Quick Answer
Illinois recognizes three kinds of depreciation: physical wear, functional obsolescence (bad design), and economic obsolescence (an outside nuisance). Mass appraisal subtracts normal age only. It misses functional and economic obsolescence unless someone documents it, so that loss can inflate your assessment.
Here is the part the assessor's computer does not tell you: the State's own assessment methodology says your property can lose value for three different reasons, but the mass-appraisal model that set your number only subtracts for one of them automatically. The other two — the awkward floor plan, the highway noise, the parcel sitting next to a nuisance — only come off your value if someone puts them on paper. Usually, nobody does. So the loss stays buried, and your assessed value sits higher than your home would actually sell for.
That is not a conspiracy. It is just how mass appraisal works. And once you understand it, you can document the depreciation the model missed and bring it to a Board of Review hearing — the exact evidence the State's own checklist invites you to bring.
The villain: a model that only sees age, not your actual house
Start with how your number gets made. The Illinois Department of Revenue (IDOR) trains assessors to estimate market value using the cost approach, and the formula is plain:
**Market Value = Land Value + (Replacement Cost New − Depreciation)**
The assessor estimates what it would cost to rebuild your home today (Replacement Cost New), then subtracts **depreciation** — which IDOR defines as "the loss of value from all causes." That last phrase is doing a lot of work, because the State recognizes that value can erode in three distinct ways.
Per IDOR's assessment-training manuals, the three types of depreciation are:
- **Physical depreciation** — loss in value due to deterioration: "wear and tear, time, and the action of the elements." The roof, the windows, the furnace getting old.
- **Functional obsolescence** — obsolescence "resulting from conditions within the property, such as an imbalance in construction features or inadequate design or arrangement that lessen its usefulness or utility."
- **Economic obsolescence** (also called external obsolescence) — obsolescence "caused by influences outside the property, such as physical, economic, social, and governmental changes that have an adverse effect on the stability and quality of the neighborhood in general."
Here is the catch. Your assessment was not set by an appraiser who walked your block and studied your floor plan. IDOR is explicit: "the assessor must estimate values within a relatively short period of time. The assessor is a mass appraiser." Mass appraisal values thousands of parcels using standard cost schedules, and it leans on **normal age depreciation** — a table that says a structure of a given age, in typical condition, has lost a typical amount of value.
That table catches physical aging. It does **not**, on its own, see that your kitchen is in the basement, or that a trucking depot opened across the street last year. Those losses are real under the State's own definitions — but they only get subtracted if someone documents them. The model's silence is the over-assessment quietly inflating your bill.
Functional obsolescence: when the house itself fights its own value
Functional obsolescence is the loss baked into the design. The State's manuals split it into two flavors, **curable** and **incurable**, and the rule is precise: depreciation is "curable when the cost to cure will add to the market value of the structure," and "incurable when the cost to cure is greater than the increase in the market value."
IDOR's own residential and commercial examples:
- **Curable functional obsolescence** — "lack of air conditioning, lack of proper electrical wiring, low-hanging pipes, and absence of proper ventilation."
- **Incurable functional obsolescence** — "extremely poor floor plan, very low or high ceilings." For income property, IDOR adds "inadequate column spacing in a warehouse, multi-story construction in older industrial buildings, and undesirable shape or location of a commercial structure on the site."
Think about what an incurable design flaw does to a buyer. A bedroom you can only reach by walking through another bedroom. A single bathroom on the wrong floor. Ceilings so low the space feels like a basement. A "very low or high" ceiling, in the State's words. Buyers discount for these. The market discounts for these. But the cost schedule that built your Replacement Cost New assumed a conventional, efficient layout — so unless the loss is documented, the model never subtracts it.
That is a method-grounded reason to review your assessment, straight from the doctrine that produced it.
Economic obsolescence: the value-killer outside your walls
This is the one homeowners most often miss, because it has nothing to do with their house at all. Economic (external) obsolescence is value lost to "influences outside the property." You did nothing wrong. The world changed around you.
IDOR's examples — and the manual labels these "usually incurable," meaning you cannot fix them at any sensible cost:
- **Location** — "change in traffic patterns and noise and air pollution," and detrimental property in the immediate area.
- **Economic** — "high interest rates and business closings."
- **Government** — "zoning changes, poor services, and high tax rate."
A new road widening that put four lanes of traffic at your front step. A commercial nuisance that moved in next door. A shuttered anchor that hollowed out the block. Each of these drags down what a willing buyer would pay — and **market value is exactly what your assessment is supposed to track.** IDOR defines it as "the most probable sale price... in a competitive and open market." If an outside nuisance suppresses that sale price, it suppresses the value your assessment should reflect. The model just cannot see it from a desk.
The State's secret decoder ring: the CDU rating
Here is the mechanism that proves all of this is real, not theory. When IDOR's depreciation table moves beyond pure age, it uses a **CDU rating** — Condition, Desirability, Utility — that the assessor assigns by comparing your property to others nearby. And the manual maps each letter directly onto the three depreciation types:
- **C — Condition** "refers to physical depreciation, such as wear and tear and the action of the elements."
- **D — Desirability** "refers to economic or external depreciation, such as lack of appeal due to location or some type of adverse influences outside the boundary lines of the property."
- **U — Utility** "refers to functional obsolescence, such as an inefficient and impractical arrangement of rooms and any super-adequacy or inadequacy that may be present."
Read that again. The State built a slot — **D** for desirability, **U** for utility — specifically to capture the highway noise and the bad floor plan. The CDU rating feeds an **effective age**, which IDOR defines as an age "based on the improvement's CDU rating" that "does not always equal actual age." A property with a worse CDU carries a higher effective age, which means more depreciation and a lower value.
So the doctrine is on your side. The only question is whether anyone actually rated your **D** and **U** to reflect your real situation — or whether the mass-appraisal model just defaulted to "average" and moved on to the next of its thousands of parcels. If it defaulted, the depreciation you are owed never came off.
How to document the depreciation the model missed
This is the best part. You do not have to argue the State's methodology — you can use the State's own evidence checklist. IDOR's list of "Evidence Needed" to support a claim of unfair assessment includes, word for word:
"a photograph of elements detracting from the value of the property not shown on the [property record card] and an estimate, in terms of dollars, of their negative effect on the market value."
That sentence is an open invitation. The assessor is literally asking you to photograph the obsolescence and put a dollar figure on it. So build the record:
- **Pull your Property Record Card (PRC).** IDOR confirms "taxpayers have the right to inspect property record cards and other assessment records." Check what the assessor actually recorded — and what they missed. Is there any notation for your floor plan flaw or the nuisance next door? Usually not.
- **Photograph the detractor.** The low ceiling. The bedroom-through-a-bedroom layout. The four-lane road at your lot line. The commercial nuisance across the street. These are your "elements detracting from the value."
- **Estimate the dollar effect on market value.** This is the step that wins. For curable functional issues, the cost to cure can anchor the number. For incurable design flaws and external nuisances, comparable sales tell the story — homes with the same problem sell for less, and the gap is your evidence.
- **Frame it as the depreciation the model never applied.** Your house is not just old. It carries functional and/or economic obsolescence that mass appraisal's normal-age table did not subtract. That is a defined loss of value under IDOR's own three-type framework.
IDOR is clear that the Board of Review exists for exactly this: on a written complaint "that any property is over assessed," the board "shall review the assessment, and correct it, as appears to be just." A documented obsolescence claim is precisely the kind of substance the board is built to weigh.
Where Censum fits
Before you spend a weekend pulling records, it is worth knowing where your assessment actually stands relative to the State's method. Censum is independent property-tax intelligence — we read the assessor's own data and the State's own methodology and show you whether there is a method-grounded reason to review, including signals that the model may have applied only normal age depreciation to a property that carries more.
We are not the county, not a law firm, and not your counsel of record — and a flat fee, never a slice of any result. When the evidence supports an appeal, you file the complaint, and for entity-owned property (an LLC, corporation, trust, or condo association) you generally need to work with an attorney to appear before the Cook County Board of Review; confirm the board's current representation rules. For your own home you have more latitude, and a clean, documented obsolescence packet is what gives any appeal its teeth.
The depreciation is real. The doctrine recognizes it. The only thing standing between you and a fair number is documentation the model never gathered.
FAQ
What is the difference between functional and economic obsolescence?
Functional obsolescence comes from conditions **inside** the property — IDOR defines it as "inadequate design or arrangement that lessen its usefulness or utility," like an extremely poor floor plan, very low or high ceilings, or missing air conditioning. Economic (external) obsolescence comes from "influences outside the property," such as a change in traffic patterns, noise and air pollution, business closings, or zoning changes. One is your house; the other is the world around it. Both are recognized losses of value under the State's assessment methodology.
Does Cook County's mass appraisal automatically account for obsolescence?
Mass appraisal applies **normal age depreciation** using standard cost schedules — that catches typical physical aging. IDOR is explicit that the assessor "is a mass appraiser" valuing thousands of parcels in a short window. Functional and economic obsolescence are not subtracted automatically; per the State's method they come off your value when documented, often through the assessor's CDU (Condition, Desirability, Utility) rating. If your Desirability and Utility were never rated to reflect a real flaw or nuisance, that loss may not be in your number.
What is curable versus incurable depreciation?
IDOR draws the line by economics. Depreciation is **curable** "when the cost to cure will add to the market value of the structure," and **incurable** "when the cost to cure is greater than the increase in the market value." Lack of air conditioning is a curable functional issue; an "extremely poor floor plan" is incurable. External nuisances — a highway, a neighboring nuisance, a zoning change — are described by the State as "usually incurable," because you cannot fix what is outside your lot lines at any reasonable cost.
How do I document obsolescence for a property tax appeal?
Use the State's own evidence checklist. IDOR lists, as supporting evidence, "a photograph of elements detracting from the value of the property not shown on the property record card and an estimate, in terms of dollars, of their negative effect on the market value." So pull your Property Record Card, photograph the detractor (the design flaw or the outside nuisance), and put a dollar figure on its effect on market value — anchored by cost-to-cure for curable issues or by comparable sales for incurable ones. That packet is what you bring to the Board of Review.
What is a CDU rating and why does it matter for my home's value?
CDU stands for Condition, Desirability, and Utility — a rating the assessor assigns to set how much depreciation your property carries. The State's methodology maps each letter to a depreciation type: Condition is physical depreciation, **Desirability is economic/external depreciation** (lack of appeal due to location or adverse outside influences), and **Utility is functional obsolescence** (impractical room arrangement, super-adequacy or inadequacy). CDU drives your "effective age," which can differ from your actual age and determines remaining economic life. A CDU that ignores a real flaw or nuisance can leave your assessment too high.
Can a nearby nuisance really lower my assessment?
Under the State's definitions, yes — that is exactly what economic obsolescence is. Your assessment is meant to track market value, which IDOR defines as "the most probable sale price" in an open market. If an outside influence — IDOR cites changed traffic patterns, noise and air pollution, or detrimental property in the immediate area — suppresses what a buyer would pay, it suppresses the value your assessment should reflect. The loss is recognized; the work is documenting it with photographs and a dollar estimate, then bringing it to the Board of Review.