger units in the 700 to 900 square foot range with more custom design work, higher-end kitchens, or specific site challenges like sloped lots, difficult soil conditions, or longer utility runs can push toward or above $200,000.

The payoff, though, is significant. A detached ADU in Kennewick or Richland that rents for $1,200 to $1,600 per month adds meaningful ongoing income and long-term property value. Many of our Tri-Cities clients see their detached ADU pay for itself within 8 to 12 years through rental income alone and that's before factoring in the appreciation boost to the main property.

Typical Detached ADU Cost in Tri-Cities, WA: $120,000 – $220,000

Permit Fees and Utility Hookups: The Costs People Forget to Budget

This is where a lot of first-time ADU buyers in Kennewick, Pasco, and Richland get caught off guard. The construction costs are one thing, but permits, utility connections, and site work add up on top of that and they're not optional.

Permit fees in Benton County and Franklin County for an ADU project vary based on the size and scope of the project, but homeowners should generally expect to budget $2,000 to $6,000 for building permits alone. If the ADU requires a new water meter, a sewer lateral, or an upgraded electrical service connection from the utility which is common for detached ADUs those utility hookup costs can add another $5,000 to $20,000 depending on how far the runs are and what Benton PUD or your local utility requires.

Site work is another line item that doesn't always make it into early budget conversations. Grading, clearing, concrete work for a new foundation, and driveway or pathway access to a detached ADU all add to the total. A good contractor will include these in your project estimate upfront rather than presenting them as surprises halfway through the build.

What Affects ADU Cost the Most in the Tri-Cities

Beyond the type of ADU, there are a handful of factors that have the biggest impact on where your project lands in the cost range:

• Existing structure condition A garage or basement in good shape with adequate framing, no moisture issues, and good bones will always cost less to convert than one that needs significant structural work first.
• Plumbing proximity Adding a bathroom or kitchen is cheapest when there's existing plumbing nearby to tie into. The farther the new fixtures are from existing lines, the higher the plumbing cost.
• Finish level Standard builder-grade finishes keep costs down. Custom cabinetry, tile work, high-end appliances, and specialty flooring push costs up. Most Tri-Cities ADU rental units do just fine with clean, durable mid-grade finishes.
• Site conditions, sloped lots in West Richland or Benton City, rocky soil, or limited access for equipment can all add to the cost of a detached ADU build.
• Permit and code requirements Benton and Franklin County building departments have specific requirements for ADUs. Working with a contractor who knows local code means no surprises, no re-work, and no failed inspections.

Is an ADU Worth the Investment in the Tri-Cities?

In most cases, yes, especially right now. The Tri-Cities rental market is active, and well-finished ADUs in Kennewick, Richland, and Pasco are renting consistently. A garage conversion that costs $65,000 and rents for $1,100 a month generates over $13,000 a year in gross rental income. That's a strong return, and it doesn't account for the added value the ADU puts on your overall property.

For homeowners who aren't looking to rent, maybe they want a space for aging parents, a grown child coming back, or a dedicated home office or studio the return is more personal but no less real. Having that extra square footage adds flexibility to your home that you just can't get any other way without moving.

The key is going in with realistic numbers and a builder who's going to be straight with you about costs before you commit. At Tiny Cottage Builders, every estimate we provide is detailed, transparent, and based on what it actually takes to build an ADU in the Tri-Cities, not what sounds good to get you to sign.

ADU Cost Summary for Tri-Cities, WA Homeowners:

• Garage Conversion ADU: $40,000 – $90,000
• Basement Conversion ADU: $50,000 – $100,000
• Detached ADU Construction: $120,000 – $220,000
• Permit Fees (Benton & Franklin County): $2,000 – $6,000
• Utility Hookups (water, sewer, electric): $5,000 – $20,000+
• Typical rental income potential in Tri-Cities: $1,000 – $1,600/month

Ready to Find Out What Your ADU Would Actually Cost?

Every property is different, and the best way to get a real number is to have someone come out, walk the space, and put together an actual estimate not a ballpark pulled from a website. That's exactly what we do for homeowners across Kennewick, Richland, Pasco, West Richland, and throughout Benton and Franklin County.

Tiny Cottage Builders is a locally owned ADU builder and home remodeling contractor serving the Tri-Cities area. Owner and builder Brandon Lott personally handles every estimate and oversees every project so you're always talking to the person who's actually doing the work, not a salesperson.

Call or text Tiny Cottage Builders at (509) 430-4244 or request a free quote at tinycottagebuildersnw.com.

Tiny Cottage Builders NW, ADU Construction, Garage & Basement Conversions, Home Remodeling, Deck Building, and Handyman Services. Serving Kennewick, Richland, Pasco, West Richland, Benton City, Prosser, and surrounding Benton & Franklin County, WA.By Tiny Cottage Builders NW  |  ADU Builder & Home Remodeling Contractor Serving Kennewick, Richland, Pasco & the Tri-Cities