How to Adjust Comps Like an Appraiser: A Practical Guide

Why Adjustments Matter More Than Comp Selection

Finding comparable sales is only the first step in a credible valuation. No two properties are identical, which means every comp requires adjustments to account for the differences between it and the subject property. The accuracy of these adjustments is what separates a reliable value opinion from a guess.

Professional appraisers spend a significant portion of their analysis on adjustments. Under USPAP, each adjustment must be market-supported — meaning there is data showing that buyers in the local market actually pay more or less for the feature in question. This guide walks through the adjustment process the way it is done in practice.

The Basic Principle

Adjustments are always made to the comparable sale, not to the subject property. The logic works like this: if a comp has something the subject does not, you subtract value from the comp. If the subject has something the comp does not, you add value to the comp. The goal is to answer the question: what would this comp have sold for if it were identical to the subject?

Common Adjustment Categories

Gross Living Area (GLA)

Square footage differences are among the most common and impactful adjustments. The adjustment is typically expressed as a dollar-per-square-foot figure derived from the local market. For example, if paired sales analysis shows that buyers pay approximately $150 per additional square foot in a given neighborhood, and a comp is 200 square feet larger than the subject, you would subtract $30,000 from the comp's sale price.

It is important to note that the dollar-per-square-foot adjustment is not the same as the overall price per square foot of the property. The adjustment reflects the marginal value of additional space, which is almost always lower than the average price per square foot.

Lot Size

In markets where land contributes meaningfully to value — particularly suburban and rural areas — lot size differences require adjustment. In dense urban markets like parts of New York City, lot size adjustments may be minimal because the building itself drives value more than the land underneath it.

Condition and Quality

Condition adjustments account for differences in the physical state of the property — updated kitchens and bathrooms, new roofs, modern mechanicals, or deferred maintenance. Quality adjustments reflect differences in materials and construction grade. These adjustments require judgment and market knowledge, as there is no universal formula.

Garage and Parking

In many suburban markets, the presence or absence of a garage is a significant value factor. The adjustment should reflect what buyers actually pay for garage space in the local market, which varies substantially by area.

Basement

Basements are adjusted based on whether they are finished or unfinished and how much usable space they provide. Finished basement space is typically valued at a lower rate per square foot than above-grade living area because it lacks natural light and may have lower ceiling heights.

Time (Market Conditions)

If a comp sold several months ago and the market has appreciated or depreciated since then, a time adjustment accounts for the difference. Appraisers calculate time adjustments by analyzing price trends in the local market over the relevant period.

How to Derive Adjustments

The most common methods appraisers use to derive adjustment amounts include:

• Paired sales analysis: Compare two sales that are nearly identical except for one feature. The difference in sale price indicates the market value of that feature.
• Regression analysis: Statistical modeling that isolates the contribution of individual property characteristics to sale price across a larger dataset.
• Cost approach cross-check: Use the depreciated cost of a feature (like a garage or finished basement) as a reasonableness check against market-derived adjustments.
• Market interviews: Conversations with local agents and buyers about how much specific features influenced their purchasing decisions.

Avoiding Over-Adjustment

One of the most common errors in comp adjustments is applying too many or too large adjustments to a comparable that was not a good match in the first place. A general rule of thumb in appraisal practice is that net adjustments (the total of all adjustments accounting for positive and negative values) should not exceed 15 percent of the comp's sale price, and gross adjustments (the sum of all absolute adjustment values) should not exceed 25 percent. Comps that require adjustments beyond these thresholds are usually not strong comparables.

How Comp Pro Handles Adjustments

Comp Pro generates suggested adjustments for each comp based on local market data and appraisal-informed logic. The platform calculates adjustment amounts for GLA, lot size, condition, garage, basement, and time, and presents them alongside the raw comp data so you can review, modify, or override any suggestion. This approach saves time while keeping the professional in control of the final analysis.