Finishing a basement can increase your property taxes in Washington, because state law lets the county assessor add the value of a remodel to the assessment rolls, and a building permit for the work prompts the assessor to appraise it. How much your own bill changes is a question for the King County Department of Assessments and a tax professional. We are a remodeling contractor, not tax advisers, so this article only explains what the law and the Assessor say, quoted from the source, and how a permitted basement remodel works.
What Washington law says about remodels and assessed value
Four sections of the Revised Code of Washington (RCW) set out how a remodel reaches the assessment rolls.
- Every property is assessed every year. RCW 84.40.020 says: "All real property in this state subject to taxation shall be listed and assessed every year, with reference to its value on the first day of January of the year in which it is assessed."
- Value means full market value. RCW 84.40.030 says: "All property must be valued at one hundred percent of its true and fair value in money and assessed on the same basis unless specifically provided otherwise by law."
- A building permit leads to an appraisal. RCW 36.21.070 says: "Upon receipt of a copy of a building permit, the county assessor shall, within twelve months of the date of issue of such permit, proceed to make a physical appraisal of the building or buildings covered by the permit."
- Added value can go on the rolls the same year. RCW 36.21.080 says: "The county assessor is authorized to place any property that is increased in value due to construction or alteration for which a building permit was issued, or should have been issued, under chapter 19.27, 19.27A, or 19.28 RCW or other laws providing for building permits on the assessment rolls for the purposes of tax levy up to August 31st of each year. The assessed valuation of the property shall be considered as of July 31st of that year."
Note the words "or should have been issued" in RCW 36.21.080. The law covers work that needed a permit whether or not one was pulled.
Why the Assessor inspects homes
Assessors also inspect every property on a regular cycle, remodel or not. RCW 84.41.041 says: "Each county assessor must cause taxable real property characteristics to be reviewed in accordance with international association of assessing officers standards for physical inspection and valued at least once every six years". The King County Department of Assessments explains this in its Property Field Inspection Frequently Asked Questions: "Pursuant to RCW 84.41.041, & WAC 458-07-015, the Assessor is required to physically inspect every property no less than once every six years. The purpose of this inspection is confirming property characteristics in the Assessor's database reflect the actual characteristics of King County properties."
What happens during a King County assessor visit
According to the same King County FAQ:
- Visits are often unannounced. "The short answer is yes. With over 700,000 properties in King County making an appointment to visit each one is not feasible. RCW 84.40.025 allows Assessors reasonable access to the exterior of properties without owner permission."
- What they look at. "Deputy Assessors update photo records, confirm property characteristics (both land and buildings), and look for changes to the property not captured previously."
- How to recognize them. "Department of Assessments employees all have King County ID, business cards and identifying clothing."
- Not every visit involves you. "Deputy Assessors do not always need to engage property owners. It is possible in past years all needed information could be gathered from public vantage points such as streets or sidewalks."
Should you let the assessor in after a basement remodel?
That is your decision about your own property, and we do not advise either way. Here is what the law and the Assessor say about access. RCW 84.40.025 says: "For the purpose of assessment and valuation of all taxable property in each county, any real or personal property in each county shall be subject to visitation, investigation, examination, discovery, and listing at any reasonable time by the county assessor of the county or by any employee thereof designated for this purpose by the assessor." The King County FAQ adds: "RCW 84.40.025 allows Assessor's reasonable access to properties. The Assessor has created detailed property access guidelines that balances common sense, staff safety and property owners' privacy rights. Assessor staff are instructed to comply when asked to leave a property and seek answers to questions using other sources and means."
The Assessor also has other ways to learn about a remodel. RCW 84.41.041 says: "The assessor may require property owners to submit pertinent data respecting taxable property in their control including data respecting any sale or purchase of said property within the past five years, the cost and characteristics of any improvement on the property and other facts necessary for appraisal of the property." If you think the Assessor's record of your home is wrong, the King County FAQ says you can ask for an Assessment Review, and that "Often characteristic updates can be handled informally with sharing information through phone conversations or e-mail."
The home improvement exemption in Washington law
Washington has a limited property tax exemption for improvements to single-family homes. RCW 84.36.400(1) says: "Any physical improvement to single-family dwellings upon real property, including constructing an accessory dwelling unit, whether attached to or within the single-family dwelling or as a detached unit on the same real property, shall be exempt from taxation for the three assessment years subsequent to the completion of the improvement to the extent that the improvement represents 30 percent or less of the value of the original structure."
The same subsection sets the timing: "A taxpayer desiring to obtain the exemption granted by this section must file notice of his or her intention to construct the improvement prior to the improvement being made on forms prescribed by the department of revenue and furnished to the taxpayer by the county assessor. The exemption in this subsection cannot be claimed more than once in a five-year period." Whether a project qualifies, and how the exemption is applied, is decided by the King County Department of Assessments. Ask the Assessor and a tax professional before you rely on it.
Why permitted basement work matters
A finished basement is a real part of the house, with framing, wiring, heat and often plumbing, so in Seattle it needs a building permit. The 2021 Seattle Residential Code, Section R105.1, says "a building permit shall be obtained from the building official for each building or structure prior to erecting, constructing, enlarging, altering, repairing, moving, improving, removing, changing the occupancy of, or demolishing such building or structure, or allowing the same to be done." The permit brings inspections at each stage, and the approved plans and final inspection are the record that the rooms were built to code.
Permits come from the city the house is in. A Seattle basement goes through the Seattle Department of Construction and Inspections for the building and electrical permits, with plumbing permitted by Public Health, Seattle and King County. A Bellevue basement goes through Bellevue Development Services. For more on when a permit is needed, read Do you need a permit to remodel in Seattle?
How we handle a basement remodel
We walk the basement with you, measure it, look for signs of water and note the height under the beams and ducts. Then we lay out the rooms, price every line in writing and file the permit, including any escape window and plumbing work. One project manager runs the framing, rough-ins, inspections and finishes, and walks the finished rooms with you at the end.
One example is a Finished Basement Remodel in Seattle, WA, completed in July 2024: new clothes and storage closets, one light oak-look plank floor through the whole level, and a bedroom suite behind glass-paned French doors. The project page shows the approximate price, about $120,000. Every project is priced in writing, line by line, for its own home.
See Basement Remodeling Across the Greater Seattle Area for what we build below grade, and the Basement Remodeling Portfolio Across the Greater Seattle Area for finished rooms. For questions about your own assessment, contact the King County Department of Assessments. For tax questions, ask a tax professional. For the remodel itself, Book a Consultation.
