Thornton vs. Westminster ADU rules
Thornton and Westminster both adopted their ADU rules in 2025, but only one passed state review. Westminster's rules (WMC 11-4-19), effective January 2025, drop owner-occupancy entirely, require a new parking space only in the statute's narrow case, and let an ADU be short-term rented with a license. Thornton's 2025 reenactment (§18-339(l), (n)(1)(c)) still ties the ADU's certificate of occupancy to an owner living in either unit under a recorded deed restriction, mandates one parking space per ADU on the site plan, and only allows short-term renting the ADU if the owner occupies the main house — conflicts that got Thornton listed as non-compliant with state law in October 2025.
✓ Verified against code · August 19, 2026
Thornton rules run under Development Code §18-339 (reenacted Chapter 18), adopted by Ordinance 3742, in effect since September 1, 2025.
✓ Verified against code · August 19, 2026
Westminster rules run under WMC 11-4-19 (Westminster Municipal Code), adopted by Ordinance 4272 (amended by Ord 4335), in effect since January 1, 2025.
Rule by rule
| Rule | Thornton | Westminster |
|---|---|---|
| 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 | One ADU per lot — interior (basement/attic), attached, or detached — with any single-family detached home, in any zone where such homes are permitted.WMC 11-4-19(B)–(D) |
| 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) | The lesser of 1,200 sq ft or 50% of the home's gross floor area, minimum 190 sq ft, maximum two bedrooms — and where the home is under 1,500 sq ft, the ADU may still be up to 750 sq ft (codified March 2026 by Ordinance 4335).WMC 11-4-19(H); Ord 4335 |
| 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 | Same maximum height as the principal dwelling on the lot.WMC 11-4-19(G) |
| 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 | Front and side: same as the principal dwelling (per the applicable PDP/ODP). Rear: the greater of the district's accessory-building setback or 5 ft. No building over easements.WMC 11-4-19(F) |
| 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) | None in most cases — the home keeps its own minimum. One new space only if no existing off-street spot could serve the ADU AND on-street parking is prohibited on the block. Statute-verbatim.WMC 11-4-19(I) |
| 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 | None — the city's own FAQ confirms you may live in the ADU and rent out the main house.WMC 11-4-19 (full section); city ADU guide FAQ |
| 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 | The combined footprint of the home and ADU must stay within the principal dwelling's maximum lot coverage; where PUD documents are silent, no maximum applies.WMC 11-4-19(E) |
| 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) | Administrative only — staff-approved building permit via the eTRAKiT portal, no public hearing, no ODP amendment, and no HOA approval required. Published staff review timeline: up to 4 weeks.WMC 11-4-19(K)–(N); city ADU guide |
Thornton: 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.
Thornton: 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.
Westminster: worth knowing
- No impact fees — no park, school, or land-dedication charges on ADUs, unusual for the region.
- Architectural match: 3 of 7 listed elements must match the main house, with a simplified single-criterion variance path.
- Short-term rental of an ADU is allowed with a license.
- Permanent foundation required — no tiny homes on wheels or RVs; engineered plans required.
- The March 2026 small-home fix came at the state's insistence — an example of the compliance-review process actually working.
Westminster: on our watchlist
A citywide Unified Development Code rewrite is in drafting — ADU provisions could move or change on adoption; we re-check each pass.
Full detail: Thornton · Westminster · back to the Colorado hub. 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.