The age-life method of depreciation calculates a building’s accrued depreciation by dividing its effective age by its total economic life, then multiplying that ratio by the replacement cost new of the improvements. A structure with a 20-year effective age and a 50-year economic life carries a depreciation ratio of 0.40, or 40 percent. If replacement cost new is $500,000, the dollar depreciation is $200,000 and the depreciated improvement value is $300,000. Add the separately estimated site value, and you have the property’s total value under the cost approach.
The formula is easy. The inputs are where the work happens, and small differences in how effective age or economic life is set can swing the result by tens of thousands of dollars.
Setting the Effective Age
Effective age is not the year on the building permit. It’s a judgment about how old the building looks and functions relative to its condition and remaining utility. A 1985 house with a full kitchen remodel, new HVAC, and a new roof completed in 2020 may carry an effective age of 10 or 15 years even though it is chronologically 40. A 20-year-old building with water damage, an aging roof, and neglected mechanical systems can have an effective age that exceeds its actual years.
Appraisers set the figure through a physical walk-through and a review of records. Major systems get compared against expected lifespans: architectural asphalt shingles run about 30 years, central air conditioning 7 to 15 years, and conventional water heaters 6 to 12 years. When several major components are nearing the end of their useful lives at the same time, effective age rises sharply. When those systems have been recently replaced, it drops.
Documentation supports the estimate. Invoices for a $60,000 modernization pull the effective age down; a building with no maintenance records and a failing roof that would cost $15,000 to replace pushes it up, because a buyer would price that liability into any offer. Under the Uniform Standards of Professional Appraisal Practice, the effective age figure has to be backed by observable evidence and documented reasoning, not simply anchored to the construction date.1Assessorcasc.org. USPAP Record Keeping
Setting Total Economic Life
Economic life is the span during which the improvements contribute more value to the property than the cost of maintaining them. It ends when keeping the building standing costs more than it adds to the land’s worth, whether or not the structure is still physically intact.
Most appraisers start with published cost service data. Marshall Valuation Service, now part of CoreLogic, publishes life expectancy tables organized by construction class and occupancy type. A Class A steel-frame office building in good condition typically carries a 60-year economic life. An average-quality wood-frame single-family home comes in around 55 years. A light industrial manufacturing building of average quality might sit at 35 to 50 years depending on construction class.
Occupancy type matters as much as construction quality. A warehouse and a medical clinic built with identical materials will have different economic lives because their mechanical systems, tenant improvement cycles, and functional demands differ. Picking the wrong occupancy category from a cost manual is an easy error to make and a hard one to catch in review.
Market Extraction as a Cross-Check
Published tables are a starting point. Local market conditions can override them, and market extraction is the standard cross-check. The appraiser identifies sales of comparable improved properties, subtracts the estimated land value from each sale price, and compares the remaining improvement value against replacement cost new. The gap reveals how much depreciation the market is actually imposing on similar buildings in that area.2International Right of Way Association. Extracting Depreciation
In a steel mill town where industrial demand has collapsed, extracted economic lives may run far shorter than a national manual suggests. In a booming market, older structures may retain useful economic life well beyond textbook expectations.
Running the Calculation
Once effective age, economic life, and replacement cost new are set, the arithmetic is one step. For a single-family home with an effective age of 20 years, an economic life of 50 years, and a replacement cost new of $500,000:
- Depreciation ratio: 20 ÷ 50 = 0.40, or 40 percent
- Dollar depreciation: $500,000 × 0.40 = $200,000
- Depreciated improvement value: $500,000 − $200,000 = $300,000
If the land is separately valued at $120,000, the total property value is $420,000.
The 40 percent figure is meant to capture all forms of value loss the building has experienced, bundled together. That single-ratio simplicity is the method’s strength for standard residential work and mass appraisal, and it’s also the source of the method’s biggest limitations.
The Modified Age-Life Method
When a building has specific, identifiable problems that are economically worth fixing, those curable items get subtracted first as dollar amounts before the age-life ratio is applied to what remains. Order of operations matters, and getting it backward produces a wrong answer.
The correct sequence is: start with replacement cost new, subtract curable items in dollars, then apply the age-life percentage to the remainder. Consider a building with a $1,500,000 replacement cost new that needs a $100,000 roof replacement (a curable item), an effective age of 20 years, and a 50-year economic life:
- Replacement cost new: $1,500,000
- Less curable deterioration (roof): −$100,000
- Subtotal: $1,400,000
- Age-life depreciation at 40 percent: −$560,000
- Depreciated improvement value: $840,000
Reversing the order, applying the 40 percent first and then subtracting the roof, gives $800,000. The $40,000 gap is not rounding error. It reflects that the curable item’s full cost belongs off the top, before the incurable deterioration percentage is applied to the remaining building systems.
Replacement Cost New vs. Reproduction Cost New
The formula needs a cost figure to depreciate, and there are two versions. Replacement cost new is the cost to build a structure with equivalent utility using modern materials, current techniques, and current codes. Reproduction cost new is the cost to build an exact replica of the existing structure, including original materials, design quirks, and features that would be considered outdated today.
The choice changes how much depreciation the appraiser then has to measure. A replacement cost estimate typically runs lower, because modern construction is more efficient and the hypothetical replacement wouldn’t include superadequate or obsolete design features. Less functional obsolescence has to be separately deducted. A reproduction cost estimate preserves every original feature, so the appraiser must measure and deduct any functional obsolescence caused by outdated design elements that a modern equivalent wouldn’t have.
For most residential and standard commercial appraisals, replacement cost is the more practical starting point. Reproduction cost becomes necessary for historic structures, insurance policies that require exact restoration, or legal contexts where measuring every form of depreciation separately matters.
What the Single Ratio Is Actually Measuring
The age-life percentage bundles three distinct sources of depreciation into one number. Knowing what’s inside it helps explain why two buildings with identical construction costs can carry very different depreciation estimates, and when the standard method needs to be modified.
Physical deterioration is wear and tear: aging roofs, cracked foundations, corroding pipes, worn mechanical systems. It splits into curable items, where repair cost is less than or equal to the value the repair adds back, and incurable items, where repair cost exceeds the value restored. Replacing a $15,000 roof that adds $20,000 in value is curable. Foundation settling that would cost $80,000 to fix but add $30,000 is incurable.
Functional obsolescence is value loss from outdated design rather than physical wear. The building works, but its layout or systems no longer match what the market expects: a walk-through bedroom, a kitchen walled off from the living area, inadequate electrical capacity for modern tenants. Functional obsolescence can be curable (removing a wall to open a floor plan) or incurable (a structural layout that can’t be changed without rebuilding).
External obsolescence comes from forces outside the property line: a highway rerouted away from a commercial strip, a factory closure that collapses local demand, a rezoning that limits potential use. It is almost always incurable. External factors affect land values first, and if the existing structure is still the property’s highest and best use, external influences are captured in the land value estimate rather than deducted again from the building.3Appraisal Institute. Land Values and External Obsolescence Double-counting external obsolescence, once in the land value and again in the building depreciation, is a common error.
Where the Straight-Line Assumption Breaks Down
The age-life method assumes a building loses value at a constant rate over its entire economic life. Year one and year forty produce the same percentage of depreciation. Buildings don’t behave that way. A well-maintained structure may lose little functional value in its first two decades, then face accelerating costs as major systems reach the end of their useful lives at roughly the same time.
The effective age input is supposed to compensate for this, but it’s a blunt instrument. Assigning an effective age of 25 to a 35-year-old building with original mechanical systems is a judgment call about how much deferred maintenance should accelerate the depreciation curve. Reasonable appraisers can disagree on that number by five or ten years, and each year changes the final valuation by thousands of dollars.
For properties where depreciation clearly doesn’t follow a straight line, the breakdown method, which estimates depreciation separately for each building component, often produces a more accurate result. The tradeoff is time and cost. The age-life method can be completed in a fraction of the time, which is why it remains the default for standard residential work and mass appraisal by tax assessors.
This Is Not Tax Depreciation
Appraisal depreciation and the depreciation schedule on a tax return solve different problems. The age-life method attempts to measure actual loss in market value. Tax depreciation is a cost recovery mechanism that lets property owners deduct improvement costs over a fixed schedule, regardless of whether the building is actually losing value. Under the Modified Accelerated Cost Recovery System, residential rental property is written off over 27.5 years and nonresidential real property over 39.4Internal Revenue Service. Publication 946, How To Depreciate Property Those periods have nothing to do with a building’s economic life under the age-life method. A rental property fully depreciated for tax purposes can still appraise for hundreds of thousands of dollars, and that gap is expected.