ADU rules in Summerville

Summerville's Unified Development Ordinance lists the Accessory Dwelling Unit as a Conditional Use (C) in the N-R, N-MX, GR-2, GR-3, GR-5, MF-R and MH-R districts and as a Permitted Use (P) in D-MX, UC-MX, N-B, G-B and AC; only L-I, H-I and PL bar it. The UDO defines a Conditional Use as one 'permitted by right, provided that the additional use standards' of Chapter 3 are met, so a detached ADU (a carriage house, guest house, pool house or garage apartment) is allowed by right on single-family lots that meet the Section 3.4.2.B standards. One ADU is allowed per lot, or two in N-R, N-MX and GR-2. The lot must be served by public water and sewer, and the ADU must sit to the rear or side of the main house.

The UDO defines an Accessory Dwelling Unit as 'an auxiliary dwelling unit either attached to a primary dwelling unit or located within an accessory structure', so attached and internal units follow the same use table and Section 3.4.2.B standards as detached ones. They are by right in N-R, N-MX, GR-2, GR-3, GR-5, MF-R and MH-R once the standards are met (public water and sewer, owner-occupied primary dwelling, two bedrooms, half the main house's floor area, two adult residents), and permitted outright in D-MX, UC-MX, N-B, G-B and AC.

Section 3.4.2.B.6 caps an ADU at 'no more than 50% of the gross floor area of the primary building', and the same section limits it to two bedrooms and two adult residents (persons over 18). Height comes from Chapter 2: accessory buildings may be 20 feet in N-R, N-MX and D-MX and 30 feet in UC-MX, and in GR-2, GR-3, GR-5, MF-R and MH-R they may be no taller than the main structure. Under Note 7, an accessory building may not be taller than the principal structure, except that a two-story accessory building is allowed in the rear yard behind a one-story house. An amendment pending at first reading (October 8, 2026) would measure the cap as 50% of the primary dwelling's footprint instead of its gross floor area. Each of two ADUs, where two are allowed, would get that full allowance.

Section 3.4.2.B.3 states: 'The primary dwelling must be owner-occupied and the ADU shall have the same ownership as the primary dwelling.' The owner must live in the main house, not just in one of the two units, and this is a continuing use standard rather than a one-time application test. South Carolina has no state law that overrides it. The requirement does not reach the D-MX, UC-MX, N-B, G-B and AC districts, where the ADU is a plain Permitted Use and the Section 3.4.2.B conditions do not attach.

The Section 7.4.1 minimum parking table requires '1 per unit' for an Accessory Dwelling Unit, on top of the one space required for the single-family house. Section 7.4.2 removes every minimum in the Downtown Mixed-Use district (D-MX). Section 3.4.2.B.8 separately caps parking at two off-street spaces per ADU.

Section 13.7.1 makes a Zoning/Development Permit (administrative review by Town Staff, appealable to the Board of Zoning Appeals) 'the only type of approval required' for Permitted Uses and Conditional Uses that meet the standards of Sections 3.3 and 3.4. An ADU therefore needs a staff zoning permit and then a building permit, with no hearing. The exception is the Summerville Historic District and individually listed landmarks. There, Section 13.8.2 requires a Major Certificate of Appropriateness from the Board of Architectural Review (discretionary design review) before a building permit for any new structure or expansion, and the BAR regularly reviews garage-to-ADU conversions and carriage houses.

The UDO does not bar long-term rental of an ADU. However, the main house must be owner-occupied, the ADU must stay in the same ownership as the main house (so it cannot be sold off separately), and no more than two adults may live in it. The UDO defines a Short Term Rental as 'the leasing or rental of an entire or extra dwelling unit by a Permanent Resident' for less than a month, so renting an ADU nightly is a separate lodging use. That use is a Conditional Use in N-R, N-MX, GR-2, MF-R and MH-R, a Special Exception decided by the Board of Zoning Appeals in GR-3 and GR-5, and permitted in D-MX, UC-MX, N-B and G-B. Section 3.4.3.E carries over the home-stay rules: the owner must reside on site at least 183 days a year, and the operator needs a business license, liability insurance, a Town safety inspection, and records kept for two years. A rewrite of the short-term rental rules (renaming the uses STR-1 and STR-2, an annually renewed STR permit, and the permit number in every advertisement) was at council workshop stage on October 15, 2026.