Denver carries the Petroleum Building at 16th and Broadway on its 2026 roll as commercial office with an actual value of $6.8 million and an assessed value of $1.7 million, which is 25%. In November 2025 the owners won approval for a $14 million Denver Downtown Development Authority loan to turn twelve floors into 178 apartments. The day the county classifies that building as residential, the assessed value on the same $6.8 million drops to $479,400 for the school levy and about $458,000 for city, county and special district levies. Roughly three-quarters of the assessed value disappears. Multiply that across the 700 units planned at High Fidelity Plaza, the 386 proposed at the Denver Energy Center and the 120 at the University Building, and the classification date is worth more to those projects than any value argument they will make.
So when does the switch happen? The statute says the class holds until actual use changes. The assessor's manual says a change after January 1 waits for the next January 1. Neither says what a building mid-conversion is on that date, and the manual used to have a sentence on partially complete structures that it has since dropped. The cases give each side a reading, and none of them involved a conversion.
The rule Colorado does have: actual use on January 1, and a change waits a year
Colorado classifies property by what it is doing on the assessment date. The Division of Property Taxation's manual, which is binding on every county assessor, puts it in one sentence: "Property is classified according to its actual use on January 1." The statute locks the class in place after that. Under C.R.S. § 39-1-103(5)(c), "once any property is classified for property tax purposes, it shall remain so classified until such time as its actual use changes or the assessor discovers that the classification is erroneous."
The manual then answers the timing question for an ordinary change of use, and its own example is a conversion: "When the use of a property changes after January 1, the assessment date, the classification assigned to the property as of January 1 remains in place until the following January 1." The illustration is a house turned into a store in February; it stays residential for the whole year and becomes commercial the next January 1.
Run that in reverse and the first part of the answer is fixed. A conversion that starts in any month after January 1 cannot move the building off the office rate for that year, no matter how far the work gets. If the Petroleum Building is still an office on January 1, 2027 and the contractor mobilizes that month, as the owners have said they expect, the 2027 tax bill is an office bill. The earliest the class can change is January 1, 2028.
Where the "foundation" instinct comes from, and why it does not reach a conversion
Anyone who has worked Colorado new construction has heard that a project becomes residential once the foundation is in. That rule is real. The manual says a "completed structural foundation for a residential improvement must be in place on January 1" for a property to be classified residential, and traces the requirement to Vail Associates, Inc. v. Board of Assessment Appeals, 765 P.2d 593 (Colo. App. 1988), where platted, zoned lots with roads, sewer and ski-way access were held not to be residential because no dwelling unit stood on them.
It is a floor, not a trigger, and it is written for vacant land getting its first house: you need at least a foundation before the parcel can leave the vacant-land class. An office tower already has a foundation. The rule says nothing about what an existing nonresidential building becomes while it is being gutted, and the court opinion it cites never mentions a foundation at all. The foundation line is the Division's operating test for the constitution's dwelling-unit requirement, and it is binding on assessors, but it answers a different question than the one a conversion raises.
The sentence the Division deleted
The 2010 edition of the same manual chapter said this: "Structures (improvements) that are partially complete on January 1 are classified according to their intended use when completed." That is the sentence a conversion owner would build a case on. A gutted office with residential permits is partially complete, and its intended use when completed is apartments.
The current edition, dated September 2026, does not contain that sentence. The "Partially Completed Structures" section now consists of the constitutional dwelling-unit requirement, the Vail Associates summary and the foundation minimum, with a cross-reference to the valuation manual's guidelines for valuing partially completed structures. The phrases "intended use" and "when completed" do not appear anywhere in the chapter. I compared the 2010 edition and the current one; I have not seen the editions in between, and I could not find a change memo explaining when or why the sentence came out. The direct statement is gone. What remains in the current chapter is indirect, and it is the whole of the owner's administrative case: the manual lists "the use for which improvements were constructed or later modified" as evidence of actual use, and it directs that where "the actual use cannot be determined, the property should be classified according to its most probable use."
What the courts have said, and what they have not
The statutory definition is the one the fight will be about. "Residential improvements" means "a building, or that portion of a building, designed for use predominantly as a place of residency by a person, a family, or families" (C.R.S. § 39-1-102(14.3)). Two appellate readings of "designed for use" matter here.
The owner's reading comes from Mission Viejo Co. v. Douglas County Board of Equalization, 881 P.2d 462 (Colo. App. 1994). The court held that "designed" does not refer "only to the original, architectural design of the building" and that actual use is not the whole test: "designed for use" contemplates that a structure is "devoted" to or "intended" for a particular use at the time its status is under review. The same opinion notes that homes standing empty do not lose their residential classification just because nobody is living in them. The taxpayer lost that case; the court upheld reclassifying a mansion used as a community center from residential to commercial because its actual use had changed. But the language stands, and a building with approved residential plans, a residential building permit and the last office lease terminated is, on that reading, devoted to residency on the next January 1 even if no unit is finished.
The county's reading comes from the more recent cases, and from the constitution. Article X, section 3(1)(b) defines residential real property as "residential dwelling units and the land, as defined by law, on which such units are located." A shell with framed demising walls and no certificate of occupancy contains no dwelling unit on January 1, and the manual's "most probable use" fallback cannot manufacture one; that is the first argument a county will make. In HCPI/CO Springs Ltd. Partnership v. El Paso County Board of Commissioners, 2024 COA 82, the Court of Appeals reclassified a rehabilitation hospital out of the residential class and stated the method plainly: "To determine the proper classification, we start with the property's actual use." The Supreme Court in Mook v. Board of County Commissioners of Summit County, 2020 CO 12, read the present tense in the residential-land statute to mean assessors classify by how the owner "presently uses the land," not by future plans. And the manual's closest analogy, partially destroyed structures, keeps a damaged building in its previous class: they "are classified for the current year according to their previous use." An assessor who wants to hold a stripped office tower at 25% has language to stand on.
Neither line of cases involved a building under conversion. The closest decision I found is not precedent at all. In 2013 the Board of Assessment Appeals kept the converted half of a Denver duplex on Holly Street classified as commercial for tax year 2011. That half had been converted into a medical office in 2007, and the owner argued it had been vacant since before January 1, 2011. The Board credited the assessor's evidence that the medical tenants did not leave until later that year, noted that no change-of-use permit was filed until August 2012, and endorsed the assessor's testimony that property is valued and classified "as it stands January 1st of each year at noon." It added that "it is the responsibility of the taxpayer to notify the Assessor's Office of any change in use to the property each year." An empty space on the assessment date did not change the class. The documented 2007 conversion, and the absence of any permit showing a change back, kept it commercial.
Three positions an assessor can take on January 1, 2028
Put those pieces together for a tower that was an office on January 1, 2027, went to demolition that spring and is a shell with framed demising walls the following January. Nothing in the statute or the manual names the event that moves the class, so three answers are defensible, and what is in the assessor's file decides between them.
- It is still an office. Actual use governs, the building's last actual use was office, a partially destroyed structure keeps its previous class, and there is no dwelling unit in the building on the assessment date. This is the default outcome if nothing is filed and nobody asks.
- It is residential. The manual counts "the use for which improvements were constructed or later modified" as evidence of actual use, and where "the actual use cannot be determined, the property should be classified according to its most probable use." A shell that can no longer function as an office, under a residential permit, has residential as its only probable use. That is the Mission Viejo argument, and it has to be made in writing to the assessor before the assessment date, not discovered on a notice of value in May.
- It is both. If some floors are occupied by residential tenants while others are still under construction, C.R.S. § 39-1-103(9) and Manor Vail Condominium Ass'n v. Board of Equalization of Eagle County, 956 P.2d 654 (Colo. App. 1998), require the building to be apportioned, with each portion classified by its own use and the land allocated in proportion to the value of each portion. Phased conversions should expect this, and should plan the phasing around the calendar.
The class also decides the valuation method. Colorado's constitution limits residential property to the market and cost approaches, and the statute narrows that to the market approach alone. Once the building is residential, the county can no longer value it on income, so the vacancy-adjusted income argument the owner has been making against the office value goes away on the same January 1 that the rate drops.
What to do if you own one
- Write to the assessor before the January 1 that matters. Send the residential building permit, the approved site development plan, the demolition permit and the date the last office lease ended, and ask in writing how the property will be classified as of the coming assessment date. The manual counts "correspondence with the owner" as evidence of actual use. If nothing is in the file on January 1, expect the building to stay an office; its last documented use is the only evidence the assessor has.
- Protest the classification, not only the value. The two are separate questions on a Colorado petition. The value of a gutted shell is low on any approach: Denver carries the improvements on the 418,846-square-foot tower at 621 17th Street, one of the two High Fidelity Plaza buildings, at $1,000. But a low value at 25% is still a bill roughly three and a half to three and three-quarters times the size of the same value at the residential rates.
- Use the intervening year. 2028 is not a reappraisal year, but C.R.S. § 39-1-104(11)(b) lets the assessor revalue for "the addition to or remodeling of a structure" and "a change of use of the land." A conversion is both. Do not assume the 2027 value or class carries into 2028 untouched.
- Sequence the occupancy if you can. A temporary certificate of occupancy and signed leases on any floor before January 1 settle the question for that floor.
Other states wrote the rule down. Colorado has not.
Arizona's statute is the rule Colorado owners imagine they have. A.R.S. § 42-12051 classifies "partially completed or vacant improvements" by "their intended use as demonstrated by objective evidence" and deems an improvement partially completed "when the foundation of the structure or structures to be located on the property is in place." Washington, D.C. moved in the other direction in 2025. Its tax office had adopted a policy that a commercial building could not become residential until construction was "100% complete" and the building "in actual use." The Council overrode it, and a building can now be reclassified as Class 1A residential, on the owner's application, once a permit issues to substantially rehabilitate it for exclusively residential use, with a clawback plus penalty and interest if the building is not in predominantly residential use by the earlier of a new certificate of occupancy or three years from the permit, or if the permit lapses. That rule currently rides on temporary acts while the permanent version waits for funding, but the Council's choice of trigger is the point: the permit, not the last coat of paint. Utah, New York City and Cook County each settled the question too, in three more ways, by statute or written policy; the citations are in the note below.
Colorado's legislature has not touched the question. The only conversion bill I found in the last five sessions, HB24-1125, was an income tax credit for conversion costs, and it died in House Appropriations in May 2024. I found nothing in the 2025 or 2026 sessions that addresses classification of property under construction. The statute that governs is the one from the first section: actual use, January 1, and a change that waits for the next January 1.
Why it matters now
CBRE put downtown Denver office vacancy at 38.6% in the second quarter of 2026, off a record 38.9% the quarter before, and noted that the conversion pipeline is what is pulling obsolete space out of inventory. The Denver Downtown Development Authority has approved conversion loans for the Petroleum Building, the University Building and High Fidelity Plaza and has a request in front of it for the Denver Energy Center. Between them that is more than 1,300 apartments in buildings that are, today, on the roll as offices at 25%. Each of them will pass through at least one January 1 as a shell. Whether the county classifies those shells by what they were or by what they are becoming is a question I found no Colorado decision on, and the difference on a single tower is a seven-figure assessed value. Somebody is going to take it to the state board. The owners who get the residential rate will be the ones who sent the assessor the permits, the plans and the lease termination before January 1. Waiting for the May notice of value and protesting then is the wrong order.