Construction cost depends on the site and can't be estimated reliably without seeing it. Published Seattle figures for a new detached cottage range from roughly $250 to $650 per square foot. Most of that spread comes from what each source counts: some include design, engineering, survey, permits, utility connections, site work and financing interest, and others cover only the building. Most of the figures also come from builder marketing rather than audited data, so we don't offer another estimate here.
This page covers the charges outside the construction contract instead. All of them are published, but they often don't come up until the design is settled and there's no longer room to plan around them.
1. The King County capacity charge
Billed for fifteen years, starting after completion
The is easy to overlook because it isn't on any permit or contractor invoice. King County bills it separately each quarter, starting roughly three months after your local sewer utility reports the new connection, which is usually after the project budget has closed.
The capacity charge is not a metered utility bill. It is a growth-cost recovery charge on properties whose sewage flows to a King County treatment plant and that connect on or after 1 February 1990. Triggers include new sewer service, reconnections after five years, additions, and changes of use.
| Property type | RCE |
|---|---|
| Single family, under 1,500 sq ft | 0.81 |
| Single family, 1,500 to 2,999 sq ft | 1.00 |
| Single family, 3,000 sq ft and over | 1.16 |
| Accessory dwelling unit, attached or detached | 0.59 |
| Duplex, or any single family plus an ADU | 1.62 |
| Five or more units | 0.63 per unit |
Worked example: what a cottage costs you in capacity charge
0.59 RCE, multiplied by the 2026 rate of $77.99 per RCE per month, is $46.01 a month.
Billed quarterly, that is about $138 a quarter. Over the full fifteen year term it is roughly $8,280 in nominal terms. A lump sum payoff at a discount is available at any point during the fifteen years, but King County does not publish the discount formula, so you have to call and ask for a quote.
Two features of the charge deserve more attention than the dollar amount.
It applies to the new dwelling unit, with or without a new pipe. Sharing the existing does not avoid it. Additions and changes of use are listed triggers, and the county's own form assigns an accessory unit its own RCE figure. Plan on owing it.
It is the property owner's obligation. It can't be passed to a tenant, and no metering arrangement shifts it. It is enforced under RCW 35.58.570, which directs collection in the same manner as water-sewer district charges under RCW 57.08.081 and expressly contemplates foreclosing a lien for it. A lien under that section is subject only to the lien for general taxes, which means it takes priority over your mortgage.
2. Water and sewer connection charges
Here a single design decision can change the cost by more than twenty thousand dollars, and Seattle's rules do not carry over to water districts.
In Seattle: charged by meter size, not by dwelling unit
Seattle Public Utilities assesses its by meter size, and the water charge is triggered by a development requiring a new or larger water connection.
Share the existing three-quarter inch meter without upsizing and the water and wastewater charges are not triggered. Add a second meter and they are: on the 2026 schedule a three-quarter inch meter carries $6,900 of water charge and $2,600 of wastewater charge, $9,500 in total. A tap on a non-arterial street is listed at $14,300. A drainage charge of $1,225 per 1,000 square feet of new hard surface applies either way.
The published documents don't make clear whether the tap figure includes the system development charge, and the two could be additive. Confirm the figures with SPU's Development Services Office before you set a budget, including any figures from us.
The wastewater and drainage charges in this form began on 1 January 2026. A budget based on 2025 figures will be out of date.
In a water district: often charged per structure, even on a shared meter
Districts assess a per structure or per rather than by meter size, so the share-the-meter-and-skip-the-charge logic doesn't apply.
- Covington Water District does not require a separate meter, but charges a detached accessory unit the full future facilities and existing capital facilities charge per structure even on a shared meter, with an attached unit waived unless the meter needs upsizing. On the 2026 table that is $5,432 plus $5,993, about $11,425 for a five-eighths inch meter, plus the meter install and an added monthly flat rate per structure.
- King County Water District 90 has stated that units built after April 2023 should have a separate meter connection, and assesses an accessory unit up to 1,000 square feet at one full equivalent residential unit, plus a Seattle facility charge at the single family rate.
- Cedar River Water and Sewer District charges accessory units built after October 2019 at 50 percent of one equivalent residential unit.
Neighboring districts handle this differently, so call your own district before you budget.
The 50 percent impact fee cap does not help here
State law caps on an accessory unit at 50 percent of what the principal unit would pay, and it's reasonable to assume that covers utility charges. It doesn't.
RCW 82.02.090 defines an impact fee against a closed list of public facilities: public streets and multimodal facilities, publicly owned parks and recreation, school facilities, and fire protection facilities. Water and sewer are not on that list. The cap also applies only to a city or county, so it doesn't reach a water district's charge or King County's capacity charge either.
Several districts do discount accessory units, as above, but that is each district's own policy. The statute doesn't require it.
3. Permit fees
| Seattle SDCI | Unincorporated King County |
|---|---|
| Base review rate $292 per hour; land use review $551 per hour; 5 percent technology fee on everything. Construction and land use fees rose about 18 percent year on year. SDCI's own example moved a 1,500 sq ft house permit from $5,759 to $6,853. | New accessory dwelling application review from $4,556; site review $3,618, or $2,033 for basic review with standard conditions; inspection at $2.46 per square foot; state building code surcharge $6.50. |
| Land use pre-submittal conference from $1,102 for two hours. | Pre-application conference $1,452 per staff member; site visit $726 per trip; code interpretation $250. |
| Additional review for drainage, grading, trees and critical areas, charged at the hourly rates above. | Critical area designation from $1,643 per parcel plus $1,212 per discipline; plan resubmittals after the second are charged again. |
King County also publishes a work-without-permit investigation fee for a new accessory dwelling unit of $4,556, charged in addition to the permit, so building first and permitting afterward adds that amount to the cost.
4. Property tax, and the exemption with a deadline
The 1 percent levy limit doesn't cap your increase
Many owners know that Washington limits annual property tax growth to 1 percent and assume a cottage won't raise their bill much. That assumption is wrong, for two separate reasons.
New construction is expressly carved out of the limit. Under RCW 84.55.010 a taxing district receives an additional amount computed from the increase in assessed value resulting from new construction. It is added on top of the 1 percent, not inside it.
The limit constrains the district's total levy, not your bill. Assessed value determines your share of a fixed pot. Raise your assessed value relative to your neighbors and you take a larger slice, even if the pot itself grew only 1 percent. New construction raises the district's total levy, and it raises your share of that total as well.
On timing, the assessor may place new construction value on the rolls up to 31 August each year, valued as of 31 July. Value is added regardless of percentage of completion, so a half-built cottage standing on 31 July is assessed. State guidance also says the trigger is a building permit having been issued or having been required. An unpermitted unit is still taxed.
A three year exemption for the new value
RCW 84.36.400 exempts physical improvements to a single family dwelling from taxation for three assessment years, up to 30 percent of the value of the original structure, claimable once in a five year period. The statute explicitly names "constructing an accessory dwelling unit, whether attached to or within the single-family dwelling or as a detached unit on the same real property."
King County administers it as the Home Improvement Exemption.
The application has to be filed in time. King County's form states that applications received after completion of the project will be denied. The statute is stricter and requires notice "prior to the improvement being made," so go by the statute rather than the form. File it before you break ground. There is no retroactive relief and no appeal on the point.
For older owners and owners with disabilities: since 2024 the senior citizens and people with disabilities exemption and its related deferral programs include accessory unit value in the reduced valuation, so the reduction is larger than it would be for the primary residence alone. Any portion rented for more than six months a year can't be included, though. A senior who rents the cottage year round gets no benefit from it. One who houses family in it may.
5. Financing and what it appraises for
Before you talk to a lender, it helps to know two things.
Projected rental income counts, within limits. Both Fannie Mae and FHA now permit accessory unit rental income to be used in qualifying, but cap it at 30 percent of qualifying income, and FHA bars it entirely on cash-out refinances. In practice it does more for a borrower who already qualifies than for one close to the limit.
Appraisals are hard to support. There are few comparable sales of houses with detached accessory units in any given neighborhood, so an appraiser has limited evidence for what the cottage adds. Do not assume the appraised value increase matches the construction cost. Where sale prices for separately owned accessory unit condominiums are quoted as the argument for building, remember that those numbers depend on completing a condominium conversion whose cost is itself rarely priced.
Next is the electrical work, which is our trade: Part 6.
Sources
- King County Wastewater Treatment Division, capacity charge, and the county's Residential Sewer Use certification form for the RCE table; RCW 35.58.570 and RCW 57.08.081 for enforcement.
- Seattle Public Utilities Development Services Office charge menu and the 2026 System Development Charge update.
- Covington Water District accessory dwelling unit policy and 2026 connection charge table; King County Water District 90 ADU policy; Cedar River Water and Sewer District 2026 fee schedule.
- SDCI 2026 Fee Summary and Fee Subtitle; King County 2026 fee guides 01, 02 and 05.
- RCW 84.55.010 on the levy limit; RCW 84.36.400 and the King County Home Improvement Exemption application; RCW 36.21.080 on when new construction is placed on the rolls.
- RCW 82.02.090, the impact fee definition, and RCW 36.70A.681(1)(a), the 50 percent cap.
- Fannie Mae Selling Guide and HUD Mortgagee Letter 2023-17 on accessory unit rental income.
Every fee schedule quoted here resets on 1 January and several changed materially on 1 January 2026. Figures were read from published schedules in August 2026 and are given so you know what to ask about, not as a quotation. This is not tax advice; the property tax material describes how the statutes are written and your own situation may differ. General information from Hanco Electric.