How to Choose an ADU Contractor in Colorado (Questions to Ask)
How to hire an ADU contractor in Colorado, the questions to ask, the red flags to avoid, how licensing works here, and modular versus site-built compared.
Firestone’s real rules, taxes and rents — read from the codified ordinance, every figure sourced and dated. Here: a HUD benchmark rent of $1,238/mo for a 1-bed. Run your own numbers below, free.
Read from the Firestone Municipal Code itself — the operative legal text, not a website summary. Firestone is an HB24-1152 subject jurisdiction: since June 30, 2025, state law requires it to allow an ADU wherever it allows a single-family home, with administrative approval — the local standards below still apply.
| Maximum ADU size | 900 sq ft or 33% of the main home's habitable floor area, whichever is less; at least 500 sq ftverified October 2026Firestone Municipal Code §16.3.3.G.1.dthe ordinance textNo accessory dwelling unit shall exceed thirty-three (33) percent of the size of the habitable floor area of the principal dwelling unit, or nine hundred (900) square feet, whichever is less. In addition, accessory dwelling units shall be no less than five hundred (500) square feet. |
| Owner occupancy | No — Firestone's ADU standards contain no owner-occupancy requirementverified October 2026Firestone Municipal Code §16.3.3.G.1the ordinance textSec. 16.3.3.G.1, the full ADU section, contains no owner-occupancy requirement; the code defines a dwelling unit as one "for owner occupancy or for rental or lease on a monthly or longer basis." |
| Short-term rental | No — ADUs are a household-living use, which requires stays of a month or longerverified October 2026Firestone Municipal Code §16.11.3the ordinance textSec. 16.11.3, "Household living": "Tenancy is arranged on a month-to-month or longer basis (lodging where tenancy may be arranged for a period of less than thirty (30) days is classified under the 'public accommodation' category). ... Home occupations and accessory dwelling units are accessory uses..." |
| Extra parking | None when added to an existing house whose street frontage has on-street parking; 1 space if the abutting street has no on-street parking or the ADU is built at the same time as the houseverified October 2026Firestone Municipal Code §16.3.3.G.1.dthe ordinance text(A) On-site parking is not required if an accessory dwelling unit is built on a site with an existing single-family detached house that has on-street parking on the abutting street frontage. (B) One (1) additional parking space is required when: (1) The street abutting the site does not have on-street parking; or (2) The accessory dwelling unit is built at the same time as the single-family detached house. |
Source: Firestone Municipal Code, §16.3.3 · ordinance text retrieved 2026-08-25, re-verified October 2026 · open “the ordinance text” under any figure to read the exact sentence it comes from.
Firestone’s size cap is conditional rather than a single number: 900 sq ft or 33% of the main home's habitable floor area, whichever is less; at least 500 sq ft — worth reading closely before sizing a unit.
Short-term rental of the ADU is not permitted, so the model here is long-term tenancy or family use.
Parking: None when added to an existing house whose street frontage has on-street parking; 1 space if the abutting street has no on-street parking or the ADU is built at the same time as the house.
Property tax runs heavier than most of the state here: 104.824 mills total in the example tax area (Weld County tax area 2556), about 17% above the median of the Colorado tax areas we track (levies vary by parcel).
What an ADU adds to the tax bill in Firestone, and the federal benchmark for what it rents for.
Levies: Weld County Assessor, 2025 Mill Levy Report (Tax District and Authority Report) — example: Weld County tax area 2556; levies differ between tax areas inside Firestone; check your parcel with the county assessor. Rates: Colorado DOLA, TY2026 — the 10% reduction on the first $700,000 of value applies to the local-government portion, once per property. Rents: HUD Fair Market Rents, huduser.gov.
Two ways to build the same unit in Firestone. Both figures are all-in — design, permits, foundation, utility connections and installation.
Published Front Range ranges for a comparable one-off build, 2025–26
≈ $2,100–$2,600 / month fully financed
In Firestone, the 490 is below Firestone's 500 sq ft minimum ADU size
≈ $2,800–$3,500 / month fully financed
In Firestone, the 735 needs a main home with at least 2,228 sq ft of habitable floor area (the 33% rule)
Olerra ranges: published all-in project pricing, September 2026 — site preparation, foundation, utility connections and permitting included.
Free and specific to you. Want to go further?
Yes. The Firestone Municipal Code permits accessory dwelling units (§16.3.3), subject to the size, height, setback and use rules quoted above from the ordinance text. Firestone is also an HB24-1152 subject jurisdiction, so since June 30, 2025 state law requires it to allow an ADU wherever it allows a single-family home, with administrative approval; the local standards above still apply. Zoning district, lot size and setbacks still decide what fits on a specific lot — the free property check confirms yours.
Yes. An ADU is a new dwelling unit, so Firestone requires a building permit (plus any zoning or site-plan review the code calls for) before construction starts. Olerra's all-in price includes permitting, and factory-built units are inspected to the same building code as a site-built home.
Per the Firestone Municipal Code: 900 sq ft or 33% of the main home's habitable floor area, whichever is less; at least 500 sq ft. The Olerra Flex Flat 490 is below Firestone's 500 sq ft minimum ADU size; the 735 needs a main home with at least 2,228 sq ft of habitable floor area (the 33% rule). The free property check confirms what fits on your lot.
No — ADUs are a household-living use, which requires stays of a month or longer. Plan on long-term tenancy or family use.
Per the ordinance: No — Firestone's ADU standards contain no owner-occupancy requirement.
A one-off site build with a general contractor typically runs about $500,000–$650,000 for a ~490 sq ft unit. Olerra's factory-built Flex Flats run $300,000–$365,000 (490) and $420,000–$465,000 (735) all-in — site preparation, foundation, utility connections and permitting included.
Roughly $727 per year for every $100,000 of value the ADU adds, in the example tax area shown above: Colorado's TY2026 school (7.05%) and local-government (6.80%) assessment rates on the latest certified (TY2025) levy, a mixed-year estimate. Levies differ between tax areas, so check your parcel with the county assessor.
Same method, each city’s own ordinance. The Colorado hub lists every city, and the statewide ADU calculator covers any Colorado address.
Guides and real Colorado numbers from our team — a fresh pick every couple of weeks, or search for exactly what you need.
How to hire an ADU contractor in Colorado, the questions to ask, the red flags to avoid, how licensing works here, and modular versus site-built compared.
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Planning departments and journalists are welcome to reference this data.
Olerra ADU Cost Calculator — Firestone, CO. Page updated October 9, 2026; rules verified October 2026. https://www.olerra.com/adu-calculator/firestone/ <a href="https://www.olerra.com/adu-calculator/firestone/">Firestone ADU rules & costs</a> Page updated October 9, 2026. The oldest dated figure on the page is from August 25, 2026.
Planning estimates, not quotes or legal advice. Rules, fees, levies and rents change — confirm with the city and your lender before committing. Rules data: codified ordinance as of August 25, 2026, re-verified October 2026. Olerra pricing: published all-in project ranges, September 2026. Corrections: sales@olerra.com.
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