Columbia, SC Accessory Structures: ADU Rules (2026)
ADU rules in Columbia, SC, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Governing section
- UDO § 17-4.3(d)(1), Accessory Dwelling Unit
- Units per lot
- One ADU maximum per single-family lot
- Size cap
- Lesser of 1,000 sq ft or 50% of house
- Detached separation
- At least 10 feet from principal structure
- Parking
- One additional off-street space required
- Density
- ADUs excluded from maximum density calculations
Summary
Columbia allows one accessory dwelling unit (ADU) per single-family lot under Sec. 17-4.3(d)(1). The unit can be attached to the house or built as a detached structure like a garage apartment, but it is capped at the lesser of 1,000 square feet or 50 percent of the principal dwelling's floor area, limited to one kitchen, and does not count toward maximum density.
Accessory Dwelling Unit An accessory dwelling unit is allowed only as accessory to, and on the same lot as, a single-family detached dwelling unit, subject to the following standards:
a. There shall be no more than one accessory dwelling unit on a lot.
c. An accessory dwelling unit may be within or attached to the principal structure (e.g., a downstairs or upstairs apartment), or exist within or as a detached structure (e.g., an apartment above a detached garage or a guesthouse).
e. The floor area of an accessory dwelling unit shall be no more than the lesser of 1,000 square feet or 50 percent of the floor area of the principal dwelling unit (excluding carports, garages, and unfinished basements).
g. Only one kitchen is allowed per accessory dwelling unit.
h. At least one off-street parking space shall be provided in addition to those required for the primary dwelling.
j. Accessory dwelling units shall not count toward the maximum density standards.
k. Home occupations are not allowed in accessory dwelling units.
Full Breakdown
The standard governs accessory dwelling units in every base zoning district that permits them by right, T/C, LL-R, RSF-1 through RSF-3, RD, RD-MV, and RM-1/RM-2 among them, per the Accessory Use/Structure Table in Sec. 3(b). An ADU is allowed only as an accessory to a single-family detached dwelling and only on the same lot, and a lot may have no more than one. It can be built two ways: attached to the principal structure with an operative interconnecting door, such as an upstairs or downstairs apartment, or detached, such as an apartment over a garage or a standalone guesthouse; a detached unit must sit at least ten feet from the principal structure and remain in the same base zoning district.
The floor area is capped at the lesser of 1,000 square feet or 50 percent of the principal dwelling's floor area, excluding carports, garages, and unfinished basements, and only one kitchen is allowed inside the unit. An ADU must share the principal dwelling's street address and mailbox, cannot be subdivided or separately owned, must use the same water, sewer, gas, and electric utilities as the principal dwelling, and generally must share its driveway unless it has separate access from an alley or a second street frontage on a corner lot.
It must also match the principal dwelling's architectural design, materials, facade treatment, and roof pitch. At least one off-street parking space is required in addition to what the primary dwelling needs, and home occupations are barred inside an ADU. Because ADUs do not count toward the lot's maximum density standards, they are the city's mechanism for adding a second unit to a single-family lot without triggering a rezoning to a multifamily district.
Violations & Fines
Building a second accessory dwelling unit on a lot, exceeding the 1,000-square-foot/50-percent floor-area cap, installing a second kitchen, or running a home occupation inside an ADU violates Sec. 17-4.3(d)(1). The Zoning Administrator can deny or revoke the certificate of occupancy, issue a stop-work order, or pursue remedies under Sec. 17-8.6, and continued noncompliance is a misdemeanor under Sec. 1-5, fined up to $500.00 or 30 days' imprisonment per day of violation.
Frequently Asked Questions
How big can an ADU be in Columbia?
Can I build a detached ADU over my garage in Columbia?
Does an ADU count toward my lot's density limit in Columbia?
Sources & Official References
Other rules in Columbia
How Columbia compares: Easiest Cities to Build an ADU·Compare Columbia to another location·View the South Carolina accessory structures overview
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