Comparable sales—commonly called "comps"—are arm's-length transactions involving properties that resemble a subject property closely enough to support a value estimate. They form the empirical foundation of the sales comparison approach, which is the most widely applied valuation method in residential real estate. Understanding how comps are selected, filtered, and adjusted is essential for anyone involved in pricing, lending, or investment analysis.
What Qualifies as a Comparable Sale
Not every nearby transaction qualifies as a comp. Appraisers and analysts apply several screening criteria before a sale enters the analysis:
Arm's-length transaction. The buyer and seller must have acted independently, without duress or a pre-existing relationship that distorted the price. Foreclosure sales, estate sales, and inter-family transfers are frequently excluded or flagged.
Proximity. A comp should be in the same neighborhood or a competing location. The acceptable radius varies by property type and market density: a quarter-mile in urban infill versus ten or more miles in rural settings.
Recency. Sales from within the prior six months are standard. Lenders following Fannie Mae guidelines generally require at least three comps within that window, with a preference for three months in volatile markets.
Physical similarity. Gross living area (GLA), lot size, age, condition, style, and room count should all be reasonably close to the subject. Analysts commonly accept GLA within 15–20% as a starting threshold, though no hard rule applies universally.
Verified data. Comps are sourced primarily from the multiple listing service, supplemented by public records, deed transfers, and third-party data aggregators. Tools such as Tophap Explorer draw on layered data sources to surface transaction history, prior listing details, and property characteristics for candidate comps.
The Adjustment Process
Selecting a comp is only the first step. Because no two properties are identical, each comp's sale price must be adjusted to reflect how it differs from the subject. This adjustment grid is the core of the sales comparison approach.
Adjustments are directional: if a comp has one fewer bathroom than the subject, a positive adjustment is added to the comp's price (the comp sold for less than it would have with the extra bathroom). If the comp is larger by 400 square feet, a negative adjustment reduces its price. The result is an adjusted sale price for each comp, and those adjusted values are then reconciled into a final value indication.
Adjustment amounts are derived from market evidence, not arbitrary rules. Paired sales analysis—comparing two nearly identical transactions that differ in only one feature—is the preferred method for extracting unit adjustments. Statistical regression on large transaction datasets is increasingly used as a cross-check, and platforms like REI-litics apply data-driven models that can support this kind of paired analysis at scale.
Common adjustment categories include:
- Location: corner lot, busy street, proximity to amenities or nuisances
- Site size: lot area differences beyond a de minimis threshold
- Gross living area: typically the single largest adjustment
- Room count: bedrooms, bathrooms, half-baths
- Age and condition: effective age often matters more than chronological age
- Amenities: garages, pools, finished basements, view
- Concessions: seller-paid closing costs must be backed out of the comp's gross price
Comps in Lending vs. Investment Contexts
In a mortgage transaction, the appraiser's comp selection and adjustment methodology is subject to lender review and secondary market guidelines. Fannie Mae, Freddie Mac, FHA, and VA each publish guidance on acceptable comp characteristics, and underwriters can challenge individual comps or request additional sales.
In investment analysis, comps serve a different purpose. An investor analyzing a fix-and-flip will use after-repair-value comps to estimate the exit price, while a buy-and-hold investor may use comps primarily to validate that the purchase price is reasonable relative to the market. Tools like ACC AI Deal Assistant integrate comp data into deal underwriting workflows, enabling faster scenario analysis without replacing the underlying need to verify comp quality.
The 2026 Guide to AI Tools in Real Estate covers how automated comp selection tools fit into broader valuation and deal analysis workflows.
Limitations of Comparable Sales Analysis
The sales comparison approach depends entirely on the availability of recent, relevant transactions. In thin markets—rural areas, luxury segments, or highly unique properties—finding adequate comps is the primary challenge. When the market shifts rapidly, even six-month-old comps may not reflect current conditions without explicit time adjustments.
Automated valuation models address this by weighting large transaction pools statistically, but they trade individual comp transparency for breadth. A traditional comp grid remains the standard where lenders require a credible, defensible value opinion. Understanding the automated valuation model alongside comparable sales methodology helps practitioners know when each tool is appropriate.
For buyers and sellers, reviewing the comps behind any valuation—whether from an agent's CMA or a formal appraisal—is a basic due-diligence step. Price-per-square-foot comparisons, days-on-market trends, and list-to-sale price ratios all add context that a single adjusted value figure cannot convey on its own.