Garages, Detached Structures and ADUs in Columbus

A second building on a residential lot is governed by a different set of rules than the house it sits behind. Accessory structures — garages, workshops, pool houses, and accessory dwelling units — have their own setbacks, their own coverage limits, and their own code path, and on a small urban lot those rules, not the budget, usually decide what can be built.

This guide covers what changes when a structure is detached rather than attached, how slabs and frost-line footings work for a garage, how setbacks and lot coverage apply to accessory buildings, what the Columbus zoning code currently requires of an accessory dwelling unit, which building code applies to which ADU configuration, and what running utilities out to a detached structure involves. Dimensional standards and review requirements are cited to the City of Columbus documents that publish them and are current as of writing — the zoning code and the building code both change, and the standards in force for a specific project are a question for Building and Zoning Services at the time of application.

Attached, detached, or internal

Three configurations cover almost everything built as a second living space or a second structure.

• Detached. A separate building, physically apart from the house. Detached structures have their own foundation, their own roof, and their own utility runs, and they are governed by the accessory-structure rules for the district.
• Attached. New structure built against the house, sharing a wall and usually sharing systems. Cheaper to service and harder to place, because it extends the principal building’s footprint and takes the principal building’s setbacks.
• Internal. Space created entirely inside an existing building — a basement, an attic, or a converted wing. No new footprint, no new foundation, and the zoning questions largely fall away. Where the goal is living space rather than a building, this is the cheapest of the three.

For an ADU specifically, Columbus describes all three: detached ADUs as one- or two-story structures separated from the primary building, which may stand alone or sit attached to or above a detached garage; attached ADUs as new structures joined to the primary building on the side or the back, with a separate entrance; and internal ADUs as units contained entirely within an existing building [1].

Garages and detached structures

Attached versus detached

An attached garage shares a wall and a roof line with the house, which makes it cheaper per square foot to build and to heat, and keeps the car within the building envelope. It also consumes side-yard space, extends the principal building for setback purposes, and requires a fire separation between the garage and the dwelling.

A detached garage is a complete small building: its own footings, slab, framing, roof, and service run. That makes it more expensive for the same square footage, and it is often the only option on an older Columbus lot where the house already sits close to a side line. The advantages are practical — noise, fumes, and fire risk are outside the house, the structure can sit at the rear where accessory setbacks are more permissive than principal-building setbacks, and it can later carry an ADU above it.

Slabs and frost-line footings

The foundation question for a detached garage is simple to state and easy to get wrong. A slab has to be supported at its edges by footings that bear below the frost depth, so that seasonal freezing does not lift the structure. In central Ohio that means a thickened-edge or frost-wall detail rather than a floating slab poured on grade, with the footing depth set by the Residential Code of Ohio as enforced by the local building department [6][7]. The Residential Code of Ohio is built on the International Residential Code, which is the model code family the frost-depth and footing provisions come from [8]. Confirm the depth in force for the jurisdiction at design time rather than carrying a number over from a previous job.

Two related details matter on clay: drainage away from the slab edge, and a compacted, properly prepared base beneath it. Clay soils in this region shrink and swell seasonally, and a slab placed on unprepared clay with water pooling at the edge is the standard recipe for a cracked garage floor. Lot due diligence and site work covers the soil question in detail.

Setbacks, lot coverage, and the rear yard

Accessory structures are measured against three limits at once, and on a small lot at least one of them binds.

• Setbacks. The minimum distance from each property line. Accessory-structure setbacks are set by the zoning district in Title 33 of the Columbus City Code, and they are frequently different — often smaller — than the setbacks the principal building has to hold [2][3][4].
• Lot coverage. The share of the lot that all structures may occupy together. A detached garage counts toward that cap alongside the house, and patios, decks, and sheds usually do too. This is the limit that most often blocks a second structure on a small lot.
• Rear-yard coverage. A separate limit on how much of the back yard an accessory building may occupy, distinct from overall lot coverage.

Where a structure cannot be placed inside those limits, the route is a variance before the Board of Zoning Adjustment, with the same calendar and the same uncertainty any other variance carries [5]. Planning a build or addition covers that process.

Accessory dwelling units as the code stands

Columbus publishes an ADU handbook through Building and Zoning Services, which summarizes the city-specific requirements and states plainly that federal, state, and local law govern in the event of any omission or inconsistency in the guidebook [1]. What follows is that summary, and it should be read against the current code rather than treated as permanent.

The dimensional standards

• Floor area. An ADU may be 1,000 square feet or 65 percent of the net floor area of the principal building, whichever is greater, and it must be smaller than the principal dwelling [1].
• Height. 25 feet maximum [1].
• Side yard. The ADU must meet the minimum required side yard of the underlying zoning district [1].
• Lot coverage. The principal building plus the ADU may not exceed 65 percent of the lot [1].
• Rear-yard coverage. A detached ADU may occupy no more than 45 percent of the rear yard, or 55 percent where two ADUs are built [1].
• Parking. No additional parking is required for an ADU. Where the ADU does provide parking, a minimum of 20 feet of clearance is required for car maneuvering [1].
• Separation. A detached ADU must sit at least 10 feet from the principal building; placed closer, it requires fire-rated exterior walls and may be limited in windows and openings on the facing sides [1].

Where ADUs are allowed

ADUs are permitted in all residential (R) districts, all apartment residential (AR) districts, and all mixed-use (MU) districts. They are not allowed within a Planned Community District (PC) or a Planned Unit Development (PUD) [1]. How many ADUs a specific property may have is set by the zoning code rather than by a single citywide number, so it has to be read against the parcel’s district [1][2].

What makes a structure a dwelling unit

An ADU is an additional dwelling unit with a separate entrance, smaller than the principal building on the lot. It must contain a kitchen, living and sleeping areas, space for sanitation, utility connections, proper ventilation, and exit routes meeting life-safety requirements, and in some cases a sprinkler system. It must have permanent utility connections — tiny homes and mobile homes do not qualify [1]. That list is what separates an ADU from a finished garage loft or a guest room over a workshop, and it is where most of the construction cost difference lives.

Preliminary zoning review

The city recommends a preliminary zoning review before proceeding with an ADU project, specifically to determine whether a Board of Zoning Adjustment variance will be required. It takes a completed application, a detailed site plan, and a data table covering the parcel, lot area, proposed ADU living area, unit counts, stories and overall height, lot coverage, rear-yard coverage, ADU-to-principal-building percentage, required and provided parking, side-yard requirements, key dimensions, surface types, and tree requirements. There is an application fee — check the current Building and Zoning Services fee list — and city staff typically complete the review within about a week [1].

For anyone considering an ADU, this is the highest-value step in the whole sequence: it converts an open question into a documented answer before design fees are committed.

Which building code applies

ADUs are subject to the Residential Code of Ohio in the common configurations: detached ADUs behind single-unit houses or apartment buildings, attached ADUs associated with one-, two-, or three-unit dwellings, and internal ADUs within single-unit or two-unit dwellings. An ADU located over a garage will likely need fire separation [1][6]. Where the ADU is attached to or internal to a building containing four or more units, the Ohio Building Code applies instead, bringing increased fire-resistance requirements for stairs and walls, sprinklers in most situations, multiple means of egress and wider corridors, and applicable accessibility requirements [1][7].

The practical implication is that the same square footage costs materially more under the commercial code than under the residential one, and the trigger is the unit count of the building it attaches to — not the size or the design of the ADU itself.

Running utilities to a detached structure

A detached garage used for storage needs power and little else. A detached structure that people live or work in needs water, sanitary sewer, electric service, and conditioning, and each one is a trench across a yard.

The practical constraints are the distance from the house or the main to the new structure, the existing electrical service capacity — inadequate sewer, water, or electrical capacity is one of the permit issues the city names as common [1] — and the easements and buried lines already crossing the yard. Public utility lines are located free through Ohio’s 811 service; private lines are the owner’s responsibility. Where the yard carries a utility or drainage easement, the structure cannot sit on it regardless of what the setbacks allow.

Permits and inspections

A detached structure follows the same permit path as any other building work in the city: a building permit application with construction plans, a site plan, and contractor information, submitted through the city’s online portal, reviewed for zoning compliance, building-code compliance, and utility and service capacity [1][9]. Trade permits for electrical, plumbing, and mechanical work are pulled separately by registered contractors after the building permit issues, and each permit brings its own inspections through the build [9][10].

On cost: this site publishes no separate band for garages or ADUs, and a garage is commonly quoted separately from a house rather than inside the per-square-foot rate. A conditioned, plumbed, finished detached structure is priced like small-footprint new construction, with the same tier logic and a worse per-square-foot rate than a house, because every trade mobilizes for a fraction of the work. New construction cost explains that arithmetic.

Where to go next

• Planning a build or addition — zoning verification, the variance route, and the permit sequence.
• Lot due diligence and site work — easements, soils, drainage, and what the ground under a slab needs.
• New construction cost — the tier logic that a detached structure is priced against.
• Home addition cost — the attached alternative, and when it is the better answer.
• Build timeline and phasing — permit lead times and the inspection sequence on a small structure.
• The home addition cost page carries the setback and lot-coverage summary in short form.

On a second structure, the zoning envelope is the design brief. Everything else is a choice made inside it.