An accessory dwelling unit, or portion thereof, that is a designated landmark in accordance with LMC Chapter 15.36 – Historic Preservation may be up to 1,000 square feet in size for detached ADUs and up to 1,200 square feet in size for attached ADUs, provided all other requirements are met.
ADU, Rentals, and guest house requirements
Accessory dwelling units, or ADUs, are a form of secondary, independent housing that share a lot with a single-family residence. ADUs can be internal, attached to the main residence, or detached and take various forms (see above graphic). In May 2024, a new state law passed that requires the City of Louisville to allow ADUs on all single family lots in the city by June 2025.
At the May 20th, 2025 City Council meeting, Council adopted an ADU ordinance which will go into effect starting June 23, 2025. Below contains zoning requirements for constructing new ADUs in Louisville. For existing units that may qualify as ADUs, please see the "Existing Unit Information" section below.
| Topic | Requirements | |
| Attached ADUs | Detached ADUs | |
| Location |
Permitted in all zone districts, including PUDs, that allow single family detached or attached (rowhomes, townhomes) units. |
|
| Maximum Number of ADUs | One ADU, either attached or detached is permitted per single family unit. | |
| Maximum Size* | 1,000 square feet. |
800 square feet. |
| ADUs cannot exceed 75% of the square footage of the primary house, or the maximum allowed square footage, whichever is less. However, ADUs at least 500 square feet in size are allowed in all cases, provided all other requirements are met. | ||
| Maximum Height | Same as primary house. | 25 feet. |
| Setbacks | Same as primary house, except the minimum rear setback is 10 feet. | Accessory structure setbacks apply. |
| Lot Coverage and FAR | ADUs are subject to minimum lot coverage and floor area ratio (FAR) requirements for the overall lot, as applicable. However, the first 500 square feet of an ADU does not count towards lot coverage and FAR requirements. | |
| Minimum Parking | One off-street parking space shall be required for an ADU only if on-street parking is prohibited on the subject block and there are no excess parking spaces available on the subject lot. | |
| Tap Fees | Separate ADU wastewater fees may apply. Please contact Public Works for more information. | |
| Placement on Lot | Same as primary house. | Not permitted in the front yard, except in Old Town in an existing building. |
* Attached ADUs located in basements shall not count towards the maximum size limitations. Certain ADUs may be eligible for a size increase (see below).
Historic Preservation Incentives
Accessory dwelling units that are deed restricted as affordable housing in accordance with LMC Chapter 17.76 may be up to 1,000 square feet in size for detached ADUs and up to 1,200 square feet in size for attached ADUs, provided all other requirements are met.
Accessory dwelling units that meet the definition of an accessible dwelling unit according to the City’s adopted building codes may be up to 1,000 square feet in size for detached ADUs and up to 1,200 square feet in size for attached ADUs, provided all other requirements are met.
What are the fees for a new ADU?
A new ADU needs a building permit like any new residential unit would. Please review the City’s permit fee schedule. New residential units are also subject to the City’s Development Impact Fees in Louisville Municipal Code Ch. 3.16. The ADU ordinance exempts the first 500 square feet of an ADU from any Development Impact Fee calculations. If the ADU is greater than 500 square feet, Development Impact Fees are only assessed if the increase in square footage puts the subject property into another square footage “bracket” according to Appendix A: Impact Fee Schedule in Ch. 3.16.
For water and wastewater tap fee information on new ADUs, consult with the City’s Public Works Department.
What building codes will the City use to review new ADUs?
The City is currently under the building codes summarized here, including the 2018 International Code Council (ICC) codes. The City is planning to adopt the 2024 ICC codes as soon as late 2025 that include an accessory dwelling unit appendix. Until that time, the City will be evaluating new ADUs using the minimum safety requirements for ADUs in the 2024 ICC, Appendix BC in addition to the other applicable requirements in the 2018 ICC codes.
Existing Unit Information
The City recognizes that some units that may meet the definition of an ADU have been constructed prior to the adoption of the ADU ordinance. The ADU ordinance allows a pathway for those units to be considered legal. A property owner may apply for a “Certificate of Exception” by December 31, 2026 to obtain “legal nonconforming status”, which means that the unit can legally function as an ADU – specifically a separate, independent living unit (e.g., rented out). More information on this process is below.
What is a Certificate of Exception?
A Certificate of Exception is documented approval by the City of Louisville that the existing unit is considered a legal nonconforming ADU. If any such unit was in existence prior to May 20, 2025, and it meets the definition of an ADU as a separate, independent living unit (e.g., rented out), it will continue to be illegal without a Certificate of Exception.
What is the Certificate of Exception Process?
The Certificate of Exception process is voluntary. It includes an inspection by the City’s Building Division to ensure the unit meets minimum safety requirements. Here are the recommended steps:
Step 1 – Evaluate the Certificate of Exception Checklist and Process: Review the Certificate of Exception Checklist. This is what the City’s Building Inspectors will use to evaluate the unit to ensure that it meets minimum safety requirements. The City may require building permits on any of these items to address safety issues before issuing a Certificate of Exception. The City’s Building Inspectors are also available for questions and consultations prior to deciding whether to proceed. We recommend you contact them at building@louisvilleco.gov or 303-335-4584 to set something up so you can understand what your options are.
Step 2: Submit a Certificate of Exception Permit Application at the City’s Self Service Permit Portal: This includes a $200 fee and three main items – the Certificate of Exception signed checklist (form below), a site plan showing the location of the ADU, and a floor plan showing the dimensions of the rooms in the ADU. This request will be reviewed for completeness and the floor plan and site plan will be evaluated by the City’s Planning Division to see if it meets the ADU regulations. Note that existing units that do not fully comply with all the ADU zoning regulations (setbacks, height, size, location) may still get a Certificate of Exception – the priority is that the unit meets minimum safety requirements. If you have questions about this process reach out to building@louisvilleco.gov or 303-335-4584.
Step 3: Certificate of Exception Request Accepted, Request Inspection: Once you’ve received notice that the permit submittal has been reviewed and approved by staff and the permit has been paid, request an inspection from the City’s Building Division and they will coordinate with you to set up an inspection of the unit to evaluate the items listed in Certificate of Exception Checklist.
Step 4: Certificate of Exception Request Approved: If the City’s Building Inspectors deem the unit safe as a separate, independent dwelling unit based on the Certificate of Exception Checklist, the request is approved in the building permit portal. This approval acts as the Certificate of Exception in compliance with Louisville Municipal Code Sec. 17.16.035.G. If the unit is deemed unsafe, then the inspectors will notify you of what items need to be corrected, which may require a separate building permit for such items.
What if my existing unit doesn't fully meet all the new ADU zoning standards?
If the existing unit doesn’t comply with all the standards in the City’s zoning code in Sec. 17.16.035, it can still get a Certificate of Exception. Zoning standards in question may include but are not limited setbacks from property lines, building heights, location of the ADU on the lot, and size of the ADU. The focus of this process is to ensure that the unit is safe, which is related to building code requirements reflected in the Certificate of Exception Checklist. If you’re not sure about whether the existing unit meets the current ADU zoning requirements, we recommend you consult with the City’s Planning Division before submitting the formal Certificate of Exception request at planning@louisvilleco.gov or 303-335-4592. Please note that the existing unit must have been built before May 20, 2025 to be eligible for a Certificate of Exception.
What are the benefits of applying for a Certificate of Exception?
Receiving a Certificate of Exception is a voluntary process. The Certificate of Exception allows for existing units that meet the definition of an ADU built before May 20, 2025 to be legally recognized and allow for continued operation of the unit as a separate, independent dwelling unit (e.g., rented out). Starting January 1, 2027, any such unit without a Certificate of Exception or ADUs is not permitted.
Short-Term Rentals
The Louisville Municipal Code prohibits short-term rental of dwelling units, including ADUs. Properties are only allowed to be rented for a monthly or longer leasing period per the City’s definition of “dwelling unit” (Section 17.08.130).
Guest Houses and ADUs
The City’s zoning code also allows “guest houses” that are defined as an accessory structure which is physically detached from a single-family dwelling unit, does not contain cooking facilities, is serviced through the same utility meters or connections as the principal use, and is intended for occupancy only by guests of the family residing in the single-family dwelling. Guest houses continue to be allowed in all residential zone districts.
The distinction between a guest house and a detached ADU is commonly that the ADU has a full kitchen (e.g., stove), a separate address, and may be occupied as an independent dwelling unit from the primary house. Guest houses are subject to the same requirements as any other type of accessory structure relative to size, setbacks, height, etc. whereas detached ADUs have separate regulations from guest houses.