One of the smartest questions a Charleston buyer can ask is whether a home can support a carriage house, what the code calls an accessory dwelling unit, or ADU. Done right, a detached or attached second unit can house a family member, a home office, or a long-term tenant who helps carry the mortgage. Done on an assumption, it can be an expensive misunderstanding. Because in this metro, whether you can build one, and what you can do with it, depends almost entirely on which town you are standing in.
Here is the honest map of ADU rules across the Charleston area, and the one rule that surprises almost everyone.
First, what an ADU actually is
An accessory dwelling unit is a smaller, secondary home on the same lot as a main house: a converted garage, a basement or above-garage apartment, or a detached carriage house. It has its own kitchen, bath, and entrance. In historic Charleston the carriage house is the original ADU, and the form has come back into favor as a way to add flexible space and, in the right case, rental income.
The rule that changes everything: long-term rent, not Airbnb
Before anything else, understand this, because it drives the whole financial case. In the City of Charleston and in North Charleston, an ADU cannot be used as a short-term rental. The City is explicit: a lot with an ADU is not eligible for a short-term rental permit, and neither the main house nor the ADU may be used as one. Mount Pleasant effectively steers ADU short-term use into its bed-and-breakfast rules rather than allowing it by right. So underwrite an ADU as long-term rental income, monthly leases, not nightly revenue. If your model depended on vacation rates, the model is wrong.
City of Charleston
The City approved a modern ADU ordinance in 2020. The key points:
- Allowed as a conditional use in the base zoning districts, so it is permitted with conditions, not automatically by right.
- Owner-occupancy required. Either the main house or the ADU must be the owner's primary residence.
- Size caps. The ordinance limits total conditioned floor area, commonly cited at 850 square feet, and the building footprint at 600 square feet, with one ADU per lot.
- It must meet the district's dimensional rules, setbacks, height, and coverage, which are district-specific, so verify them for the exact parcel.
- Parking. One space for the ADU in addition to the parking required for the main house.
- No short-term rental, as above.
On affordability: the original 2020 rule required ADUs to rent at an affordable rate, but the City loosened that in 2022. Now rent is generally at the owner's discretion unless City grant or subsidy money was used, which triggers affordability limits. Verify current terms with the City.
On the peninsula, add a second approval: the BAR
If the home is in the historic district, a new or altered carriage house visible from the street also needs Board of Architectural Review approval, on top of the zoning conditional use. That is two tracks, design review plus zoning, which adds time and design constraints. Budget for it, and read my guide to what the BAR controls before you plan a peninsula ADU.
Mount Pleasant
Mount Pleasant allows ADUs in a specified list of zoning and planned-development districts, capped at 850 square feet of conditioned space, one per lot. Two features stand out. First, the town requires a recorded covenant affidavit certifying that no private covenant on the property prohibits an ADU, a nod to the fact that an HOA can block one even where the town allows it. Second, the rental rule is tied to where the owner lives: if the owner occupies either the house or the ADU, the other can be rented long-term; if the owner does not live on-site, the ADU cannot be rented separately from the main house. Exact setback, height, and parking figures come from the town's accessory-structure rules, confirm them with Mount Pleasant Planning.
North Charleston
North Charleston is the newest entrant, adopting an ADU ordinance in early 2025, so this is a recent change. An ADU there generally cannot exceed two-thirds the size of the main house or 800 square feet, whichever is smaller, needs a minimum lot around 4,500 square feet, and requires one additional off-street parking space. Like the City, North Charleston ADUs are not eligible for short-term rental. Owner-occupancy and the exact list of eligible districts should be confirmed against the enacted code.
Charleston County and Summerville: verify before you assume
In unincorporated Charleston County, ADUs are addressed in the county's land-development regulations. Secondary sources describe roughly one ADU per lot with a size cap near 800 square feet, plus waterfront rules if the parcel touches the coastal critical line, but the exact districts and owner-occupancy terms should be pulled from the current county code or confirmed with County Zoning. Summerville does not publish a simple blanket ADU allowance the way the cities do, and much of the surrounding corridor is unincorporated Berkeley or Dorchester County, each with its own zoning. In other words, confirm the governing jurisdiction by parcel, because a Summerville mailing address does not tell you the rules.
What it costs to build
Construction cost varies widely, so treat any figure as a starting point and get local bids. Industry estimates for a detached ADU in the Charleston area run roughly $180 to $400 per square foot, with total detached projects commonly cited from around $150,000 into the $300,000s depending on size, site access, utilities, and finishes. Peninsula and historic builds tend to run higher because of design review and site constraints. The City's ADU permit fee has been cited in the low thousands, verify current fees. The point: a carriage house is a real construction project, not a weekend add-on.
The income reality
Here is the part to be honest about. A compliant long-term ADU can add rental income for the right property in an active rental market like Charleston. But the amount depends entirely on location, size, and current market rents, and there is no reliable single number I can promise you. Underwrite it conservatively, on long-term monthly rent, not vacation rates, and confirm realistic rents with a local property manager or comparable long-term listings before you count on any figure in your offer.
Check the covenants before the code. Even where a town allows an ADU, a subdivision's recorded HOA covenants can prohibit one, and the covenant controls. Mount Pleasant even makes you certify this in writing. In any community with an association, pull the covenants before you assume you can build.
The bottom line
A carriage house can be one of the best value-adds in Charleston real estate, flexible space, a family suite, or a long-term tenant who helps with the mortgage, if the property actually supports it. The failure mode is assuming, buying on great ADU potential in a listing, and discovering the jurisdiction, the zoning, or the covenants say otherwise. That is exactly the check I run for you. Tell me the address and how you want to use the second unit, and I will confirm the jurisdiction, the current ADU rules, the historic-district track if it applies, and the covenants, before you pay for potential. None of this is legal advice, always verify current zoning and ordinances with the specific jurisdiction.
Common questions
Can you build an ADU or carriage house in Charleston?
Yes in many cases, but the rules depend on the jurisdiction. The City of Charleston allows ADUs as a conditional use with owner-occupancy and size caps, commonly cited at 850 square feet of conditioned area and a 600-square-foot footprint, one per lot. Mount Pleasant and North Charleston allow them in specified districts with their own caps, and North Charleston adopted its ordinance in early 2025. Unincorporated county and Summerville rules vary and must be verified by parcel. On the peninsula, an ADU also needs Board of Architectural Review approval.
Can you Airbnb an ADU in Charleston?
Generally no. In the City of Charleston, a lot with an ADU is not eligible for a short-term rental permit, and neither the main house nor the ADU may be short-term rented. North Charleston also prohibits short-term rental of ADUs, and Mount Pleasant steers ADU short-term use into its bed-and-breakfast rules. ADU income in the Charleston area should be underwritten as long-term rental only, not nightly vacation revenue.
How big can an ADU be in Charleston?
Size caps vary by jurisdiction. The City of Charleston limits an ADU's conditioned floor area, commonly cited at 850 square feet, and building footprint at 600 square feet, one per lot. Mount Pleasant caps conditioned space at 850 square feet. North Charleston limits an ADU to two-thirds the size of the main house or 800 square feet, whichever is smaller. Always confirm the current cap and dimensional rules with the specific jurisdiction for the exact parcel.
Does an ADU require owner-occupancy in Charleston?
In the City of Charleston, yes, either the main house or the ADU must be the owner's primary residence. Mount Pleasant ties rental rights to owner occupancy: if the owner lives on-site, the other unit can be rented long-term; if not, the ADU cannot be rented separately. North Charleston's owner-occupancy terms should be confirmed against the enacted 2025 ordinance. Requirements differ by jurisdiction, so verify for the specific property.
How much does it cost to build a carriage house in Charleston?
Construction costs vary widely, but industry estimates for a detached ADU in the Charleston area run roughly $180 to $400 per square foot, with total detached projects commonly cited from around $150,000 into the $300,000s depending on size, site access, utilities, and finishes. Historic-district builds tend to cost more due to design review and site constraints. Get multiple local contractor bids and confirm current permit fees with the jurisdiction before relying on any figure.