What Drives the Cost of an ADU in Vancouver and Clark County

An ADU's cost in Vancouver comes down to five things: official fees (impact fees, permit fees and utility charges), how hard it is to get water, sewer and power to the unit, the site itself, the unit's size, and the finishes and layout you choose. Some of those are fixed by the City and county, and you can look them up before you design. The rest are decisions, and that is where a budget is won or lost.

JDI does not publish an ADU price range, because two ADUs of the same size on two Vancouver lots can differ widely once utilities and site work are counted. What follows is how each cost driver works, so you can ask the right questions before you get a bid.

1. Impact fees: half of the main house, by law

Impact fees pay for the schools, roads and parks that new homes use. State law, RCW 36.70A.681, says cities and counties may not charge an ADU more than 50% of the impact fees the principal home would pay. Vancouver's code, in effect since July 31, 2026, matches that: impact fees for up to two ADUs of 1,000 square feet or less "shall be 50% of the principal unit but are otherwise calculated based on the ADU square footage." When the main house already exists, the City figures the fee as if the house were built today.

The City's building permit fee page lists per-unit fees by school district, traffic district and park district. As listed in fall 2026:

Two illustrations, using our own arithmetic on those published figures. A single-family home in Vancouver School District and the Columbia traffic district would pay $2,786 + $2,006 + $6,130 = $10,922, so one ADU at 50% comes to about $5,461. In Evergreen School District and the Cascade traffic district, the house would pay $8,875.50, and the ADU about $4,437.75. These are illustrations, not quotes. Because the code also bases the fee on the ADU's square footage, use the City's fee quote form for your address.

Keep the unit at 1,000 square feet or less. Above that, it is no longer an ADU under Vancouver's code and does not get the reduced fee.

2. Permit and review fees

Building permit fees in Vancouver are based on a valuation the City sets from national construction cost averages, not on your contract price. On top of the base permit fee, the City charges plan review (65% of the building permit fee), a 5.5% technology fee, a $32 issuance fee for residential permits and the state building fee ($6.50 per residential permit, plus $2 for each additional unit in the same building). When work involves two or more codes, the permit fee is 1.6 times the base amount. Fees adjust each January 1.

Two steps before the permit cost little or nothing:

• Request for Utility Services: free, and required before you apply for a building permit for any new dwelling unit.
• Pre-application conference: optional, $382 for one to six units on one lot. It also waives the utility request if held within the last 365 days.

If the utility review says your project needs a civil engineering (ENG) review, that permit has to be approved before the building permit can be issued. Budget time and design fees for it.

3. Utilities: often the biggest unknown

The City's utility review reports "availability, capacity and an overview of costs involved to connect new units to water and sewer." That report is the first real number in an ADU budget. The fee page also lists 2026 water and sewer system development charges, with base amounts of $3,783.95 for water and $3,768.42 for sewer. The utility review tells you which connection costs apply to your project.

Utility items that move the budget:

• Sewer capacity. If the existing line on your property cannot take another unit, separate or larger lines may be required.
• Distance. A detached unit at the back of a deep lot needs longer trenches for water, sewer and power than an attached unit next to the house.
• Power. A service upgrade or a separate meter goes through Clark Public Utilities.
• Separate controls. Where utilities are shared, each unit needs its own shutoffs and its own temperature control.
• Septic. On septic, Clark County Public Health decides whether the system can serve an ADU. 2026 county fees include $402 for a septic release letter, $502 for a verification and $718 for an expansion. Adding a bedroom counts as an expansion, which requires a system and reserve area that meet new-construction standards.

4. The site

Every lot is different, and site work is where two similar ADUs part ways on cost.

• Slope and drainage. Excavation, retaining and stormwater handling for a new footprint.
• Placement limits. Under Vancouver's Flex-Plex standards for lots with ADUs, side and rear setbacks are at least 5 feet (zero from an alley), impervious area is capped at 65% of the lot for fewer than three units, and at least 10% must stay open space. A detached ADU must also be 10 feet from other structures or add sprinklers or 1-hour fire-rated construction.
• Access. A narrow side yard means smaller equipment and more hand work.
• Critical areas. Wetlands, fish and wildlife habitat, geologically hazardous areas and frequently flooded areas carry their own rules, mapped or not, and can require a pre-application conference.

5. Size, type and design choices

Square footage matters, but the rooms inside it matter more. A kitchen and a bathroom carry much of the plumbing, electrical and finish cost of any dwelling, and an ADU has both in a small footprint. Choices that move the number:

• Type. A garage conversion reuses an existing shell but must be brought to full energy code. A detached unit needs a new foundation and roof. An attached unit needs 1-hour fire and STC/IIC 45 sound separation from your home. Our article on garage conversion cost covers the conversion side.
• Energy code. A new unit under 1,500 square feet with less than 300 square feet of windows generally needs 5.0 energy credits. Heating and water heating choices are part of meeting that.
• Kitchen and bath scope. A full kitchen versus a kitchenette, a tub versus a shower, accessibility features such as a curbless shower. Our aging-in-place bathroom guide covers the details if the ADU is for a parent.
• Finish level. Cabinets, counters, flooring, fixtures and windows.

6. Sales tax

In Washington, sales tax applies to the full contract price for construction, labor included. Since April 1, 2026, the combined rate is 8.9% in Vancouver and 8.0% or 8.7% in unincorporated Clark County, depending on the address. On a project of this size, that line is real money. Our page on sales tax on remodeling explains how it works.

7. Property tax

Adding a unit can raise your assessed value. The City's ADU page notes that "an ADU as part of an addition or remodel may be eligible for tax relief for up to three years," and that you must apply to the Clark County Assessor before construction is completed. The state exemption (RCW 84.36.400) is limited to improvements of 30% or less of the original structure's value and can be claimed once in five years. Whether a detached ADU qualifies is a question for the Assessor. Our page on property tax and remodeling has the filing steps; ask your tax adviser how it applies to you.

Unincorporated Clark County

The county's ADU pages (revised June 3, 2024) set different numbers. Inside the urban growth area, the county urban ADU page says the county waives 75% of transportation, school and park impact fees at the multifamily rate, and ADUs must connect to public water and sewer unless a waiver is granted. Rural ADUs get a 75% waiver of school and transportation impact fees at the multifamily rate and need a septic release from Public Health before the building permit. Check the county's current pages before you rely on these figures.

Paying for it

Common ways to pay for a project this size include borrowing against your home's equity or refinancing. The Consumer Financial Protection Bureau describes a home equity line of credit as a line of credit that "allows you to borrow against the equity of your home," and a cash-out refinance as replacing your mortgage "with a bigger mortgage" and taking the difference in cash. JDI also offers financing through Lend Home Improvements and Medallion Bank, subject to credit approval. See our financing page for how to apply.

Frequently asked questions

How much does an ADU cost in Vancouver?

It depends on the five drivers above, especially utilities and site work. JDI gives a firm written quote after a free in-home visit and preliminary design, typically two to three weeks after the visit.

Do I pay full impact fees for an ADU?

No. For up to two ADUs of 1,000 square feet or less, Vancouver charges 50% of the principal unit's fee, adjusted for the ADU's square footage. Unincorporated Clark County's pages describe a 75% waiver at the multifamily rate.

What is the cheapest kind of ADU to build?

Often a conversion of space you already have, such as a garage or part of the house, because the shell exists. A garage that needs structural repair can lose that advantage.

Will an ADU raise my property taxes?

Usually, because it adds value. You may qualify for the three-year home improvement exemption if you file with the Assessor before construction is completed. Ask your tax adviser.

Rules and fees change; confirm current requirements with the City of Vancouver Permit Center before you design.

How JDI can help

JDI designs and builds ADUs in Vancouver and across Clark County. We start with your lot, your utilities and how the unit will be used, then price it once we know what the site needs. Visit our Vancouver ADU builder page or schedule a free consultation.

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