An ADU can trigger costs beyond the base building-permit fee. Depending on the authority and project, these may include development charges, impact fees, water or sewer system development charges, connection work, capacity charges, deposits, or required upgrades. The name and trigger vary by jurisdiction.
Identify who charges and what triggers payment
Start with the local development or building authority, utility providers, and any school, park, transport, or other district with a published schedule. Check whether the charge is assessed per unit, by floor area, by bedroom count, by service size, or by a project-wide formula. Also check when payment is due and whether a waiver, exemption, deferral, or appeal requires a separate application.
Bonney Lake, Washington, publishes 2026 ADU examples: $2,613.53 per ADU for parks, $2,297.50 for schools, and $1,877.50 for transportation, plus system-development-charge examples of $9,183.79 for sewer, $1,282.05 for stormwater, and $4,439.82 for water. Both listed school districts—Sumner-Bonney Lake and White River—currently publish the same $2,297.50 ADU rate through January 31, 2027. Confirm which district applies and its payment-receipt procedure; bedroom-based calculations begin February 1, 2027, but the future amount is not established. The City’s page says the listed impact fees and system development charges are due before a building permit is issued. A single ADU may qualify to defer the cited water and sewer charges, but eligibility and approval must be confirmed. See the Bonney Lake rate guide; these figures are not Washington-wide rates or a parcel quote.
Relief may change timing or conditions, not erase the charge
Toronto’s published laneway-suite permit guide identifies a development-charge deferral route for eligible secondary dwellings. The separate Laneway and Garden Suite Development Charges Deferral Program page sets its own area, zoning, title, and administrative conditions. A deferral is not necessarily a waiver, and the amount due must be confirmed for the property.
Portland’s ADU System Development Charge waiver is another local example. It requires application and a recorded covenant, including a ten-year short-term-rental restriction that applies to all structures on the property. Portland’s separate temporary exemption for certain new housing SDCs expressly excludes ADUs; confirm the distinct ADU waiver rather than assuming the temporary program applies. Ontario has configuration-specific statutory exemptions, separate affordable-unit relief, and municipal programs that reduce some development charges. Their eligibility tests and continuing conditions differ; see the Ontario development-charge exemption guide. Local charge schedules and other project costs still require confirmation.
Napa County’s affordable-housing impact-fee table lists a $0 County fee for ADUs up to 750 square feet in each of 2026, 2027, and 2028. For larger ADUs, the table says the fee is proportionate to the primary house’s square footage but does not state a flat amount or complete calculation. This is one County fee schedule, not a full project-cost estimate or a City of Napa fee rule. See the Napa County fee guide and ask the County whether its schedule applies to the parcel.
Some jurisdictions tie impact-fee treatment to a separate ADU program. Pasadena’s current page states that an ADU of 900 square feet or less on a single-family site may be exempt from Residential Impact Fees; above that size, the listed RIF basis is proportional to the primary dwelling. A Housing Agreement or City Standard Plan changes the stated treatment, and agreement timing matters. San Diego’s ADU Home Density Bonus page describes a Community Enhancement Fee for specified under-750-square-foot bonus or affordable units, with a waiver for deed-restricted units in High or Highest Resource Areas. The City’s summary does not give the fee amount or complete code and map tests, so the City must confirm a project-specific result.
California’s SB 1117 was listed as signed in the Governor’s September 2026 announcement. The available Assembly analysis describes the introduced bill’s proposed rule for impact fees on ADUs over 750 square feet, but the exact chaptered text and effective provisions were not established from the official bill-text URL. Do not calculate a current fee from the introduced-bill summary; confirm the operative statute and local schedule. The Pasadena guide covers its local fee schedule; it does not replace the State or another city’s rule.
In England, Community Infrastructure Levy relief for a qualifying residential annex has pre-commencement claim and notice steps and can be withdrawn in specified circumstances during a three-year period. England’s Building Safety Levy is a different charge: national guidance says it applies from October 1, 2026 only when the development meets all charging tests, including the major-residential-development threshold. A small standalone ADU should not be assumed liable solely because it creates a dwelling. Hounslow publishes local rate examples of £58.29/m² and £29.15/m² for qualifying previously developed land for relevant applications submitted on or after October 1, 2026. That effective date has passed. The Hounslow levy guide explains how the local rates relate to the national charging tests; a rate does not establish whether a specific project owes the levy.
Request the parcel-specific calculation
Ask each authority or provider to confirm the property address, service configuration, unit count, project type, effective schedule, payment deadline, available relief, and required documents. For utilities, separate a published connection or system charge from a private contractor quote and from an estimate of service capacity or upgrade work. Keep ordinary application fees in the permit budget as a separate line.
A published schedule is a planning input, not a total project-cost estimate. Verify the latest rate and property classification before relying on it. Paying or deferring a charge does not establish zoning permission, code compliance, utility capacity, or approval to occupy.