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
- Req
Building Permit Application (CityView Portal)
Submitted after Planning approval of the ADU
- Opt
Accessory Dwelling Unit Handbook
City of Thornton's ADU type definitions, requirements, deed restriction template, and Plumbing Data Worksheet
- Opt
Development Plan
Required for Detached ADUs over 500 sq ft visible from the right-of-way, and Attached ADUs (additions) over 500 sq ft; approved by City Development before the Building Permit
- Req
Deed Restriction
Recorded with the Adams County Clerk and Recorder's Office, with the property's legal description attached, prior to Building Permit issuance
- Opt
Plumbing Data Worksheet
2024 IPC water fixture unit calculation, submitted with the Building Permit application if adding fixtures to the existing water service
Fee schedule
| Fee type | Amount | Notes |
|---|---|---|
| Building Permit Fee — valuation-based (same table as new construction) | Varies by valuation — see Residential Building Permit fee tiers (2026 Fee Schedule) | 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 |
| 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 | ADU Handbook confirms additional utility connection fees apply beyond the shared water/sewer connection, but does not publish a specific dollar amount |
Review timeline
~1–13 business days
Typical estimate — confirm current times with the Thornton building department
Inspection process
- 1
Foundation
Footings and foundation before concrete pour, below the 36-inch amended frost line
- 2
Framing
Structural framing, egress windows, fire separation (if attached)
- 3
MEP Rough-In
Mechanical, electrical, plumbing rough-ins before walls closed
- 4
Final
All work complete, ready for Certificate of Occupancy
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
- How big can an ADU be in Thornton?
- 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.
- Do I need to record anything before getting an ADU building permit in Thornton?
- 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.
Sources & verification
Verified against official sources. Last reviewed 2026-06-30.
- City of Thornton — City Development, Building Inspection Division — official building department
- City of Thornton — Accessory Dwelling Unit Handbook (PDF)
- City of Thornton 2026 Building Permit Fee Schedule (PDF)
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.
Other permits in Thornton
Related guides
Get the AI compliance checklist
Upload your project scope, get a jurisdiction-specific checklist of everything you need. Coming soon.
We only use your email to send permit updates. No spam, unsubscribe anytime. Privacy.