ADU rules in Columbia

A detached ADU (for example an apartment above a detached garage or a guesthouse) is allowed as an accessory use to a single-family detached dwelling in every residential district (T/C, LL-R, RSF-1, RSF-2, RSF-3, RD, RD-MV, RM-1, RM-2) plus MU-1 and MU-2, marked 'C' in the accessory use table. 'C' means allowed on the condition that the use-specific standards are met, not a hearing. A detached unit must sit at least 10 feet from the principal structure and in the same base zoning district, and accessory structures must stay at least 3 feet from side and rear lot lines and out of the required front yard. Manufactured homes, RVs, travel trailers and campers may not be used as ADUs.

An ADU may be within or attached to the house (for example a downstairs or upstairs apartment) in the same districts that allow detached units. An attached unit must have an operative interconnecting door with the principal structure. Only one ADU is allowed per lot, and only one kitchen per ADU.

The ADU's floor area may not exceed the lesser of 1,000 square feet or 50 percent of the floor area of the principal dwelling, with carports, garages and unfinished basements excluded from the principal dwelling's area. The ADU section sets no separate height cap; a detached unit is otherwise held to the general accessory-structure rules (subordinate in area, extent and purpose to the principal structure, 3 feet from side and rear lines, not in the required front yard) and the district's dimensional standards. The ADU must also match the principal dwelling's architectural design, materials, colors, windows and roof pitch.

Either the principal dwelling or the ADU must be the permanent, full-time residence of the lot's owner. The requirement is written as a standing use standard, so it applies for as long as the ADU exists, not only at application. South Carolina has no state ADU law that limits this.

At least one off-street parking space must be provided for the ADU in addition to the spaces required for the primary dwelling (2 per single-family detached dwelling outside DAC, MC, NAC, CAC, RAC, MU-1 and MU-2, where the principal dwelling has no minimum). The ADU must also share the principal dwelling's driveway unless it is reached from a right-of-way the principal use does not use, such as a rear alley or the second street of a corner or through lot. The ADU standards apply regardless of zoning district, so the one-space rule appears to apply even in MU-1 and MU-2.

ADUs are a 'C' accessory use: allowed on condition of meeting the use-specific standards in §17-4.3(d)(1), with no special exception ('S') route needed in any district that allows them. Approval runs through a Zoning Permit, which the Zoning Administrator reviews and decides (appeal optional to the Board of Zoning Appeals), followed by a building permit. In a Historic Preservation Overlay district or on a landmark site, any development also needs a Certificate of Design Approval.

The UDO bars leasing or renting an ADU for tenancies of less than 30 days, so an ADU cannot be a short-term rental even though the city runs an STR permit program for other dwellings. Long-term rental is allowed, but the owner must live in one of the two units, and any residential rental unit needs an annual city rental permit under Chapter 5, Article VIII. The ADU may not be subdivided or segregated in ownership from the principal dwelling, and it must share the principal dwelling's street address, mailbox, and water, sewer, gas and electric utilities. Home occupations are not allowed in an ADU.