Algorithms Don't Buy Houses: Making Sense of Two Very Different Valuations

Every seller we meet has already done it. Twelve seconds on a property portal, one postcode, one number on screen, and a quiet sense of what the house is worth. Then an agent walks round, sits at the kitchen table, and puts a figure on it that is either flattering or insulting by about twenty grand.

That gap is not usually a scam and it is not usually a mistake. It is two completely different methods answering two completely different questions. Once you understand which is which, the whole pricing conversation gets a lot less fraught.

What the online estimate is actually measuring

Automated valuations are, at heart, very well-dressed arithmetic. They pull together Land Registry sold prices, postcode averages, property type, bedroom count, floor area where it is available, and the movement of the wider market. Then they index the last recorded sale forward to today.

That is genuinely useful information. It is just not a valuation of your house. It is a valuation of a spreadsheet row that resembles your house.

What the model cannot see: that you extended in 2021. That the kitchen was replaced last spring. That the back garden backs onto a railway line, or alternatively onto nothing but trees. That the bathroom is original and mint green. That there is a long lease, a hefty service charge, an awkward shared access or a particularly good school two streets away.

So the online number is a starting reference point, and a decent one. Treat it as a range with soft edges. Treat it as a floor and ceiling, not a price.

What an appraisal is measuring, when it is done properly

A good agent is not valuing your house either. They are valuing the competition.

The question is never really what your house is worth in the abstract. It is what a buyer, standing in your hallway on a wet Saturday in February with three other viewings booked, would choose to pay for it instead of the alternatives. That means recent sold evidence for genuinely comparable homes, but it also means knowing what is currently on the market down the road, how long it has been there, and why it has not sold.

That is a judgement built on viewing traffic, offers, fall-throughs and the mood of the local market. You cannot scrape it from a database.

Where the twenty thousand pounds comes from

In our experience, a large gap between the two numbers almost always traces back to one of three causes.

Condition. The portal saw a four-bedroom semi from 1935. The agent saw a house that needs a new roof and a bathroom. Condition is invisible to the algorithm and decisive for the buyer.

Data lag. Sold prices take months to appear. If your street has moved twenty per cent in eighteen months, the automated figure is quoting history.

Optimism. Sometimes the higher number is simply an agent telling you what you want to hear to win the instruction. It is a business development tactic dressed up as a valuation, and it costs you more than the agent.

When the low number deserves respect

If the online estimate comes in well below what you were hoping, do not dismiss it. Look at what it is based on. If the comparable sales it is drawing from are genuinely similar properties in similar condition, it is telling you something. If it is pulling in flats from the next postcode or bungalows from a different road, it is noise.

A useful test: find the three sold prices it is leaning on and ask yourself honestly whether you would swap your house for any of them.

When the high number deserves respect

Occasionally an agent's figure sits well above the algorithm and they can prove it. Genuine scarcity is the usual reason. A particular road, a specific school catchment, a house type that rarely comes up, a plot that cannot be replicated. If they can show you three recent sales that support the number and name the buyers who would pay it, that is a valuation. If they cannot, it is a brochure.

Questions that separate a valuer from a leaflet-dropper

Ask these, and listen to how quickly the answers come.

• Which three sold properties are we actually being measured against?
• Which property currently on the market are we competing with for the same buyer?
• Who is the likely buyer, and where are they living now?
• What percentage of asking price do you typically achieve, and how long does it take?
• What is your plan if we have no offers in six weeks?
• If we price at your figure, what does that make us look like next to the neighbours?

A good agent will enjoy those questions. A poor one will start talking about marketing packages.

How to spot lazy pricing

Round numbers are the giveaway. So is a figure that exactly matches what next door sold for two years ago, or a recommendation prefaced with the words try it and see. A valuation with no written comparable evidence attached is a guess with a handshake.

Also watch for the agent who never mentions a single drawback. Any honest appraisal includes the things buyers will use to negotiate, and what they might cost you. If they cannot name one, they have not looked properly.

Using both numbers properly

Get two or three appraisals. Compare the evidence, not just the figures. Lay them against the online range and see whether each one sits inside it, above it or below it, and whether the reasoning holds up.

Then price with a strategy rather than a hope. The first fortnight of a listing is when you get your best traffic, your most motivated buyers and your strongest offers. Overprice and you burn that window, then spend the next three months explaining a reduction to people who have already decided something is wrong with the house.

In 2026, buyers arrive at viewings having already checked the sold prices on your street. They are better informed than at any point in the last twenty years, and they will not be talked into a number by a glossy brochure. Price sensibly, present it well, and let the market confirm what the algorithm could only guess at.