Most home value estimates are a black box. Zillow gives you a single number. Redfin gives you a single number. Neither one shows you how they got there — and neither one lets you challenge it.
But real estate isn't a pure math problem. Your home's value depends on things no algorithm can see: the $50,000 kitchen remodel, the quiet cul-de-sac, the view that makes buyers gasp when they walk in. A number without context is just a guess.
What's Wrong With the Zestimate
Zillow's Zestimate uses broad ZIP code averages and a proprietary algorithm. It's a starting point — but it has three fundamental flaws:
- It's a black box. You can't see which comparable sales it used or how it weighted them. If the number is wrong, you have no way to understand why.
- It can't see your upgrades. A $50,000 kitchen renovation? A new roof? Hardwood floors? The algorithm doesn't know — and can't adjust.
- It treats all locations equally. A home backing to a park and one backing to a highway in the same ZIP code get similar estimates. But the market values them very differently.
Zillow itself says its estimates have a median error rate of several percent. On a $1 million home, that's a $50,000+ margin of error.
What a Real CMA Looks Like
A Comparative Market Analysis (CMA) is what professional appraisers and managing brokers use. It pulls the closest comparable sales — not ZIP code averages — and adjusts each one for the differences between that home and yours.
ShopProp built this into a free tool anyone can use. Here's what makes it different:
- 10 closest comparable sales exposed. Every comp shows address, beds, baths, square footage, lot size, year built, sale date, sale price, and price per square foot — with over/under indicators.
- 7 interactive adjustment sliders. Condition. Location. View. Waterfront. Age/Updates. Lot Size. Demand. Move any slider and your estimated value recalculates instantly.
- Remove outlier comps with one click. That foreclosure sale three blocks away? Remove it. The new-construction spec home? Remove it. Your valuation should reflect YOUR market, not noise.
- Add your own comps. Found a recent sale the tool missed? Add it manually and it factors into your estimate.
The "20% Rule" — Is the Listing Overpriced?
For buyers, ShopProp's CMA tool does something no other platform offers: it shows you the percentage delta between what a seller is asking and what comparable homes actually sold for.
If a home is listed at $2,250,000 but the 10 closest comps averaged $1,868,000, the tool flags it: "Listed 20% above comparable sales. There may be room to negotiate."
That's not an opinion — it's data. And it gives you negotiating leverage that no traditional agent provides.
Why Your Agent Might Not Want You to Have This
Traditional real estate thrives on information asymmetry. The less you know about comparable sales, the more you depend on your agent's interpretation. They pull comps behind closed doors, interpret them privately, and present you with a recommended price.
ShopProp flips that model. We show you everything — the raw data, the math, the adjustments. Then our managing broker brings decades of experience to help you interpret it. But the data is always yours to see.
That's the difference between an agent-driven brokerage and a consumer-driven one.
Try It Free
ShopProp's CMA tool is free for any address. No sign-up required. See the comps. Adjust the sliders. Know what any home is really worth.
And when you're ready for representation: ShopProp's buyer fee starts at $1,995 flat. Seller listing from $4,995. Managing broker on every deal since 2007.