Sacramento County Zoning Code § 902-57 Substantial

Any reconstruction, rehabilitation, addition, or alteration of a building, or any part thereof, the cumulative cost of which equals or exceeds 50 percent of the market value of the building prior to the start of construction, as determined by independent appraisal. Start of construction is the point at which a building permit for the improvements is issued. Cumulative cost is calculated for the preceding five (5) year period from the date of building permit application. “Substantial improvements” includes repair of “substantial damage”, regardless of the actual repair work performed. The term does not include either:

A. Any project for improvement of a building to correct existing violations or state or local, health, sanitary, or safety code specifications that have been identified by the local code enforcement official and which are the minimum necessary to assure safe living conditions, or
B. Any alteration of a "historic building" provided that the alteration would not preclude the building's continued designation as a "historic building".

902-57.1 Urban Level of Flood Protection (ULOP)

The level of flood protection that is necessary for new development to withstand a 200-year event in accordance with Urban Level of Flood Protection criteria developed by the State of California Department of Water Resources. The ULOP is required for a development project and/or new construction located within an “applicability area” as shown in Appendix D: “ULOP Applicability in Unincorporated Areas of Sacramento County”, indicating locations where all the following criteria apply:

The project site is in a Flood Hazard Zone.
The project site is in an urban area with 10,000 or more residents, or is in an urbanizing area in which 10,000 or more residents are anticipated within 10 years; and
The project site is in a watershed with a contributing area greater than 10 square miles; and
The project site has potential flood depths greater than 3’ in the 200-year flood.