San Marcos, CA Accessory Structures: ADU Impact Fees (2026)

G directly addresses the fees a San Marcos ADU can be charged. 1 states plainly that the City does not assess utility fees or capacity charges on ADUs at all. 2 governs public facilities fees more broadly: any fee charged must be proportionate to the ADU's actual burden on that public facility, must be calculated based on the size of the primary dwelling unit, and can never exceed the reasonable cost of providing the service. Critically, the same subsection creates a hard exemption: no public facilities fees of any kind may be imposed on an ADU smaller than 750 square feet.

C put a one-bedroom ADU at exactly 850 square feet and cap Junior ADUs at 500 square feet, most Junior ADUs and many single-room ADU projects fall entirely inside the fee-free 750-square-foot threshold. 3 requires that whatever fees do apply be paid before the City issues the building permit for the ADU's construction, not later in the process. Subsection F separately requires the City to obtain a 'will serve' letter from every utility agency with jurisdiction before issuing that building permit, and it specifies that an ADU built entirely within the existing footprint of a single-family dwelling or accessory structure does not require a new or separate utility connection in the first place, which is part of why no capacity charge attaches to it.