ADU rules in Hilton Head Island

Hilton Head Island's Land Management Ordinance (LMO) has no accessory dwelling unit use. Since Ord. 2023-06 (5-2-2023), 'More than one single-family dwellings on a single lot constitutes a multifamily dwelling', so a backyard cottage is legal only where Multifamily is a permitted use and the lot has density for a second unit. The use table permits Multifamily in RM-4, RM-8 and RM-12 and most mixed-use districts, but not in the RSF-3, RSF-5 and RSF-6 single-family districts, which allow Single-Family only. In the ten PD-1 plantations (Sea Pines, Hilton Head Plantation, Palmetto Dunes and others) uses and densities follow each Town-approved Master Plan, and private covenants usually govern as well. In the Gullah historic neighborhoods, a qualifying family can also place several homes on one parcel as a Family Compound.

An internal or attached second unit (basement apartment, addition, garage apartment) makes the building one 'containing two or more dwelling units', which is the LMO's Multifamily use. It is therefore permitted only in the districts that allow Multifamily (RM-4, RM-8, RM-12 and most mixed-use districts) and only where density allows a second unit. It is not permitted in RSF-3, RSF-5 or RSF-6. The accessory use table lists no dwelling of any kind. Ord. 2023-06 also struck the only accessory dwelling use the LMO had, 'divisible dwelling units' (lock-out rooms), which had been limited to multifamily and interval occupancy developments in the CR and RD resort districts.

Because the LMO treats any second unit as multifamily, there is no accessory-unit floor-area cap or percentage-of-primary limit. Instead, every unit counts against the district's maximum density per net acre: 4 units in RM-4 (6 on lots of at least 3 acres, 8 on at least 5 acres), 8 in RM-8 and 12 in RM-12. In practice a second unit in RM-4 needs at least about half a net acre. Height limits are 35 feet in RM-4 and 45 feet in RM-8 and RM-12. The 2023 staff report noted that multifamily design standards (setbacks and buffers) apply once a second home is added.

Neither the Single-Family nor the Multifamily use, nor Small Residential Development Review for two to five units on a lot, carries any owner-occupancy requirement. The only ownership test is in the Family Compound route: the property must be in a historic neighborhood and must have been owned by the same family since 1956 or earlier, or sold to such a family, and the owner of record must request the compound. South Carolina has no state ADU law that addresses owner occupancy.

The residential district tables set minimum off-street parking by use. Multifamily, which includes any second dwelling on a lot, requires 1.4 spaces per one-bedroom unit, 1.7 per two-bedroom unit and 2 per unit with three or more bedrooms. Family Compound units require 2 spaces each. Single-Family requires 2 spaces per dwelling plus 1 per 1,250 square feet of gross floor area over 4,000. No reduction is offered for a small second unit.

Where Multifamily is allowed, two to five dwelling units on a single lot go through Small Residential Development Review. The Official decides it within five business days, and the application is deemed approved if the Official does not act. A building permit follows. Family Compound Review is also decided by the Official, within seven business days, with expedited building permit review. Neither route requires a board hearing. Adding a second unit in RSF-3, RSF-5 or RSF-6 would need a rezoning, a legislative decision by council.

The LMO has no rental restriction on a second dwelling. However, the Municipal Code treats anyone who rents one or more residential units, short- or long-term, as doing business, so an annual business license is required. Rentals of fewer than 30 consecutive days need an annual STR permit (valid May 1 to April 30) under Title 10, Chapter 2, as rewritten by Ord. 2025-20 (10-21-2025). Where several units sit on one parcel, each unit needs its own permit. A permit requires a certificate of occupancy, an HOA approval letter where applicable, a site plan for trash and up to six driveway parking spaces, a natural-person applicant, fire and life-safety equipment, and an agent who can respond within one hour. The LMO has no separate-sale rule specific to second units.