Rightmove figures miss the story: how portal data distorts prime London

In April 2026, the most expensive house sale in British history completed in Chelsea at more than £270 million. Search the portals for it and you will find nothing: no listing, no price history, no record it was ever for sale. Picture the same transaction on the open market instead. Had the house launched at an ambitious figure and sold months later for tens of millions less, the headline would have written itself: sold for £80 million below asking. That story, fair or not, would have followed the property forever. Off-market, it simply does not exist. Nobody knows what the house first aimed for, how long it took, or what was turned down along the way, and nobody ever will, including its buyer. That is precisely the protection sellers are choosing. Now consider that the same is true of a third of all £1m-plus sales, and more than half above £2m, according to Hamptons. Every market report built on portal data is describing prime London with its largest segment missing.

How big is the blind spot?

Bigger than most buyers assume, and growing for a decade. Hamptons recorded London off-market sales at under 9 per cent of transactions in 2018; by 2022 it was a record 23 per cent, a near-tripling that Mortgage Strategy charted as the habit spread beyond the traditional prime core. At the top of the market the open portal is now the minority venue: 54 per cent of £5m-plus sales in 2023 never carried public advertising. As The Sunday Times concluded, off-market sales have gone mainstream.

The distortion compounds in three ways. First, average prices: the discreet half of prime sales includes much of the best stock, so portal-derived £-per-square-foot benchmarks systematically understate what excellent properties actually achieve. Second, time on market: the fastest transactions never enter the data, because the private market moves in days, not months. Third, supply: a buyer counting portal listings in their search area is not measuring availability, only the publicly visible fraction of it.

“Off-market sales have gone mainstream, with the number of properties sold this way hitting new highs.”

The Sunday Times

Why does the gap keep widening?

Because the data itself drives sellers away. Every day on market and every price change a portal records is leverage in a buyer’s hands, and sellers have learned to deny them the ammunition. By early 2023, CNBC was reporting that London’s luxury sellers had moved to WhatsApp rather than expose their homes to public scrutiny. The portals created transparency; transparency created the incentive to leave.

Land Registry data eventually captures the prices, but months in arrears and stripped of context: no marketing history, no time-to-sale, no record of whether the price followed competition or a single quiet approach. By the time an off-market sale appears in official statistics, the market it describes has moved on.

The practical conclusion differs by reader. Analysts should treat portal-based prime London indices as partial by construction. Sellers should understand that the leverage portal data hands to buyers is optional. And buyers should accept that no amount of search refinement reveals stock that was never uploaded; for that, the answer is access. Register as a member and see the market the portals cannot show you.