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Accessory Dwelling Unit (ADU) Permit in Louisville, Colorado

Required for construction of an ADU on a single-family lot in Louisville, per Ordinance 2025-1892 (adopted May 20, 2025, effective June 23, 2025), enacted to comply with a 2024 Colorado state law mandating ADU allowance on all single-family lots by June 2025. Reviewed as a new residential building permit by the Building Safety Division; zoning compliance reviewed by the Planning Division.

Verified 2026-07-05 · Source

When you need this permit

  • One ADU (attached or detached) permitted per single-family unit, in all zone districts (including PUDs) that allow single-family detached or attached (rowhome/townhome) units
  • Maximum size: 1,000 sq ft detached / 800 sq ft attached, or up to 75% of the primary house's square footage (whichever is less); ADUs of at least 500 sq ft are always allowed if other requirements are met
  • Maximum height: same as primary house (attached) / 25 feet (detached)
  • Setbacks: same as primary house with 10-ft minimum rear setback (attached); standard accessory-structure setbacks (detached)
  • One off-street parking space required only if on-street parking is prohibited on the block AND no excess parking exists on the lot
  • Not permitted in the front yard, except in Old Town in an existing building
  • First 500 sq ft of an ADU is exempt from lot coverage/FAR calculations and from Development Impact Fee calculations
  • Historic-landmark, affordable-housing (deed-restricted per LMC Ch. 17.76), and accessible-dwelling-unit ADUs may qualify for a size increase to 1,000 sq ft (detached) / 1,200 sq ft (attached)
  • Building-code review currently uses the City's adopted 2024 ICC codes, including ADU-specific Appendix BC provisions
  • Existing non-permitted units built before May 20, 2025 may seek a voluntary Certificate of Exception (LMC Sec. 17.16.035.G) by December 31, 2026 — after January 1, 2027, any such unit without a Certificate of Exception or ADU permit is not permitted
  • Short-term rental of an ADU is prohibited by the Louisville Municipal Code; only monthly-or-longer leasing is allowed

Required documents

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Fee schedule

Building Permit Fee (same valuation-based table as new residential construction)Standard fee table applies; no ADU-specific fee variance published
$28.00 minimum, scaling per the valuation tiers in the Building Permit Fee Table
Development Impact FeesADUs, Rentals, Guest Houses page FAQ
First 500 sq ft exempt; if the ADU exceeds 500 sq ft, fees apply only if the added square footage pushes the property into a higher bracket under Appendix A, Ch. 3.16 LMC
Certificate of Exception fee (existing non-permitted units only, not new construction)Applies only to the voluntary legal-nonconforming pathway for pre-existing units, not new ADU permits
$200

Review timeline

Plan reviewLouisville’s published plan-review target
15–20 business days

How long did your Accessory Dwelling Unit (ADU) Permit permit actually take in Louisville?

Anonymous — one question, no email. Answers are combined and only shown once at least 5 people have reported.

Inspection process

  1. 1

    Foundation

    Before concrete pour (new detached ADU)

  2. 2

    Rough Frame / Rough Plumbing / Rough Electrical / Rough Mechanical

    Trade rough-ins before concealment

  3. 3

    Building Final

    All work complete; occupancy and utility-connection compliance confirmed

See the full Louisville inspection guide — scheduling & re-inspection fees →

Tips

  • Confirm your ADU's size against BOTH the flat cap (1,000 sq ft detached / 800 sq ft attached) AND the 75%-of-primary-house rule — whichever is lower governs, though a 500-sq-ft floor always applies.
  • The first 500 sq ft of any ADU escapes both lot-coverage/FAR limits and Development Impact Fee calculations — a meaningful cost break worth designing around.
  • If your ADU predates May 20, 2025 and was never permitted, the Certificate of Exception pathway (deadline December 31, 2026) is a materially different — and cheaper — process than a new-construction ADU permit; don't conflate the two.
  • Short-term rental of a Louisville ADU is prohibited citywide — only monthly-or-longer leases are allowed.

Frequently asked questions

Louisville requires an accessory dwelling unit (adu) permit for: One ADU (attached or detached) permitted per single-family unit, in all zone districts (including PUDs) that allow single-family detached or attached (rowhome/townhome) units; Maximum size: 1,000 sq ft detached / 800 sq ft attached, or up to 75% of the primary house's square footage (whichever is less); ADUs of at least 500 sq ft are always allowed if other requirements are met; Maximum height: same as primary house (attached) / 25 feet (detached); Setbacks: same as primary house with 10-ft minimum rear setback (attached); standard accessory-structure setbacks (detached); One off-street parking space required only if on-street parking is prohibited on the block AND no excess parking exists on the lot; Not permitted in the front yard, except in Old Town in an existing building; First 500 sq ft of an ADU is exempt from lot coverage/FAR calculations and from Development Impact Fee calculations; Historic-landmark, affordable-housing (deed-restricted per LMC Ch. 17.76), and accessible-dwelling-unit ADUs may qualify for a size increase to 1,000 sq ft (detached) / 1,200 sq ft (attached); Building-code review currently uses the City's adopted 2024 ICC codes, including ADU-specific Appendix BC provisions; Existing non-permitted units built before May 20, 2025 may seek a voluntary Certificate of Exception (LMC Sec. 17.16.035.G) by December 31, 2026 — after January 1, 2027, any such unit without a Certificate of Exception or ADU permit is not permitted; Short-term rental of an ADU is prohibited by the Louisville Municipal Code; only monthly-or-longer leasing is allowed. If your project isn't listed, confirm with City of Louisville Building Safety Division at (303) 335-4584 before starting work.

In Louisville, the published Building Permit Fee (same valuation-based table as new residential construction) is: $28.00 minimum, scaling per the valuation tiers in the Building Permit Fee Table. Additional published fees: Development Impact Fees — First 500 sq ft exempt; if the ADU exceeds 500 sq ft, fees apply only if the added square footage pushes the property into a higher bracket under Appendix A, Ch. 3.16 LMC; Certificate of Exception fee (existing non-permitted units only, not new construction) — $200. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Louisville's published plan-review target for an accessory dwelling unit (adu) permit is 15–20 business days.

You'll need: Building Permit Application (CSS Online Portal). Depending on your project, Louisville may also ask for: ADU Handout; Certificate of Exception Checklist. See the required-documents checklist on this page for details on each.

Louisville requires 3 inspection(s) for an accessory dwelling unit (adu) permit, in order: Foundation, Rough Frame / Rough Plumbing / Rough Electrical / Rough Mechanical, Building Final. Schedule each through City of Louisville Building Safety Division ((303) 335-4584).

Apply through City of Louisville Building Safety Division at City Hall, 749 Main St, Louisville, CO 80027. Phone: (303) 335-4584, email: building@louisvilleco.gov. Office hours: Monday-Thursday 8AM-4PM (in-person; remote 4PM-5PM); Friday 8AM-5PM remote/by appointment only; Tuesday 11AM-12PM limited staff availability (staff meeting). Inspection hours Monday-Friday 6:30AM-5PM.. Official information: https://www.louisvilleco.gov/local-government/government/departments/building-safety.

Up to 1,000 sq ft for a detached ADU or 800 sq ft for an attached ADU, capped at 75% of the primary house's square footage (whichever is less) — though an ADU of at least 500 sq ft is always allowed if other requirements are met. Historic-landmark, affordable-housing, or accessible-dwelling-unit ADUs may qualify for a size increase to 1,000/1,200 sq ft.

If it was built before May 20, 2025, you may apply for a voluntary Certificate of Exception (a $200 fee plus a Building Division safety inspection) by December 31, 2026 to obtain legal-nonconforming status. Starting January 1, 2027, any such unit without a Certificate of Exception or a proper ADU permit is not permitted.

Sources & verification

Verified against official sources. Last reviewed 2026-07-05.

Fees, timelines, and adopted codes are researched from each jurisdiction's published records — see how we verify. Requirements change and vary by project, so always confirm the current details with the Louisville building department before you submit. PermitBase is an independent reference and is not affiliated with any government agency.

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Accessory Dwelling Unit (ADU) Permit in other cities

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