ADU rules in Rock Hill

A detached ADU is allowed on any lot whose primary use is a single-family detached house, a rural farm or an urban farm. It is a 'conditional accessory use', which in Rock Hill means it is allowed once the written standards are met, with staff review and no hearing. Single-family detached homes are themselves a conditional (staff-level) use in every residential district (RR, SF-2, SF-3, SF-4, SF-5, SF-8, SR, SF-A, MF-15/MFR) and several business districts, so the ADU route reaches essentially every house lot in the city. Only one ADU is allowed per lot, and mobile homes, manufactured homes, RVs and travel trailers cannot be used as ADUs.

The ordinance defines an ADU as a secondary unit 'whether part of the same structure as the principal dwelling unit, or as a detached structure on the same lot', so an attached or internal unit (basement, addition, conversion) takes the same staff-level conditional accessory use route as a detached one. It must have its own bathroom, kitchen and sleeping quarters and be clearly subordinate to the main house. The same one-ADU limit and owner-residency rule apply.

Rock Hill sets a minimum, not a maximum, for the ADU itself: 'Accessory dwelling units must be at least 480 square feet.' A detached ADU is an accessory structure, so it falls under the general size table for all detached accessory structures on a single-family lot combined: the greater of 30% of the principal structure's heated floor area or 600 sq ft when the house has a two-car garage or carport; 800 sq ft when it has a one-car garage, provided at least 200 sq ft goes to a detached garage or carport; 1,000 sq ft when it has no garage, provided at least 400 sq ft goes to a detached garage or carport. Lots zoned RR or RC of 5 acres or more with the house at least 100 feet off the road have no limit. Accessory structures may not exceed the lesser of two stories or the height of the house. Attached units have no numeric cap beyond the definition's 'clearly subordinate' test and the district's dimensional standards for the house.

Ongoing requirement: 'The property owner must live on the property.' For the first five years after the ADU is created the owner must live in the main house; after that the owner may live in either the main house or the ADU. The one exception is an ADU built as part of a 'residential infill' use (a Zoning Board of Appeals special exception in certain districts), where the owner need not live on site. South Carolina has no state ADU law that preempts this.

The ADU standards require 'at least one off-street parking space ... for each bedroom located in an accessory dwelling unit', on top of whatever the main house needs. A one-bedroom ADU therefore needs one added space and a two-bedroom ADU two. ADUs built under the residential infill special exception follow that use's own parking standards instead.

An ADU is a conditional accessory use ('C' in the accessory use table). Rock Hill's ordinance says 'Permitted and conditional uses are a staff-level review; special exceptions go to the Zoning Board of Appeals.' A 'C' accessory use is allowed 'provided that all use-specific standards and all other applicable regulations in this ordinance are met', so the normal route is staff zoning review plus a residential building permit, with no hearing. The Zoning Board of Appeals is involved only for a variance from a dimensional standard (setbacks, height) or for the separate residential infill special exception.

An ADU 'must not be leased or rented for tenancies of less than 30 days, nor leased to more than 11 different individuals in any calendar year', unless the property is approved as a short-term rental. Short-term rentals (stays under 30 days) need a city STR permit and are allowed only as a conditional use in MX, RC, NO, NC, LC, GC, CC, CI, DTWN and MUC, not in any residential district (RR through MF), so an ADU on an ordinary single-family lot cannot be rented nightly. STR permits issued before 11-29-2023 in districts that no longer allow STRs expire on 4-30-2029. STR permits renew annually by April 30 and require a business license (as a hotel use) and local accommodations tax. Long-term rentals must be registered under the city's residential rental registration program (Code Chapter 10, Article X), and an ADU cannot be sold apart from the main house.