ADU rules in Charleston County

Unincorporated Charleston County lists Accessory Dwelling Units among the accessory uses allowed with any legally permitted residential use (ZLDR Sec. 6.5.3), and Sec. 6.5.9 allows one ADU on an existing lot subject to objective standards; the definition includes garage apartments and Sec. 6.5.1 names detached ADUs as accessory structures. In the Rural Area of the Comprehensive Plan the lot must be at least 50 percent larger than the district's minimum lot area (for example 45,000 square feet in AGR or RR instead of 30,000, 1.5 acres in AG-8). In Residential and RO districts of the Urban/Suburban Area a detached ADU needs only a 3 foot interior side setback and may sit in front of the house. Waterfront lots on an OCRM critical line face extra lot area or lot width minimums under Sec. 4.24.2.

The ADU definition covers any dwelling unit with complete independent living facilities separate from the principal unit, and Sec. 6.5.9.E expressly contemplates attached ADUs (they may not have a separate electric meter, while detached ADUs must have one). An attached ADU is part of the principal structure and must meet the principal structure's setbacks (Sec. 6.5.8.A and F). The same one-ADU-per-lot limit and Rural Area lot size gate apply. The Sol Legare overlay is the exception that allows separate meters for attached units.

Sec. 6.5.9.A caps the heated gross floor area of an ADU in the Rural Area at 1,500 square feet. Ord. No. 2375 (1-27-2026) left no fixed square footage cap for the Urban/Suburban Area; there Sec. 6.5.1.B.4 only requires non-agricultural accessory structures to be subordinate to the principal structure in gross floor area, while letting ADUs reach the district's maximum building height (35 feet in most residential districts), and all coverage and setback limits apply. Overlay and special districts set their own caps: Sol Legare 800 square feet (up to 1,500 by BZA special exception), the Mount Pleasant overlay's Sweetgrass Basket Stand area 1,200 square feet on lots of 12,500 square feet or more, Parkers Ferry 1,500, conservation subdivisions 600 (detached, non-waterfront lots only), the form-based district 1,500 in T2 and 800 in T3 to T5, and Dewees Island a 3,300 square foot combined house-plus-ADU cap; the Ten Mile overlay waives the 1,500 square foot Rural Area cap.

Neither Sec. 6.5.9 nor any other ADU provision of the ZLDR requires the owner to live on the property; an enCodePlus search for 'owner-occupied' returns only the short-term rental provisions and the affordable housing section. Owner occupancy matters only if the property is used as a Limited Home Rental short-term rental, which requires the record owner to use the property as a legal residence (4 percent assessment plus voting address or driver's license address).

Table 9.3.2, the Off-Street Parking Schedule, has no Accessory Dwelling Unit row, and Sec. 6.5.9 says nothing about parking. Under Sec. 9.3.3 a use not listed in the table takes the standard of the most functionally similar listed use as the Zoning and Planning Director decides, and lots with more than one use provide the total for all uses; the closest rows are 'Dwelling, Single-Family Detached' at 2 per dwelling unit and Manufactured Housing Unit at 2 per unit. How staff actually apply that rule to ADUs was not confirmed in writing. An ADU used as a short-term rental adds 1 space per permitted bedroom (Ord. No. 2409 of 10-13-2026 extends this to every designated sleeping area).

ADUs are an accessory use listed in Sec. 6.5.3, not a principal use in Table 6.1-1, so they need no conditional use or special exception: the route is a county zoning permit (the residential addition and accessory structure zoning permit packet, revised 10-2026, with a restrictive covenants affidavit) followed by a building permit. A BZA special exception enters only in the Sol Legare overlay, to go from 800 to 1,500 square feet. Sec. 3.8.7 still mentions five-year administrative renewal of special exceptions for ADUs, a leftover from older rules that the current Sec. 6.5.9 does not use; a temporary manufactured home ADU for an ill relative (Sec. 6.6.1) is a separate one-year renewable permit.

The ZLDR places no limit on renting an ADU long-term. Sec. 6.8.3.A.1 lets legally permitted principal dwellings and ADUs be used as short-term rentals (29 days or fewer), even on the same property, but accessory structures that are not dwelling units may not be. Table 6.8.2 sets the types: Limited Home Rental (owner-occupied property, 72 days a year, staff review) in RM, AG, AGR, RR, S-3, R-4, MHS and UR; Extended Home Rental (no owner occupancy, 144 days, Limited Site Plan Review plus a BZA special exception) in AG-8 and AGR (bona fide agricultural uses with the owner living there), S-3, R-4 and MHS; and Commercial Guest House (no limit) in RO, GO, NC, RC and CC. Rental days count per lot, not per dwelling, so an ADU and the main house share one allowance. Every STR needs a zoning permit and business license renewed by December 31 each year, with the permit number on every advertisement. Dwellings in dwelling groups may not be STRs. The ZLDR's base ADU rules do not bar selling an ADU separately, but it must sit on the same lot as the principal use (the form-based Chapter 7 expressly bars separate sale).