Thornton ADU rules
✓ Verified against code · August 19, 2026
Current rules under Development Code §18-339 (reenacted Chapter 18), adopted by Ordinance 3742, in effect since September 1, 2025. Thornton reenacted its entire Development Code effective September 1, 2025 — mostly statute-shaped, with one glaring exception: it kept an ongoing owner-occupancy requirement enforced by a recorded deed restriction, which Colorado's ADU statute has barred for cities like Thornton since June 30, 2025. The state's October 2025 compliance review listed Thornton among the non-complying jurisdictions.
Common questions, answered from the code
Does Thornton still require me to live on the property?
The code says yes — ongoing owner-occupancy of either unit, enforced by a recorded deed restriction — but Colorado's ADU statute has barred that requirement for cities like Thornton since June 30, 2025, and the state listed Thornton as non-compliant in October 2025. State law controls on paper; in practice, expect the city to apply its code until it's amended. Get the city's position in writing and know the statute exists.
§18-339(l), (n)(1)(c); C.R.S. 29-35-403(2)(b) · Permalink
How big can a Thornton ADU be?
Generous: the greater of 1,000 sq ft or half the home's gross floor area, with garages and porches excluded from the math — one of the roomier allowances on the Front Range.
§18-339(k) · Permalink
How tall can a detached Thornton ADU be?
Genuinely ambiguous in the current tables: 35 ft clearly covers ADUs above garages, while other accessory structures cap at 16 ft — and a ground-level detached ADU sits between the definitions. Ask the city for a written determination before designing; we track this on our watchlist.
Table 18-76.1 fn 7 · Permalink
Do I need parking for a Thornton ADU?
The code requires one space per ADU on the lot. Note the statute limits when a city can demand a *new* space — if you already have a driveway spot to designate, raise C.R.S. 29-35-403(3) at the counter.
Table 18-650.1 · Permalink
The rules at a glance
| ADUs allowed per lot | One ADU per lot — interior, attached, or detached — accessory to a single-unit detached dwelling, permitted in the RE, RL, RM, RH, AG, and SFD-L districts.Dev. Code §18-339(b),(c),(e); Table 18-334.1 |
| Maximum size | The greater of 1,000 sq ft or 50% of the home's gross floor area — garages, porches, and similar areas excluded from the calculation. Also exempt from the 5% accessory-use area cap.§18-339(k)(1)–(3) |
| Detached ADU height | Same standards as the principal dwelling 'unless otherwise noted' — and the district tables note a split (e.g., RL: 35 ft for the house and for ADUs above a garage, 16 ft for other accessory structures), leaving ground-level detached ADU height ambiguous between the two numbers. Get the city's reading in writing.§18-339(f); Table 18-76.1 fn 7 |
| Detached ADU setbacks | Same as the principal dwelling unless noted. RL example: interior side 5 ft; rear 15 ft principal / 5 ft accessory on front-loaded lots; alley-loaded 10/0 ft (20 ft for an alley garage with driveway).§18-339(f); Table 18-76.1 |
| Off-street parking | One space per ADU, on the same lot and shown on the development-plan site plan. The statute only allows requiring a new space in a narrow three-condition case; how Thornton applies this is untested.Table 18-650.1; §18-339(h); contrast C.R.S. 29-35-403(3) |
| Owner-occupancy | Still required, ongoing: the owner of record must occupy either the house or the ADU, and a recorded deed restriction ties the ADU's certificate of occupancy to continued owner occupancy. This directly conflicts with C.R.S. 29-35-403(2)(b), which bars ongoing owner-occupancy requirements for subject jurisdictions — Thornton is one, and the state listed it as non-compliant in October 2025.§18-339(l), (n)(1)(c); DOLA Oct 2025 determination |
| Lot coverage / floor area | All accessory buildings combined, ADU included, must stay within the district's maximum lot coverage (RL 85%, RE 45%).§18-336(b)(2); Tables 18-75.1/76.1 |
| Permit path & timeline | Administrative development plan (director-approved) plus building permit; the ADU's certificate of occupancy requires the home's CO first, and water/sewer capacity verification is required. Small or non-visible projects (≤500 sq ft, or not visible from the right-of-way) skip the development plan. No published timeline.§18-339(g)–(j); §18-41(b) |
Worth knowing
- Maximum one kitchen in the ADU and one in the principal home — an unusual anti-duplex provision.
- Separate exterior entrance and 'Unit 1 / Unit 2' addressing required.
- Short-term renting the ADU is allowed only if the principal home is the owner's primary residence, and not while any part of the principal home is an STR.
- Home occupations are now expressly allowed inside ADUs.
- The reenacted code is otherwise statute-shaped — the owner-occupancy holdover in subsections (l) and (n) is the specific conflict.
On our watchlist
No corrective ordinance for the owner-occupancy conflict found through August 2026 — the only 2026 Chapter 18 amendment was an unrelated affordable-housing fast-track. Top watchlist item.
The ground-level detached-ADU height ambiguity (16 vs. 35 ft) awaits a city determination.
Check your parcel
The rules above are citywide — what you can build depends on your lot’s zone, size, and setbacks. Look yours up on the official viewers:
Primary sources
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Not legal advice. Every fact above cites the code section it comes from — check the cite, or ask your city’s planning department to confirm your specific lot. Spotted an error or a new ordinance? See about to reach us; corrections are logged in the changelog.