ADU rules in Mount Pleasant

A detached ADU is an accessory use of a detached house dwelling, allowed with a zoning permit in any district where detached houses are allowed (Table 156.436-1). The code defines an ADU as a separate unit 'within, attached to, or detached from a detached house dwelling' with its own outside entrance, kitchen and bathroom. One ADU per lot, subject to building coverage limits. ADUs are not tied to the Old Village anymore: the zoning code rewrite adopted 1-14-2025 carries them townwide, as did the prior code (the town's older handout listed RC-1, RC-2, RR, CC, CL, R-1 to R-4, WG-W and PD).

Attached ADUs and ADUs inside the principal house are allowed on the same terms as detached ones (the definition covers a unit 'within, attached to, or detached from' a detached house), with a separate 850 square foot floor-area cap. ADUs attach only to a detached house dwelling, not to a duplex, townhouse or multiplex. An accessory structure connected to the house by conditioned space, a shared wall, roof or foundation counts as part of the principal structure and must meet the district's principal setbacks.

Detached ADUs follow the accessory building footprint table: 850 square feet in CC, R-1 and R-2, 600 square feet in R-3, R-4, TH and RM, 850 square feet in all other districts except PD, and no maximum in RC-1, RC-2 and RR if building coverage is not exceeded; a PD without its own figure borrows the comparable district's number. The cap is on footprint, not floor area, so a two-story detached unit (for example over a garage) is limited by footprint and height. Attached and internal ADUs are capped at 850 square feet of gross floor area. Major residential accessory structures may be 25 feet tall with 6 foot side and rear setbacks. In the Ten Mile Community Overlay (adopted 6-11-2026) the ADU square-foot limit does not apply. A proposed 40 percent of the principal footprint cap was struck by council before adoption of Ord. 25051 on 2-10-2026.

Ongoing requirement: a covenant recorded to run with the property must state that either the principal house or the ADU is owner-occupied as the owner's primary residence. The same covenant limits the ADU to three occupants and bars converting the property to a horizontal property regime (condominium). South Carolina has no ADU statute that preempts this.

One private off-street space is required for the ADU's occupants on top of all spaces required for the principal house, shown on the site plan. Tandem parking is allowed only if the tandem spaces serve the ADU alone. An ADU used as a short-term rental also needs at least one additional off-street space for each unit rented short term.

An ADU needs a zoning permit (Table 156.436-1), which the Zoning Administrator approves or denies with no notice or hearing, plus a scaled site plan following the ADU Site Plan Checklist and the recorded covenant. New ADU construction also needs a building permit allocation: the growth-management system reserves Category B permits for ADUs, 10 per half-year cycle (20 a year, 100 for 2024-2029), handed out first come, first served from January 1 and July 1 with a waiting list. In the Old Village Historic District, exterior work also needs a certificate of appropriateness from the Historic District Preservation Commission.

The code does not bar renting either unit long term, but the recorded covenant keeps one unit owner-occupied, caps ADU occupancy at three persons and forbids a horizontal property regime, so the ADU cannot be sold separately. Short-term rentals (under 30 consecutive days) need an annual STR permit and a business license. Townwide STRs are capped at 400, a permitholder may hold two permits, and new applicants join a waitlist. A legally established ADU may be rented short term in its entirety to one guest, or the main house may be; if the owner does not live on site, the ADU and the main house may never be rented separately. The ADU and whole-house permit fee is $500, and operators must show at least 24 nights and four bookings. Short-term rental of any residentially zoned parcel in the Old Village Historic District is prohibited.