Why online property value estimates never agree — and how much to trust any of them

Search the same address on a few different property websites and you'll often get a few
different answers — sometimes tens of thousands of dollars apart, sometimes more. That's not a
glitch on one site or a sign that something's wrong with your property. It's a predictable result
of how these numbers actually get made. This page explains the mechanism, how wide the gap can
genuinely get, and — because it's a fair question to ask of us too — how to read Delora's own
suburb-level fair-value figure with the same healthy scepticism.

What's actually generating that number

An online "estimate" is almost always an automated valuation model (AVM) — a computer-driven
statistical model that estimates a property's value from its recorded characteristics (land size,
bedrooms, property type), recent comparable sales nearby, and broader market trends. Critically,
an AVM does not include a physical or visual inspection of the property by an appraiser, and each
provider's confidence-score system — the "how sure are we" rating some estimates carry — differs
and isn't standardised across the industry. That's not Delora's characterisation of AVMs; it's
stated directly by one of the AVM providers itself, Cotality (formerly CoreLogic), in its own
consumer disclosure.

In practice, that means an AVM can't see the renovated kitchen, the busy road out front, the
north-facing block, or the fact that the "4-bedroom comparable sale" down the street was actually
a knock-down deal. It's working from records and patterns, not a walk-through.

Three different companies, three different answers

A large part of why estimates disagree is simpler than any modelling detail: there isn't one
Australian AVM — there are several, run by different companies on different data. PropTrack,
which provides the automated valuation and digital-valuation tools used across REA Group's
platforms (REA Group owns realestate.com.au), is a distinct business from Cotality (formerly
CoreLogic), which is in turn a separate data provider from Domain's own valuation model. Three
different companies, drawing on their own comparable-sales data and their own statistical
approach, will not produce identical numbers for the same property — that's expected, not a bug
in any one of them.

Common mistake: treating agreement between two estimates as independent confirmation. If two
tools you're comparing are ultimately licensing data from the same underlying provider, seeing a
similar number twice isn't two opinions converging on the truth — it's one opinion appearing
twice.

How wide can the gap actually get

Genuinely wide. In a test run by CHOICE — Australia's consumer advocacy body, not a real estate
business — comparing online valuation tools against one Sydney inner-west terrace ahead of its
April 2017 auction, the tools produced a $560,000 spread between their lowest and highest
estimated price ranges, and a $355,000 difference between the tools' individual point
estimates. That test is from 2017 — we're citing it as a real, documented illustration of how
large the gap between AVMs can get, not as a current statistic. But the underlying mechanism that
produced it — different companies, different comparable sales, no physical inspection — hasn't
changed, which is exactly why sellers and buyers still routinely notice the same kind of
disagreement between today's estimates.

The one alternative that actually states its margin of error

If you need a number to actually rely on — for a sale, a purchase decision, or a dispute — the
honest comparison point is a professional valuation from a Certified Practising Valuer, not
another online estimate. CHOICE's guidance: expect to pay around $500, and expect an accepted
margin of error of about ±10% — a figure a valuer will actually put in writing, because they
physically inspect the property and stake their signature on the number. That's the real contrast
worth noticing: a professional valuation states its own uncertainty; most free online estimates
don't state one at all.

How Delora's own suburb estimate is different — and how you should read it too

Every Delora suburb page shows a modelled "fair value" for that suburb, alongside the actual
current median. We built it because we think the AVM problem above deserves an honest answer, not
just a fourth competing number — so our model is different in two specific, checkable ways.
First, it doesn't claim property-level precision it can't back up: it's a suburb-level model,
explicitly not a valuation of any individual property, shown with its assumptions and a stated
confidence range rather than a single bare figure. Second, we publish our own scorecard: a
public backtest of the model against real subsequent sale prices,
across five separate historical periods back to 2013 — including the tests where the model's
call was wrong. Across those five tests, the model's "undervalued" and "premium" calls were
right roughly 51–62% of the time — a real, moderate edge over a coin flip, and nowhere close to a
promise. See what actually drives the number, suburb by suburb
for the underlying methodology.

That's the standard we'd ask you to hold any estimate to, including ours: does it say what it's
built from, does it admit what it can't see, and does it show you how often it's actually been
right? Most online property estimates answer none of those three questions. Read Delora's own
number the same way you should read any of them — as modelled context with a stated confidence
range, not a verdict.

What to actually do with any of these numbers

• Treat every online estimate — including Delora's — as a rough starting range, not a number to
  negotiate around.
• If two estimates agree, check whether they're actually independent, or just licensing the same
  underlying data.
• For a decision that has real money attached to it (a sale, an offer, a dispute with a lender or
  an insurer), a Certified Practising Valuer's physical inspection and stated margin of error is
  worth more than any number of free online estimates combined.
• If your own home's estimate looks wrong, remember the model doesn't know about the renovation,
  the noisy road, or the block's actual orientation — that's a limitation of the method, not
  necessarily an error to fight with the provider.

Frequently asked questions

Which online property estimate is the most accurate?
None of them publish a directly comparable, standardised accuracy figure, so there's no verified
answer to "which is best." What's actually established is that different providers (PropTrack,
Cotality, Domain) use different data and models and will legitimately disagree — treat the spread
between them as more informative than any single number.

Should I ignore online estimates completely?
Not necessarily — they're a reasonable, free starting point for a rough sense of range. Just don't
treat one as a valuation, a negotiating anchor, or a reason to be alarmed if it's lower than you
expected.

Why did my property's estimate change even though nothing happened to the house?
AVMs update as new comparable sales occur nearby and as the underlying model is retrained — a
sale down the street can move your own estimate even though nothing about your property changed.

Is Delora's fair-value figure more accurate than realestate.com.au's or Domain's estimate?
We don't claim that, and you shouldn't assume it. Ours is a suburb-level model with a published,
independently checkable track record — theirs are property-level estimates from proprietary
models we can't audit. They're answering different questions at different levels of precision;
neither is a substitute for a professional valuation when real money is on the line.

See also: our fair-value model's public track record
· which property data tools are actually worth paying for
· what actually drives Melbourne house prices ·
how Delora sources and verifies its content

Delora provides general information, not legal, financial or investment advice, and is not a
substitute for a licensed valuer, conveyancer or financial adviser. Delora's suburb-level fair-
value figure is a modelled estimate, not a valuation of any individual property. Always obtain a
professional valuation before relying on any property value for a sale, purchase or financial
decision.