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Residential Addition / Remodel Permit in Thornton, Colorado

Required for room additions, basement finishes, structural alterations, and remodels to existing homes in Thornton. Reviewed against the 2024 IRC with Thornton amendments; uses the same valuation-based fee table as new construction.

Verified 2026-06-30 · Source

When you need this permit

  • Room additions, basement finishes, structural modifications, and attached/detached garages require a building permit
  • Remodel/renovation/alteration project valuation is calculated as labor + material cost, or material cost x2, whichever method the applicant provides support for
  • Plans must show scope, dimensions, structural connection to the existing structure, and compliance with the amended Table R301.2 design criteria where structural work is involved
  • For Accessory Dwelling Units (ADUs) created via basement/garage/attic finish (Integrated ADU), contact Planning before building permit submittal (303-538-7295) — additional zoning requirements apply
  • Applications submitted via the CityView Portal

Required documents

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Fee schedule

Building Permit Fee — valuation-based (same table as new construction)City of Thornton 2026 Building Permit Fee Schedule; remodel/renovation/alteration valuation = labor + material cost, or material cost x2
Varies by valuation — $30.00 minimum up to $5,615.25 + $3.75/$1,000 over $1,000,000
Plan Review Fee
65% of the Building Permit Fee
Additional Plan Review (changes, additions, or revisions to plans)
$75.00 or actual hourly cost to the jurisdiction, whichever is greater
Mechanical, Electrical & Plumbing trade permit feesApplies when a remodel/addition includes plumbing, mechanical, and/or electrical work — each trade charged separately based on that trade's valuation
$30.00 for the first $1,000 of trade valuation, plus $10.00 for each additional $1,000 or fraction thereof, per trade

Review timeline

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

How long did your Residential Addition / Remodel 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 (if new footings)

    Before concrete pour, below the 36-inch amended frost line

  2. 2

    Framing

    After framing complete, before insulation or wall cover

  3. 3

    MEP Rough-In

    Electrical, plumbing, mechanical rough-in before walls closed

  4. 4

    Final

    All work complete per approved plans

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

Tips

  • Thornton calculates remodel/renovation/alteration valuation as labor + material cost, or material cost x2 — different from the ICC-table method used for new construction.
  • Basement, garage, or attic finishes that create an Integrated ADU trigger additional zoning review — contact the Planner of the Day (303-538-7295) before applying for the building permit.
  • Additional plan review triggered by applicant-requested changes is billed at $75.00 (or actual hourly cost if greater).

Frequently asked questions

Thornton requires a residential addition / remodel permit for: Room additions, basement finishes, structural modifications, and attached/detached garages require a building permit; Remodel/renovation/alteration project valuation is calculated as labor + material cost, or material cost x2, whichever method the applicant provides support for; Plans must show scope, dimensions, structural connection to the existing structure, and compliance with the amended Table R301.2 design criteria where structural work is involved; For Accessory Dwelling Units (ADUs) created via basement/garage/attic finish (Integrated ADU), contact Planning before building permit submittal (303-538-7295) — additional zoning requirements apply; Applications submitted via the CityView Portal. If your project isn't listed, confirm with City of Thornton — City Development, Building Inspection Division at 303-538-7250 before starting work.

In Thornton, the published Building Permit Fee — valuation-based (same table as new construction) is: Varies by valuation — $30.00 minimum up to $5,615.25 + $3.75/$1,000 over $1,000,000. Additional published fees: Plan Review Fee — 65% of the Building Permit Fee; Additional Plan Review (changes, additions, or revisions to plans) — $75.00 or actual hourly cost to the jurisdiction, whichever is greater; Mechanical, Electrical & Plumbing trade permit fees — $30.00 for the first $1,000 of trade valuation, plus $10.00 for each additional $1,000 or fraction thereof, per trade. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Plan review for a residential addition / remodel 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); Site Plan; Construction Drawings. See the required-documents checklist on this page for details on each.

Thornton requires 4 inspection(s) for a residential addition / remodel permit, in order: Foundation (if new footings), 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.

For remodel, renovation, or alteration projects, Thornton calculates valuation by adding labor and material cost, or by doubling the material cost — as opposed to new construction, which may use ICC square-footage valuation tables.

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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