The headline number from Carbondale's April ADU vote is 1,000 square feet with two bedrooms. Most houses in town will never reach it. Under the town's Unified Development Code, an accessory dwelling unit can't be larger than 40 percent of the main house's gross floor area. Only a main house of about 2,500 square feet can support the full 1,000-square-foot unit. A 1,200-square-foot house in Old Town tops out near 480.
If you're comparing houses in Carbondale and a second unit is part of the plan, for family, a caregiver, a long-term tenant or a future rental, that ratio should guide the search more than the headline does. The amendment matters most in the older parts of town, and its size limit depends on the house you buy.
What the Trustees Changed on April 14
The Carbondale Board of Trustees approved the ordinance unanimously at its Tuesday, April 14, 2026 meeting, the Sopris Sun reported. The paper reported these changes:
- Review moved from a Planning & Zoning Commission minor site plan review to an administrative site plan review by town staff.
- ADUs became a permitted use in Old Town Residential, Residential/Low Density, Residential/Medium Density and agricultural zones. Before, they were special or conditional uses.
- The bedroom limit went from one to two.
- The size ceiling went from 850 to as much as 1,000 square feet.
- Studios and one-bedroom ADUs no longer need off-street parking. Two-bedroom units need one space.
The town's consolidated code, labeled "Amended May, 2026," lists the change as Ordinance No. 2, Series of 2026. Mayor Ben Bohmfalk described the goal to the Sopris Sun as streamlining "the process for adding accessory dwelling units to allow for housing as infill."
The section of the code those changes feed into, §4.4.4.A of the Unified Development Code, is what sets the size.
The 40 Percent Line
The code reads: "Square footage of ADUs shall not be more than 40 percent of the gross floor area of the Primary Structure, up to a maximum of 1,000 square feet."
Here's what that formula produces at common house sizes:
| Main house gross floor area | Largest ADU the 40% rule allows |
|---|---|
| 1,000 sq ft | 400 sq ft |
| 1,200 sq ft | 480 sq ft |
| 1,500 sq ft | 600 sq ft |
| 2,000 sq ft | 800 sq ft |
| 2,500 sq ft | 1,000 sq ft |
| 3,000 sq ft and up | 1,000 sq ft, the hard cap |
The parking rule interacts with this table. A studio or one-bedroom ADU needs no off-street space. A two-bedroom needs one. On a smaller house, the 40 percent limit already points toward a compact unit, and keeping it at one bedroom or less avoids the parking requirement. On a larger house, the full two-bedroom unit is available, along with the parking space it requires.
When you tour two houses with similar list prices, the square footage of the main house tells you how much second unit the lot can legally carry.
Which Houses Qualify at All
The code adds limits that remove many properties before the formula even applies:
- Only one ADU is allowed per property.
- The property must already have a single-family detached house.
- ADUs are prohibited for duplexes, multifamily dwellings and live/work units.
- An ADU must be attached to or inside the main house unless it meets the detached standards. Detached units go to the side or rear of the main house.
- Every ADU needs its own exterior entrance and its own cooking, sleeping and sanitary facilities.
Rule 3 matters for comparing market segments. In the Colorado Association of REALTORS® Local Market Update for August 2026, current as of September 3, Carbondale townhouse and condo sales ran 39 year-to-date through August at a $895,000 median, against 79 single-family sales at a $1,650,000 median. That's about a third of the town's closings through August in the townhouse and condo segment. The code allows ADUs only on lots with a single-family detached house and bars them for duplexes and multifamily dwellings, so buyers shopping that segment are mostly looking at homes this ordinance doesn't reach.
A single-family house is the starting point, and the zoning has to match too. The Sopris Sun printed a correction on its April story: the amended code does not permit ADUs in River Valley Ranch, Hendrick Ranch or Roaring Fork Village. Those neighborhoods' zoning would have to be amended first. River Valley Ranch's March 2026 board packet, which predates the vote, shows no discussion of ADU zoning, and no later proposal turned up in local reporting.
State law doesn't automatically change that. Colorado's HB24-1152 requires "subject jurisdictions" to allow one ADU administratively wherever single-unit detached homes are allowed, effective June 30, 2025. In subject jurisdictions it also bars PUD ordinances from restricting ADUs more than the rest of local law does. A subject municipality has at least 1,000 residents and sits inside a metropolitan planning organization. The Colorado Department of Local Affairs list of subject jurisdictions doesn't include Carbondale. Carbondale reached its own ADU rules through a local ordinance, and the boundaries those rules draw around the PUD neighborhoods are the ones that apply today.
This sorts the single-family inventory into two groups. The single-family figures in the August report were 15 sales at a $1,599,000 median, 116 days on market and 84 homes in inventory. The report doesn't split those numbers by zone, so it doesn't show how many of them sit in ADU-eligible districts. Before you assume a single-family listing can carry a second unit, find out which zone it's in.
Old Town Gets the Permission and a Second Look
Old Town Residential covers 16 downtown blocks south of Main Street, according to the Sopris Sun. It moved from special use to permitted use for ADUs, the biggest procedural jump in the ordinance. That night the trustees also expanded the Historic Downtown Area to include Old Town Residential, which brings a courtesy review to projects there.
The ADU section reflects this. Any ADU in the Old Town Residential zone, and any ADU on a property anywhere in town with a Structure of Merit, follows the Historic Preservation Design Guidelines and goes to the Historic Preservation Commission for a courtesy review.
How the work is done decides how much review it gets. An ADU built entirely inside an existing house, with no exterior changes except access stairs, skips site plan review but still needs a building permit. An ADU that changes the exterior, or stands detached, needs administrative site plan approval. In Old Town both kinds also get the historic courtesy review. Older Old Town houses also tend to be on the smaller side of the 40 percent table, so a converted basement or a portion of an existing floor can be a different project from a new backyard structure, both in review and in the size the formula allows.
What the Code Leaves Open
The ADU standards say a lot about size and placement and very little about use. The code text includes no owner-occupancy requirement, no long-term rental requirement and no deed restriction on the unit. Short-term rentals fall under Carbondale's existing rules. Under the Town's short-term rental FAQ, renting a whole property or part of one for less than 30 days requires a Short Term Rental License. Those stays carry a 6% short-term rental tax approved by voters in November 2022, a separate 2% lodging tax and the 3.5% sales tax on lodging. The town says revenue from the 6% tax goes primarily to affordable and attainable housing.
Trustees discussed this gap. Trustee Chris Hassig raised whether the amendment would lead to more short-term rentals, and the Sopris Sun reported Bohmfalk's answer: "We can watch, and say, 'Oh my gosh, we approved 10 ADUs, and they all become short term rentals, and that's not exactly what we were thinking.'" He said the town could analyze the amendment's effect on short-term rentals through annual data collection.
The other open item is cost. The code says the town won't provide separate water or sewer service to an ADU, though other utilities can be metered separately. The town's September 2026 building permit application lists these fee categories:
- A building permit fee, plus a plan review fee of 65% of the permit fee
- Use-tax lines at 3% and 0.5%
- An 8.4% water-sales-tax line
- Meter and backflow charges
- Water and wastewater system-impact fees
- Water dedication
- A development fee
The form leaves the dollar amounts blank, and the town's posted materials don't say which of these charges apply to an ADU that shares the main house's water and sewer. A buyer pricing a property around a future ADU can get those numbers from the Town of Carbondale before the inspection period ends. No published count of ADU applications filed since April was available either.
A Few Straight Answers
Can I buy in River Valley Ranch and add an ADU under the new rules? Not under this amendment. The Sopris Sun's correction says ADUs in River Valley Ranch, Hendrick Ranch and Roaring Fork Village would require amendments to those neighborhoods' zoning first.
Could I sell the ADU separately later? No. The code says ownership of an ADU can't be legally separated from the property and the main house, whether through subdivision, condominiumization, a ground lease or any other method.
How much ADU does a mid-size house allow? A 1,500-square-foot main house allows up to 600 square feet, and the unit still needs its own entrance, kitchen, sleeping area and bathroom.
If a second unit is part of your plan for a Carbondale home, we can check the zoning, measure the main house against the 40 percent rule and line up the fee questions for the Town before you write an offer. Schedule a free consultation with Aspen Valley REI, in English or en español.
This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.