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Accessory Dwelling Unit (ADU) Permit in Thornton, Colorado

Required for construction of a Detached, Attached, or Integrated ADU on a residential property in Thornton, per Thornton City Code Section 18-339. ADUs are allowed on most lots zoned RE, RL, RM, RH, AG, or SFD-L that permit a Dwelling, 1-Unit Detached, unless the primary dwelling already has two kitchens.

Verified 2026-06-30 · Source

When you need this permit

  • Property must be zoned RE, RL, RM, RH, AG, or SFD-L with an existing or planned primary use of Dwelling, 1-Unit Detached
  • Primary dwelling must have only one kitchen (properties with an ADU may have only one full kitchen in the principal dwelling)
  • At least 3 parking spaces required on the property, with 1 space in a garage
  • ADU size limited to no more than 50% of the primary dwelling size or 1,000 sq ft, whichever is GREATER (garage, porch, and similar areas excluded from the calculation)
  • ADU requires a separate exterior access and its own address
  • Detached ADUs must be set back 5 feet from side and rear property lines, with a maximum height of 16 feet (different standards apply in the RE zone)
  • Detached ADUs over 500 sq ft and visible from the right-of-way require an approved Development Plan from City Development; under 500 sq ft or not visible from the right-of-way, no Development Plan required
  • Attached ADUs (building additions) over 500 sq ft also require a Development Plan; attached ADUs must meet the primary use's zone-district setbacks/height and cannot have an interior entry to the primary dwelling
  • Property owner's primary residence must be in either the primary dwelling or the ADU
  • A deed restriction (template provided in the City's ADU Handbook) must be recorded with the Adams County Clerk and Recorder's Office prior to Building Permit issuance
  • ADU utilizes the same water/sewer connections as the primary dwelling, but additional utility connection fees apply (contact Utility Billing, 303-538-7370)

Required documents

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Fee schedule

Building Permit Fee — valuation-based (same table as new construction)Thornton's 2026 Fee Schedule does not publish an ADU-specific fee variance; ADUs use the same valuation-based Building Permit fee table as other residential construction
Varies by valuation — see Residential Building Permit fee tiers (2026 Fee Schedule)
Plan Review Fee
65% of the Building Permit Fee
Utility Connection FeeADU Handbook confirms additional utility connection fees apply beyond the shared water/sewer connection, but does not publish a specific dollar amount
Not published as a fixed dollar figure in sources reviewed — contact Utility Billing (utilbilling@thorntonco.gov, 303-538-7370) for current ADU utility connection fee amount

Review timeline

Plan reviewTypical estimate — confirm current times with the Thornton building department
~1–13 business days

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

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

Inspection process

  1. 1

    Foundation

    Footings and foundation before concrete pour, below the 36-inch amended frost line

  2. 2

    Framing

    Structural framing, egress windows, fire separation (if attached)

  3. 3

    MEP Rough-In

    Mechanical, electrical, plumbing rough-ins before walls closed

  4. 4

    Final

    All work complete, ready for Certificate of Occupancy

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

Tips

  • Thornton's ADU size cap is whichever is GREATER of 50% of the primary dwelling's gross floor area or 1,000 sq ft — this is more generous than a flat cap, and excludes garage/porch area from the calculation.
  • A recorded deed restriction with the Adams County Clerk and Recorder is mandatory before the Building Permit is issued — budget time for this recording step, distinct from the permit process itself.
  • Detached ADUs under 500 sq ft (or not visible from the right-of-way) skip the Development Plan step entirely and go straight to Building Permit; larger or visible units need Planning's Development Plan approval first.
  • Contact the Planner of the Day (303-538-7295 or POD.info@ThorntonCO.gov) early to confirm zoning eligibility before designing your ADU.

Frequently asked questions

Yes. Thornton requires a deed restriction, using the template in the City's ADU Handbook, to be recorded with the Adams County Clerk and Recorder's Office (with the property's legal description attached) before a Building Permit for the ADU can be issued.

In Thornton, the published Building Permit Fee — valuation-based (same table as new construction) is: Varies by valuation — see Residential Building Permit fee tiers (2026 Fee Schedule). Additional published fees: Plan Review Fee — 65% of the Building Permit Fee; Utility Connection Fee — Not published as a fixed dollar figure in sources reviewed — contact Utility Billing (utilbilling@thorntonco.gov, 303-538-7370) for current ADU utility connection fee amount. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Plan review for an accessory dwelling unit (adu) permit in Thornton typically runs 1–13 business days. This is an estimate, not a published figure — confirm current review times with City of Thornton — City Development, Building Inspection Division at 303-538-7250.

You'll need: Building Permit Application (CityView Portal); Deed Restriction. Depending on your project, Thornton may also ask for: Accessory Dwelling Unit Handbook; Development Plan; Plumbing Data Worksheet. See the required-documents checklist on this page for details on each.

Thornton requires 4 inspection(s) for an accessory dwelling unit (adu) permit, in order: Foundation, Framing, MEP Rough-In, Final. Schedule each through City of Thornton — City Development, Building Inspection Division (303-538-7250).

Apply through City of Thornton — City Development, Building Inspection Division at 9500 Civic Center Drive, Thornton, CO 80229. Phone: 303-538-7250, email: buildings@thorntonco.gov. Office hours: Mon–Fri, 8:00 a.m.–5:00 p.m. (counter payments 8:00 a.m.–4:30 p.m.). Official information: https://www.thorntonco.gov/business-development/city-development/building-permits-inspections.

Thornton, Colorado has adopted: 2024 International Building Code (IBC) — with Thornton amendments; adopted by ordinance May 13, 2025, effective July 1, 2025 (replaced the 2021 IBC); 2024 International Residential Code (IRC) — with Thornton amendments (Table R301.2 climatic/geographic design criteria repealed and reenacted with Thornton-specific values); effective July 1, 2025; 2024 International Plumbing Code (IPC) — with Thornton amendments; effective July 1, 2025; 2024 International Mechanical Code (IMC) — with Thornton amendments; effective July 1, 2025; 2024 International Fuel Gas Code (IFGC) — with Thornton amendments; effective July 1, 2025; 2024 International Energy Conservation Code (IECC) — adopted in full (residential provisions); includes Colorado Model Electric Ready and Solar Ready Code per HB22-1362; effective July 1, 2025; 2024 International Existing Building Code (IEBC) — with Thornton amendments; effective July 1, 2025; 2024 International Swimming Pool and Spa Code (ISPSC) — adopted by reference; effective July 1, 2025; 2023 National Electrical Code (NEC) / NFPA 70 — Thornton adopts the NEC edition currently adopted by the State of Colorado automatically and simultaneously (Thornton City Code Sec. 10-162); matches the statewide electrical code; 2021 editions of the above I-Codes and the 2017 NEC — the prior code cycle, superseded for new submittals as of July 1, 2025 but referenced for projects permitted under it (e.g., the city's 2023-dated Solar PV submittal-requirements handout still cites 2020 NEC/2021 IRC/2021 IBC/2021 IFC and should be read as superseded by the current 2024/2023 code cycle for design-code citations). Local amendments apply — see the Thornton overview page for the full list.

An ADU in Thornton may be up to 50% of the primary dwelling's size or 1,000 sq ft, whichever is greater. Garage, porch, and similar areas are excluded from this size calculation.

Sources & verification

Verified against official sources. Last reviewed 2026-06-30.

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 Thornton 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

Arvada, ColoradoAurora, ColoradoBoulder, ColoradoColorado Springs, ColoradoCommerce City, ColoradoCortez, ColoradoDenver, ColoradoFort Collins, Colorado

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